-------
Table 17. Coupon-Weight-Loss Data for 1020 Carbon Steel in 50 Weight Percent STPP-Based Detergents
Detergent Concentration, Hardness,
weight percent ppm Coupon
0.06 15 H45
H46
150 H47
H48
0.12 15 Hll
H12
H13
£ H14
150 H21
H22
H23
H24
0.18 15 H25
H26
150 H27
H28
=====
Temp,
F
130
130
130
130
160
160
130
130
160
160
130
130
130
130
130
130
Exposure,
days
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
Corrosion E
mdd
60.7
56.0
26.4
23.8
43.9
70.2
41.4
40.1
167.9
213.3
109.5
138.3
273.8
271.6
68.8
58.0
ropy Remarks
11.1 Dark brown deposit. General
10.2 corrosion plus heavier localized
corrosion.
4.83 Bright appearance. General
4.35 corrosion.
8.03 Local etching corrosion.
12.9
7.58 Local etching corrosion.
7.34
30.7 Mostly heavy general etching
39.0 plus some localized.
20.0 Mostly general mild etching.
25.3
50.1 General even corrosion
49.7
12.6 Local etching corrosion.
10.6
f\
(a) mdd = milligram/(decimeter) /day
mpy = mils penetration/year
-------
Table 18. Coupon-Weight-Loss Data for 1020 Carbon Steel in 50 Weight Percent NTA-Based Detergents
Detergent Concentration, Hardness,
weight percent ppm Coupon
0.06 15 H41
H42
150 H43
H44
0.12 15 H9
H10
HIS
H19
150 H15
H16
H17
H20
0.18 15 H29
H30
150 H31
H32
Temp,
F
130
130
130
130
160
160
130
130
160
160
130
130
130
130
130
130
Exposure,
days
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
Corrosion
mdd
161
157
116
85.8
152
140
277
311
180
207
130.1
110.0
97.9
81.6
185
137
Rate(a)
mpy
29.5
28.7
21.2
15.7
27.8
25.6
50.6
56.9
32.9
37.8
23.8
20.1
17.9
14.9
33.9
25.1
Remarks
General and some heavier
localized corrosion.
Reddish-brown film. Even
general corrosion.
Localized shallow etching
(2-mil deep) under corrosion
product.
General corrosion.
Intense local etching corrosion.
Local etching corrosion.
Local etching corrosion.
Localized and general corrosion.
(a) mdd = milligram/(decimeter) /day
mpy = mils penetration/year
-------
P416
FIGURE 27.
1020 CARBON STEEL COUPON H26
AFTER EXPOSURE
Note general corrosion.
P639
FIGURE 28. 1020 CARBON STEEL COUPON H41
AFTER EXPOSURE
Note general corrosion and
heavier localized corrosion.
P419
FIGURE 29. 1020 CARBON STEEL COUPON H27
AFTER EXPOSURE
Note corrosion-deposit-free
localized corrosion.
P638
FIGURE 30. 1020 CARBON STEEL COUPON H46
AFTER EXPOSURE
Note corrosion-deposit-covered
localized corrosion.
56
-------
70
60
50
15 ppm 150 ppm
Hardness Hardness
50 weight' percent STPP based detergent
50 weight percent NTA based detergent
o
A
0.06
denotes localized corrosion
0.12
Detergent Concentration, weight percent
0.18
FIGURE 31. CORROSION RATE OF 1020 CARBON STEEL AS A FUNCTION
OF DETERGENT CONCENTRATION IN 15 PPM AND 150 PPM
WATER HARDNESS AT 130 F
57
-------
60
10
15 ppm
Hardness
0.06 weight percent detergent concentration
0.12 weight percent detergent concentration
0.18 weight percent detergent concentration
denotes localized corrosion
0
50
_J
50 STPP
0 NTA
STPP and NTA Concentration, weight percent
FIGURE 32 (a). CORROSION RATE OF 1020 CARBON STEEL AS A FUNCTION
OF STPP AND NTA CONCENTRATION AT 15 PPM WATER
HARDNESS AND 130 F
58
-------
60
50
o
O)
o>
Q.
tf)
40
- 30
o:
c
g
'to
o
i_
o
O
10
0.06 weight percent detergent concentration
0.12 weight percent detergent concentration
0.18 weight percent detergent concentration
L---denotes localized corrosion
150 ppm
Hardness
0
50
_J
50 STPP
0 NTA
STPP and NTA Concentration, weight percent
FIGURE 32 (b).
CORROSION RATE OF !020 CARBON STEEL AS A FUNCTION
OF STPP AND NTA CONCENTRATION AT 150 PPM WATER
HARDNESS AND 130 F
59
-------
0)
CL
100 -
80
60
40
20
10
8
6
o
cc
c
I I0°
t 80
o
° 60
4O
20
10
8
6
0.12 weight percent of 50 STPP detergent
0.12 weight percent of 50 NTA detergent
0.12 weight percent of 37.5 STPP-12.5 NTA detergent
15 ppm
Hardness
o
A
a
I 2
Exposure Time, days
(a)
3
150 ppm
Hardness
0.12 weight percent of 50 STPP detergent
0.12 weight percent of 50 NTA detergent
0.12 weight percent of 37.5 STPP-12.5 NTA detergent
Exposure Time, days
(b)
FIGURE 33. CORROSION RATE OF 1020 CARBON STEEL AS A FUNCTION OF EXPOSURE
TIME IN (a) 15 PPM AND (b) 150 PPM HARDNESS SOLUTIONS AT 130 F
AND USE OF THE LINEAR POLARIZATION METHOD
60
-------
the corrosion rate increased over the first day's exposure to steady state.
>ehavior paralleleda change in corrosion potential from about -0.6 to
• volt versus SCE. The linear polarization results which were evaluated
:he determined values of bA = 0.14 volt and bc = ooand Equation 5 Rave
corrosion rates between about 15 and 40 mils per year which were in good
with weight loss data. The linear polarization results appeared
to indicate that STPP solutions were more corrosive than NTA solutions with
the mixed STPP-NTA solutions of intermediate behavior. The STPP solution
•n the .ft-water condition appeared to be the most corrosive. In several
ons, the linear polarization method gave responses similar to that
ihown in Figure 4(a) indicating a localized form of corrosion, as evidenced
on the electrode after experiments.
A limited number of studies were made of corrosion products by X-ray diffraction
Coupons H27/H28 and H45/H46 from STPP solutions were found to contain a
1)3 product as did coupons H29/H30 from an NTA solutions. Coupons H43/
from a NTA solution showed the presence of a 6 -FeOOH product with a
for NaFe02 and a very faint pattern for Fe304. It appears that
OH)3 which was sharper from samples off coupons H45/H46 was aided by the
soft-water condition of the low-STPP concentration. This solution corresponded to
the lowest PH value of 10.0 of these solutions producing this corrosion product.
Chemica1 Lead
Tables 20 and 21 summarize the weight-loss data for chemical lead in STPP
NTA solutions, respectively. Weight losses varied between 11 and 223 mg
per coupon over the 5-day exposure periods. Corrosion rates between 1.3 and
29 mils per year were moderately low and very high, respectively. The corrosion
J material appeared to be general and a dull white or gray deposit
typi::ied in Figure 34 was observed on the specimens.
P415
FIGURE 34.
(a)
(b)
CHEMICAL LEAD COUPON L26 (a) BEFORE AND
Note fine white deposit.
(b) AFTER EXPOSURE
61
-------
Table 20. Coupon-Weight-Loss Data for Chemical Lead in 50 Weight Percent STPP-Based Detergents
Detergent Concentration, Hardness,
weight percent ppm Coupon
0.06 15 L45
L46
150 L47
L48
0.12 , 15 Lll
L12
L13
L1A
c* 150 L21
L22
L23
L24
0.18 15 L25
L26
L27
L28
Temp,
F
130
130
130
130
160
160
130
130
160
160
130
130
130
130
130
130
Exposure,
days
5
5
5
. 5
5
5
5
5
5
5
5
5
5
5
5
5
Corrosion
mmd
34.4
32.8
10.1
13.4
25.1
25.4
54.3
62.3
72.1
78.9
65.3
57.3
177.2
150.8
95.0
100.2
Rate(a)
mpy
4.37
4.17
1.28
1.70
3.19
3.23
6.90
7.91
9.16
10.0
8.29
7.28
22.5
19.2
12.1
12.7
Remarks
Dull greenish-gray film. General
corrosion.
Dull light gray film. General
corrosion.
Dull gray. General corrosion.
Dull gray with yellow spotted
areas. General corrosion.
Dull gray and traces of white
and brown deposits. General
corrosion.
Dull gray with traces of white
deposit. General corrosion.
Fine dull white deposit. General
corrosion.
Dull gray with trace of white.
General corrosion.
(a) mdd = milligram/(decimeter) /day
mpy = mils penetration/year
-------
Table 21. Coupon-Weight-Loss Data for Chemical Lead in 50 Weight Percent NTA-Based Detergents
Detergent Concentration, Hardness,
weight percent ppm Coupon
. 0.06 15 L41
L42
150 L43
L44
0.12 15 L9
L10
L16
^ L17
W 150 L15
LI 8
L19
L20
0.18 15 L29
L30
150 L31
L32
Temp,
F
130
130
130
130
160
160
130
130
160
160
130
130
130
130
130
130
Exposure,
days
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
(a)
Corrosion Rate
mdd
104
119
43.8
28.3
219
226
112
137
169
155
109
101
146
174
131
127
mpy
13.2
15.2
5.56
3.59
27.8
28.7
14.2
17.4
21.4
19.7
13.8
12.8
18.5
22.0
16.6
16.1
Remarks
Fine white deposit. General
corrosion.
Dull light gray film. General
corrosion.
Fine crystalline deposit. General
corrosion.
Fine crystalline deposit. General
corrosion.
Coarse crystalline white deposit.
General corrosion.
White crystalline deposit.
General corrosion.
Fine white deposit. General
corrosion.
Dull gray with traces of
yellowish-green white
deposit. General corrosion.
(a) mdd = milligram/(decimeter) /day
mpy = mils penetration/year
-------
50
40
0>
ex
(A
0)
"5
o:
g
'in
o
*_
o
o
30
20
10
50 weight percent STPP based detergent
50 weight percent NTA based detergent
15 ppm 150 ppm
Hardness Hardness
o •
A A
0.06
0.12
Detergent Concentration, weight percent
0.18
FIGURE 35.
CORROSION RATE OF CHEMICAL LEAD AS A FUNCTION
OF DETERGENT CONCENTRATION IN 15 PPM AND 150
PPM WATER HARDNESS AT 130 F
64
-------
30 i—
20 9=
0.06 weight percent detergent concentration
0.12 weight percent detergent concentration
0.18 weight percent detergent concentration
15 ppm
Hardness
A
o
D
a>
o.
a>
"a
a:
c
o
o
o
STPP and NTA Concentration, weight percent
(a)
50 STPP
0 NTA
30
20
150 ppm
Hqrdness
0.06 weight percent detergent concentration A
0.12 weight percent detergent concentration •
0.18 weight percent detergent concentration •
STPP and NTA Concentration, weight percent
(b)
50 STPP
0 NTA
FIGURE 36. CORROSION RATE OF CHEMICAL LEAD AS A FUNCTION OF
STPP AND NTA CONCENTRATION AT (a) 15 PPM AND (b)
150 PPM WATER HARDNESS AND 130 F
65
-------
0.2
O.I
0.08
0.06
0.04
0.02
s
Q, 0.01
j« 0.008
0.12 weight percent ot 50 STPP detergent
0.12 weight percent of 50 NTA detergent
0.12 weight percent of 37.5 STPP- 12.5 NTA detergent
15 ppm
Hardness
o
a
a
cc.
c
o
"8
o
o
0.2
0.1
0.08
Q06
0.04
0.02
0.01
0.008
Exposure Time, days
(a)
0.12 weight percent of 50 STPP detergent
0.12 weight percent of 50 NTA detergent
0.12 weight percent of 37.5 STPP-12.5 NTA detergent
150 ppm
Hardness
I 2
Exposure Time, days
(b)
FIGURE 37. CORROSION RATE OF CHEMICAL LEAD AS A FUNCTION OF EXPOSURE
TIME IN (a) 15 PPM AND (b) 150 PPM HARDNESS SOLUTIONS AT 130 F
AND USE OF THE LINEAR POLARIZATION METHOD
66
-------
Figure 35 summarizes the weight-loss data in Tables 20 and 21. The data
indicated that the corrosion rate of chemical lead increased with increase
of detergent concentration and that NTA solutions appeared to be more
corrosive than the corresponding STPP solutions. The most corrosive
solutions were those based upon NTA in the soft-water condition.
Figure 36 shows the weight-loss data from which the increased corrosivity
of the NTA solutions could be determined. These results are summarized in
Table 22 which shows that the corrosivity factor varied between 1.0 and 3.1.
Values for both soft- and hard-water conditions are similar and increased
with decreased detergent concentration.
Table 22. Increased Corrosivity Factors for NTA Solutions
Over STPP Solution with Chemical Lead
Detergent Concentration,
weight percent 15-ppm Hardness 150-ppm Hardness
0.06 3.1 " 3.0
0.12 2.1 1.6
0.18 1.0 1.3
The linear polarization measurements on chemical lead are summarized in
Figure 37. As seen from Figure 37, large changes in corrosion rates were
measured over the first-day exposure period. Generally the corrosion
potentials varied between -0.5 and -0.6 volt versus SCE but did not parallel
the marked changes in corrosion rate. The corrosion rates were calculated
using Equation 5 and the determined values of b^ = 0.30 volt and b^ = °°
from polarization curves. It was apparent that the corrosion rates measured
by the linear polarization method were very much smaller than those determined
by weight loss. Again, it would appear that poorly, conducting films produced
on the material in these solutions gave a low assessment of corrosion rate.
X-ray diffraction identified corrosion products of
and N32S04 on coupons L25/L26 from a STPP solution. Coupons L29/L30 from a
NTA solution indicated products of Pb3(C03)2(OH)2 and PbO(red). The product
PbO (red) was also identified on the coupons L31/L32 from another NTA solution.
Lead was the only material in which phosphate deposits were identified on
materials exposed to STPP-based detergents.
Cast-iron Soil-Pipe Material
The weight- loss data for this material are shown in Tables 23 and 24 for as-
prepared and soil STPP and NTA solutions respectively. Soil-pipe studies
were conducted at 130 F since this temperature related to studies of other
materials and was only slightly higher than the usual temperature (115 to
120 F) of detergent solutions goitg to drain. As shown in Table 23, weight-
loss data on this material was obtained over exposure periods of 2 and 7
days. Two exposure periods were used because 2-day corrosion rates were
very high and, therefore, there was a need to determine if these corrosion
rates became stiff led on longer exposure. The corrosion rates of this
material were always very high (16 to 120 mils per year) in the fresh
detergent solutions (Table 23).
67
-------
Table 23. Coupon-Weight-Loss Data for Cast Iron Soil Pipe in STPP and NTA-Based Detergents
ON
00
Detergent Concentration, Hardness,
weight percent ppjn Coupon
0.12 of 50 wt. percent 15
NTA-based detergent
150
0.12 of 50 wt. percent 15
STPP-based Detergent
150
(a) mdd « milligran/ (decimeter) 2/day
mpy - mils penetration/year.
Ml
M2
M9
M10
M3
M4
Mil
M12
M5
M6
M13
M14
M7
M8
M15
M16
*
Temp, F
130
130
130
130
130
130
130
130
130
130
130
130
130
130
130
130
Exposure,
days
2
2
7
7
2
2
7
7
2
2
7
7
2
2
7
7
Corrosion
mdd
602
595
391
416
144
150
513 .
325
80.9
100
175
267
246
180
233
230
Rate(a)
mpy Remarks
120 Mottled brown color. General
119 corrosion.
78.2 Light and dark brown. General
83.1 corrosion.
28.8 Mottled brown color. Mostly
30.0 general corrosion with some
pitting. (<0.5 mil depth).
103 Light reddish-brown. General
65.0 corrosion.
16.2 Mottled brown color. Mostly
20.0 general corrosion with trace
of pitting (<0.5 mil depth).
34.9 Light-brown and black deposit.
53.3 Mostly general corrosion
with a little localized.
49.0 Localized and general corrosion.
35.9
46.5 Encrusted brown, gray, and black
45.9 deposit. Mostly general
corrosion with more intense
localized.
-------
Table 24. Coupon-Weight-Loss Data for Cast Iron Soil Pipe in Soiled STPP and NTA-Based Detergents
Detergent Concentration, Hardness,
weight percent ppm Coupon
0.12 of 50 wt. percent 15 M17
NTA-based detergent M18
150 M19
M20
0.12 of 50 wt. percent 15 M21
STPP-based detergent M22
150 M23
M24
Temp, F
130
130
130
130
130
130
130
130
Exposure,
days
2
2
2
2
2
2
2
2
(a)
Corrosion Rate
mdd
383
354
227
147
175
280
74.8
45.2
mpy
76.6
70.8
45.4
29.4
35.1
56.0
15.0
9.0
Remarks
Light and reddish-brown.
Localized corrosion up to
0.4 mil deep and pits up
to 0.2 mil deep.
Light brown non-adherent
deposit. Large areas of
localized corrosion.
Yellow and light brown de-
posit. Localized corrosion
up to 0.4 mil deep and pits
up to 0.2 mil deep.
Pale yellow, poorly adherent
deposit. Mild general
corrosion.
(a) mdd = milligram/(decimeter) /day.
mpy = mils penetration/year.
-------
In the 2-day exposure period, the soft-water condition was more corrosive
in NTA solutions at 120 mils per year than STPP solutions with a rate of
about 18 mils per year. In the hard-water condition, however, the STPP
solution was slightly more corrosive at 43 mils per year compared to 30
mils per year with NTA solution. Figure 38(a) shows the typical appearance
of coupons M1/M2, M3/M4, M5/M6, and Figure 38 (b), the appearance of coupons
M7/M8 in this series.
P425 (a) (b)
FIGURE 38. CAST-IRON COUPONS (a) M4 and (b) M8 AFTER EXPOSURE.
Note general corrosion of M4 and mixed corrosion of M8.
From the data in Table 23, it was apparent that the 7-day exposure in NTA
solutions showed a decrease from 120 to 81 mils per year in soft-water
solution and an increase from 30 to 84 mils per year in hard-water solutions.
It appeared that, in 7 days, corrosion rates of 81 to 84 mils per year
obtained were independent of water hardness. The decrease in corrosion
rate from 120 to 81 mils per year in the soft-water solutions was un-
doubtedly due to the formation of a graphitized layer as shown by comparison
of Figures 39 and 40. The formation of a thin graphitized layer was also
noted on coupons M13/M14 in the STPP solution.
70
-------
, ^ - • •
*.?
•v. *
-
*
-r
v
•
*
'
-
C-3924
FIGURE 39. CROSS SECTION OF COUPON Ml
250X
Note genera 1, even corrosion and only thin graphitized layer.
C-3925
FIGURE 40. CROSS SECTION OF COUPON MlO
Note the thick graphitized layer.
250X
71
-------
In STPP solutions, the 7-day exposure indicated an increase of corrosion
rate from 18 to 44 mils per year in soft-water solutions and from 43 to 46
mils per year in hard-water solutions. Thus, as for NTA solutions, the
7-day exposure period gave corrosion rates that were independent of water
hardness. The rates of 44 to 46 mils per year in the STFF solutions were
much lower, however, than the rates of 81 to 84 mils per year in the NTA
solutions. The independence of corrosion rate with water hardness was
probably due to controlled dissolution through surface films which more
readily occurred in NTA solutions. X-ray diffraction of the deposit from
coupons M7/M8, however, failed to show the presence of crystalline corrosion
products.
The weight-loss data in Table 23 is summarized in Figure 41 to allow
evaluation of the increased corrosivity factors of NTA solutions over
corresponding STPP solutions. These factors are summarized in Table 25.
The data in Table 25 indicated, with the exception of the hard-water
condition and 2-day exposure period, the NTA solutions were more corrosive
Table 25. IncreasedCorrosivity Factors for NTA Solutions
Over STPP Solutions With Cast Iron
Detergent Concentration,
weight percent
0.12
0.12
Exposure,
days
2
7
15-ppm
Hardness
6.7
1.8
150-ppm
Hardness
1.8
(a) STPP solution more corrosive than NTA solution
than corresponding STFP solutions with increased corrosivity factors between
1.8 and 6.7. It appeared that, with the occurrence of graphitized layers
within the 7-day exposure period, the corrosivity factor became equal in
both hard-water and soft-water solutions at a value of 1.8.
A number of coupons have been exposed to STPP and NTA solutions which have
been deliberately contaminated in order to simulate spent or soiled solutions
which go to the drain. These solutions were made up using metal-salt
additions of Fe2(804)3 • 6H20, CuSO^ - 5H20, ZnS04 • 7H20, PbS04, and A12(30^)3
18H20 to 0.12 weight percent detergent solutions in proportion to the pre-
determined corrosion rates of the corresponding metals in these solutions.
These data are summarized in Table 26. The soft-water and hard-water NTA
solutions were adjusted to pH values of 8.8 and 9.3, respectively, after
preparation and the STPP solutions were adjusted to pH 9.3. The pH values
corresponded to mean values of the 1020 steel and die-cast zinc used solutions
in the corresponding detergents. These pH values were selected since 1020
carbon steel and die-cast zinc suffered the most corrosion and would,
therefore, determine the pH conditions of soiled solutions.
72
-------
15 ppm 150 ppm
Hardness Hardness
0.12 weight percent detergent concentration o •
o
a>
a>
CL
in
a>
"b
o:
c
o
'vt
o
w
O
o
50 STPP
0 NTA
STPP and NTA Concentration, weight percent
FIGURE 41
CORROSION RATE OF CAST IRON AS A FUNCTION OF
STPP AND NTA CONCENTRATION AT 15 PPM AND
150 PPM WATER HARDNESS AND 130 F
73
-------
Table 26. Summary of Data for Preparation of Soiled Detergent Solutions
Base Detergent Solution
0.12 wt. percent of
50 wt. percent NTA
and 15-ppm hardness
0.12 wt percent of
50 wt. percent NTA
and 150-ppm hardness
0.12 wt. percent of
50 wt. percent STPP
and 15-ppm hardness
0.12 wt. percent of
50 wt. percent STPP
and 150-ppm hardness
Metal
Fe
Cu
Zn
Pb
Al
Fe
Cu
Zn
Pb
Al
Fe
Cu
Zn
Pb
Al
Fe
Cu
Zn
Pb
Al
Corrosion Rate,
mdd
294 (a)
11.8
178
125
2.87
120 (a)
10.9
96.7
105
1.64
r \
6.0
104
58.3
2.65
124(a)
3.0
56.5
61.3
1.62
Metal Loss /Coupon
In 2 Days, mg
117.3
4.7
76.0
52.9
1.1
47.8
4.4
41.2
44.4
0.6
16.2
2.4
44.4
24.7
1.0
49.4
1.2
24.2
26.0
0.6
Equivalent Weight of Metal Salt Added
To 150 Ml Detergent Solution, mg
537.2 Fe2(S04>3 6H20
18.5 CuS04 5 H20
334.4 ZnS04 7H20
77.2 PbS04
13.6 A10(SO,)0 18H00
2 4 j i
218.9 Fe2(S04)3 6H20
17.3 CuS04 5H20
181.3 ZnS04 7H20
64.8 PbS04
7.4 A1_(SO,)0 18H00
243 2
74.2 Fe2(S04)3 61^0
9.4 CuS04 5H20
195.4 ZnS04 7H20
36.1 PbS04
12.3A12(S04)3 18H20
226.3 Fe_(SO,)~ 6H.O
£, *T "^ *•
4.7 CuSO, 5H 0
4 *£
106.5 ZnSCh 7H 0
38.0 PbS04
7.4 A12(S04>3 18 H_0
(a) 1020 Carbon steel.
-------
The coupon-corrosion data for simulated soiled solutions are given in Table
24. Comparison of this weight-loss data to the 2-day weight-loss data in
Table 23 for unsoiled solutions showed no consistent trends due to soiling.
Thus, in NTA solutions in the soft-water condition, the corrosion rate
decreased from 120 to 74 mils per year, but in hard water the corrosion rate
increased from 29 to 37 mils per year with soiling. In STPP solutions and
soft water the corrosion rate increased from 18 to 46 mils per year, but in
hard waters the corrosion rate decreased from 45 to 12 mils per year with
soiling. In the soiled solutions, it was evident, however, that the NTA
solutions were more corrosive than the corresponding STPP solutions. In
the soft-water solutions, the increased corrosivity factor was 1.6, and
in hard-water solutions, 3.1. A greater increase in corrosivity of NTA
solutions was observed in hard-water solutions, i.e., solutions of greater
Cl" content.
Corrosion products on coupons M17/M18 and M21/M22 from the NTA and STPP
solutions, respectively, were found to contain principally 6 -FeOOH.
However, M17/M18 contained Fe-jO^ anda-NaFeC^ in small additions to the
main phase- of 6-FeOOH.
75
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SECTION IX
DISCUSSION
Corrosion studies have been conducted on a variety of materials of construction
associated with laundering: 1100 Aluminum, 260 Brass, electrolytic copper,
die-cast zinc, Type 304 stainless steel, 201 Nickel, Type 420 stainless steel,
chemical lead, and cast iron in a number of STPP- and NTA-based detergent
solutions. The solutions were prepared as representative of heavy-duty
granular detergents; i.e., 50-weight percent STPP or NTA at detergent con-
centrations of 0.06, 0.12, and 0.18 weight percent as likely to be employed
by the average housewife in washing machines. The use of water of hardness
values 15 and 150 ppm to relate to soft and hard water respectively, and
solution temperatures of 130 and 160 F was also designed to represent con-
ditions most likely to be met under laundering conditions. Coupon-weight-
loss data were the main source of corrosion-rate information but was supple-
mented by limited electrochemical "linear polarization" studies.
It was readily apparent from the corrosion results that the materials in-
vestigated could be classed into four groups with regard to corrosion
behavior in NTA and STPP-based detergents.
Group I included materials having very low corrosion rate in NTA and STPP
solutions of the order of 0.01 to 0.15 mil per year. In this group were
the materials in the following order of decreasing corrosion resistance:
Type 304 stainless steel, 201 Nickel, and Type 420 stainless steel.
Group II included materials of moderate corrosion resistance with pene-
tration of 0.2 to 3 mils per year. In this group were the materials:
-260 Brass, electrolytic copper, and 1100 Aluminum which were quite similar
with respect to their corrosion rates.
Group III included materials having poor corrosion resistance on the order
of 2 to 60 mils per year. In this group were chemical lead, die-cast zinc,
and 1020 carbon steel. Of these, chemical lead was slightly more corrosion
resistant.
Group IV was represented by cast-iron soil pipe which had an extremely
poor corrosion resistance material giving corrosion rates between 16 and
120 mils per year in the uncoated condition.
The materials within each of the above groups and the order of the groups
with regard to corrosion behavior are as expected considering the usual
corrosion behavior of the materials studied. Thus, it has been found that
the usually very inert stainless steel and nickel materials are also the
most corrosion resistant in the detergent solutions. On the other hand,
the active base materials such as zinc alloys and carbon steel have been
found to have poor corrosion resistance in the detergents. Copper, brass,
and aluminum have shown the expected intermediate character for these materials.
76
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As might be predicted from electrochemical theory, the group classification
of the materials with regard to corrosion rates was closely paralleled by
the corrosion potentials of the various materials. Thus, Group I materials
exhibited noble corrosion potentials of about -0.2 to -0.3 volt, Group III
materials exhibited active corrosion potentials of about -0.6 to -0.9 volt,
and Group II were of intermediate behavior with corrosion potentials of
-0.2 to -0.4 volt.
In almost all experiments, it was found that the soft-water condition of
15-ppm hardness was more corrosive than the corresponding solution with
hard water of 150-ppm hardness at a given detergent concentration. As
discussed in Appendix B, the greater corrosivity of softer water is well
established; the above results follow the usual trend. The greater
corrosivity of the softer water was due to the greater concentration of
builder component; i.e., STPP or NTA available from the detergent. In
harder water, some of the STPP or NTA is used to soften the water by
sequestering the magnesium and calcium ions and, therefore, is no longer
available as a corrosion stimulator. Also, in softer water, the formation
of protective magnesium and calcium scales is diminished.
In addition to the softer water being more corrosive for the investigated
materials, it was also apparent that for any given water hardness, the
corrosivity of the solutions increased with increase of detergent concen-
tration from 0.06, 0.12 to 0.18 weight percent. This increase in corrosion
rate was, in part, due to the increased availablility of sequestering
compound for the corrosion process but probably also due to the associated
increase of pH and decrease of specific resistance of the solutions as
shown in Table 27. The increase of pH with increase of concentration was
Table 27. Specific Resistivity and pH Data for Detergent
Solutions at Room Temperature
50 Wt. Percent NTA
Detergent Concentration,
weight
percent
0.06
0.12
0.18
Hardness,
ppm
15
150
15
150
15
150
pH
10.3
9.9
10.6
10.4
10.6
10.6
Detergent
Specific Resis-
tivity, ohm cm
1800
1800
1000
950
650
700
PH
10.0
9.8
10.4
10.1
10.3
10.2
50 Wt. Percent
STPP Detergent
Specific Resis-
tivity ohm cm
1600
1500
1000
900
650
600
probably due to the greater amount of OH formed through the hydrolysis of
the STEP and NTA which are salts of weak acids and strong bases. It is
apparent from Table 27 that NTA solutions were always slightly more
alkaline by about 0.2 to 0.4 pH units than the corresponding STPP
77
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solutions. This behavior was not due to a higher molar concentration of
NTA at 50 weight percent builder since the molar ratio was 1.34 in favor
of STPP, but was the result of the greater hydrolysis of the NTA detergent.
The slightly smaller specific resistivities of the STPP solutions are
probably associated with the larger ionic mobilities of the electrical
species. The increased pH of the solutions would, of course, be less
favorable to the amphoteric metals; e.g., aluminum, die-cast zinc, lead,
and, tentatively, brass because of its zinc content, since these materials
are strongly corroded in very alkaline solutions. Increased conductance
favors corrosion processes by more readily allowing ionic transport in the
solutions and the establishment of electrochemical cells on the surface
of the materials. These data indicate that in either STPP-or NTA-based
detergents, the use of higher-than-average detergent concentration (1 cup
per 17-gallon washing machine) would favor increased corrosion. Lower-
than-average detergent concentrations would be less corrosive. In all
cases, however, soft make-up water could be more corrosive.
In addition to the soft water and increase of detergent concentration
giving rise to higher corrosion rates, it was apparent that the NTA
solutions in these conditions were almost always more corrosive than the
corresponding STPP solutions. As discussed earlier, this behavior may be
due, in part, to the higher pH values of these solutions. However, the
most plausible explanation is that it is related to the greater sequestering
power of NTA discussed in Appendix B. The increased corrosivity of the
NTA solutions over STPP solutions was readily shown by the calculated
increased corrosivity factors for the materials. These factors can be
used to indicate how much more corrosive NTA detergent solutions are than
corresponding STPP detergent solutions. These factors would thus also
relate to the increased metal-ion pickup of the NTA detergent solutions
or STPP detergent solutions as seen by a sewer treatment plant. The
corrosivity factors are summarized in Table 28 for the least corrosion
Table 28. Summary of Increased Corrosivity Factors for NTA
Solutions Over Corresponding STPP Solutions
Metal Increased Corrosivity Factor
1100 Aluminum 1.0 to 1.3
260 Brass 1.3 to 5.0
Electrolytic Copper 1.5 to 3.8
Die-Cast Zinc 1.3 to 4.7
1020 Steel 1.0 to 7.1
Chemical Lead 1.0 to 3.1
Cast Iron 1.6 to 6.7
resistant metals of the construction materials employed in laundering. These
factors were determined from weight-loss data for detergent solutions most
likely to be employed by the housewife and at the average hot-water wash
temperature of 130 F. The data clearly show the increased corrosivity of
the NTA solutions. The increased corrosivity was particularly marked for
cast iron and 1020 carbon steel and to a lesser extent for die-cast zinc
78
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and 260 Brass. Electrolytic copper and chemical lead showed a smaller in-
crease in corrosion rate and 1100 Aluminum was only very slightly effected.
It is apparent, that for conditions corresponding to this work, the NTA
detergent solutions could contain as much as 7 times more metal ions due to
corrosion that would be brought about by similar STPP detergent solutions.
As shown in Table 28, cast-iron material, representative of uncoated cast-
iron soil pipe, showed the greatest corrosivity factor. The high initial
corrosion rates of 16 to 120 mils per year were reduced to 44 to 84 mils
per year on longer exposure periods as graphitization occurred. It should
be noted, of course, that the sewer pipe is supplied with a thin asphalt
coating which affords some degree of protection for a short period. The
corrosivity factors and corrosion rates, however, are applicable to the as-
supplied pipe in undiluted detergent solutions. Corrosion of cast iron was
still high at rates between 12 and 74 mils per year with simulated soiled
detergent solutions thus indicating the inherent high corrosion susceptibility
of this uncoated material.
It should be noted that the NTA and STPP detergent formulations used in these
studies did not contain corrosion inhibitors as might normal detergents.
Thus, the formulations used here would probably be more corrosive than the
corresponding commercial detergents. However, since neither NTA nor the
STPP detergent formulations used in the studies contained corrosion in-
hibitors, the relative corrosivity of the solutions should be significant
and relate to the behavior of the corresponding commercial detergent
formulations.
With the exception of die-cast zinc and 1020 carbon steel, the corrosion
behavior of the investigated materials was usually in the form of general
even corrosion for both STPP and NTA solutions. However, die-cast zinc
exhibited pitting corrosion in many experiments. The pitting was usually
more severe in the solutions representative of the average detergent strength
used by the housewife (i.e., 0.12 weight percent detergent). Furthermore,
the pitting corrosion extended as fine corrosion paths below the pits. It
is doubtful that the pitting rates of 180 to 540 mils per year measured
over 2 days' exposure would be sustained at this rate, but the inherent
susceptibility of this material is indicated. 1020 steel exhibited localized
attack in STPP and NTA solutions representative of average and greater than
average detergent concentrations (i.e., 0.12 and 0.18 weight percent).
This corrosion, however, was usually not deep and the associated etched
appearance did not penetrate into the material intergranularly as the Surface
appearance suggested. Although 260 Brass corroded in many STPP solutions,
it was apparent that some dezincification occurred. The dezincification
was confined to the surface and did not penetrate into the material. The
dezincification may be associated with a limited solution pH range of about
9.9 to 10.4.
Linear polarization measurements to evaluate corrosion rates did not
correlate well with weight-loss data with the exception of results for
1020 carbon steel and probably Type 304 and 420 stainless steel and 201
Nickel. Because of good overall precision of the weight-loss data using
duplicate samples, it was concluded that weight-loss data were the most
significant. The poor correlation with weight-loss data in terms of
measured corrosion rates was probably due to the formation of poorly con-
ducting films on the materials. The films gave a low assessment of corrosion
rate in this method which effectively depends on measuring the electrical
79
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resistance of the corrosion process. Using the results of this method to
compare behavior within a given system, it appeared that the use of mixed
37.5 STPP-1.25 NTA-based detergents did not significantly change corrosion
behavior over that noted for the pure 50 STPP- or 50 SNTA-based detergents.
Since the joint statement (Appendix A) on December 18, 1970, by the En-
vironmental Protection Agency Administrator and the Surgeon General, con-
cerning the voluntary removal of NIA from detergents, a variety of phosphate
and NTA "free" detergents have appeared on the market. These products
appear to be based upon soda ash, silicates, borax, or polyelectrolytes
or mixtures of these with other minor ingredients.^ The corrosivity of
such detergent formulations to materials used in laundering and sewer systems
was not determined from a scan of the open literature, and thus some con-
cern must be expressed for their use.
80
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SECTION X
ACKNOWLEDGMENTS
This work was performed under EPA, WQO Contract No. 14-12-943. Liaison
was maintained with Mr. Charles E. Myers, Project Officer, of the Division of
Process Research and Development, WQO, EPA. The support of the project by
the WQO, EPA, under the guidance of Mr. Charles E. Myers, is acknowledged
and appreciated.
The provision of the basic ingredients for preparation of the heavy-duty
granular detergents and useful advice by The Procter & Gamble Company,
Cincinnati, Ohio, is appreciated.
81
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SECTION XI
REFERENCES
1. "World Soap, Detergent Production", Soap and Chemical Specialities, 46,
40 (1970).
2. "More Information on Detergents/Phosphates Issued", Clean Air and Water,
News, 2, (38), 9 (1970).
3. "Preparing, Cleaning, and Evaluating Corrosion Test Specimens", ASTM,
Gl-67 (1967).
4. Stern, M., and Geary, A. L., "Electrochemical Polarization", J. Electro-
chem. Soc., 104, 56 (1957).
5. "Standard Reference Method for Making Potentiostatic and Potentiodynamic
Anodic Polarization Measurements", ASTM, G5-69 (1969).
6. "Moving in Fast on a Market that is Up for Grabs", Chemical Week, 11,
January 6 (1971).
82
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SECTION XII
GLOSSARY
Builder - A substance added to or used with detergents to increase their
cleaning action.
Descaling - The removal of corrosion products from a corroded specimen, with
a minimum removal of base metal so that underlying base metal can be examined
and/or metal weight loss can be determined.
Detergent - Any of numerous synthetic water-soluble or liquid-organic prep-
arations that are chemically different from soaps but resemble them in the
ability to emulsify oils and hold dirt in suspension.
Dezincifica tion - Corrosion of a zinc containing alloy, usually brass, in-
volving loss of zinc and a residue or deposit in-situ of one or more less
active constituents, usually copper.
General Corrosion - Corrosion producing uniform penetration of a surface.
Graphitization - Corrosion of gray cast iron in which the metallic constituents
are converted to corrosion products leaving the graphite intact.
Linear Polarization - A method in which small potential changes AE of the
order of lOmV applied to an electrode and the resultant current flow AI are
related through the linear relationship AE = RAI where R is a constant.
R, termed the "polarization resistance", is related to the corrosion rate
of the electrode material. Thus, the linear polarization method enables
corrosion rates to be determined.
Localized Corrosion - Corrosion producing nonuniform penetration of a surface
which ranges from small areas of general corrosion to intense attack at one
or small areas.
pH - A measure of the hydrogen-ion concentration'of a sample which represents
the logarithm of the reciprocal (negative logarithm) of the activity of
hydrogen ions calculated as follows:
PH = log 1/(H+) = - log (H+),
where H = activity of hydrogen ions.
Pitting Corrosion - An extreme form of localized corrosion giving rise to
cavities or pits in the surface which have a small width-to-depth ratio
(approximately less than 6:1).
Potentiodynamic - The technique of varying the potential of an electrode in
a continuous manner at a preset rate. It is frequently employed to prepare
polarization plots.
83
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Sequestration - The formation of a soluble complex chemical from the
reaction of a simple metal ion with a complexing chemical species.
Specific Resistivity - The resistance in ohms measured between two
electrodes which cover opposite faces of a centimeter cube in aqueous
solution at a specific temperature.
Tafel Slope - Application of large potentials to an electrode frequently
yields a current-potential relationship over a region which can be
approximated by
E = a + bin/I/,
where E is the electrode potential
I is the observed current density
a and b are constants.
The constant b is known as the Tafel slope.
84
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SECTION XIII
APPENDIX
A. STATEMENT ON NTA, DECEMBER 18, 1970
Page
BACKGROUND 86
RECOMMENDATIONS 87
B. CORROSION EFFECTS OF NTA WITH SPECIAL REFERENCE 87
TO ITS USE AS A BUILDER TO REPLACE POLYPHOSPHATES
IN DETERGENT FORMULATIONS
INTRODUCTION 88
CHEMISTRY OF AMINO POLYCARBOXYLIC ACIDS 88
NTA VERSUS POLYPHOSPHATES IN DETERGENTS 90
CORROSION REACTIONS OF NTA AND POLYPHOSPHATES 91
Factors Affecting Corrosivity 91
Use of Inhibitors 91
Examples of Corrosion Behavior 92
DISCUSSION 93
REFERENCES 94
85
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A. STATEMENT ON NTA, DECEMBER 18, 1970
Environmental Protection Agency Administrator, William D. Ruckelshaus, and
Surgeon General, Jesse L. Steinfeld, said today in a joint statement:
"We commend the major detergent manufacturers for their voluntary
action to discontinue use of NTA (nitrilotriacetic acid) in the
manufacturing of detergents, pending further tests and review of
recently completed animal studies, in order to protect the Nation's
health from any potential hazard. NTA, a chemical being substi-
tuted for phosphates in some detergents, has been used in gradually
increasing quantities to reduce the possible contribution of de-
tergent phosphates to accelerated aging of surface waters in the
environment. The aging process, known technically as eutrophi-
cation, results from excessive growth of aquatic plant life;
overgrowth of algae and nuisance aquatic weeds has a deleterious
effect on aquatic life and can have severe impact on water
quality. Although the industry was urged to make the substi-
tution of NTA for phosphates, and NTA was subjected to extensive
testing, recently completed studies of the biological activity
of NTA combined with heavy metals have raised concern that the
projected uses of NTA may constitute a hazard to health."
BACKGROUND
Early studies, both those conducted by the Federal Water Quality Admin-
istration (FWQA) and the industry, concentrated on the more conventional
kinds of toxicology studies using NTA alone. The studies which provide
the basis for today's action were initiated in the National Institute of
Environmental Health Sciences of the National Institutes of Health of the
Department of Health, Education, and Welfare last spring at the request of
the FWQA, formerly of the Department of the Interior and now an agency of
the Environmental Protection Agency. The NIEHS was asked to examine the
possibility of adverse reactions (birth abnormalities) secondary to ex-
posure to NTA itself or to NTA administered simultaneously with heavy
metals. At the dosages employed in the NIEHS studies, which were consid-
erably higher than would ordinarily be encountered by the human population,
the administration of two heavy metals (cadmium and methyl mercury) si-
multaneously with NTA to two species of animals (rats and mice) yielded a
significant increase in embryo toxicity and congenital abnormalities in the
animals studied over the results with the same dosage of the metals alone.
NTA alters the toxicity of metals by affecting their entry, distribution,
and concentration in the tissues. Of particular concern is the increased
rate of transmission of metals across the placenta to the fetus which
accounts for the fetal toxicity and congenital abnormalities produced in
animals in the NIEHS studies. This potentiating effect of NTA with mecals,
together with the projected scale of use of NTA from today's beginning 100-
200 million pounds per year to as much as a billion or more pounds per
year over the next few years, was the basis for today's action.
86
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There is no evidence at this time to indicate that anyone has been or is
being harmed by the combination of NTA with metals in the environment.
However, prudence dictates not permitting a situation to develop in which
harm may occur to man from the effects of the projected uses of NTA.
RECOMMENDATIONS
Representatives of the major detergent manufacturers, though they have not
had an opportunity to analyze fully the NIEHS data, nevertheless have agreed
to discontinue use of NTA in the manufacture of soap and detergent products,
pending further tests and review of these and other studies. Existing
supplies of NTA-detergent products may be depleted through normal dis-
tribution. We want to reemphasize that we have no evidence to indicate
that anyone has been harmed or is being harmed by these products.
However, there are certain areas where prudence would dictate that these
products should not be used. ¥e recommend that NTA-detergent products
not be used in certain limited areas which have both well water supplies and
septic tanks, and in which these treatment systems are operating under
completely anaerobic conditions and where short-circuiting of septic tank
effluent directly into well supplies is occurring. The products are:
Amway SA-8, Cheer, Gain, H.L.D., K-50, Laundri-Maid, Liquid All, Loft,
Phos-Free, Roundy's, Sav-us, Ultra, and Valley-Dew. (This list is not
complete).
In responding to one environmental problem, great care must be exercised
to assure that the alternative does not pose equal or greater hazards to
the environment or to human health. This is certainly the case with de-
tergents in view of the massive quantities produced and ubiquitous nature
of their distribution. It should be recognized that regulatory efforts by
Federal, State, and local officials must be conducted intelligently with
full awareness of potential secondary effects of those efforts.
As important as the prompt reduction of phosphates in detergents is in
reducing the load of algae nutrients in water, we must reluctantly conclude
that NTA does not appear to be an appropriate alternative at this time.
We continue to urge that phosphates and other nutrients be removed from
waste waters by the application of proper waste treatment methods across
the country. Additional studies of NTA are now underway, both by the
industry and within the Government. Intensive study of other phosphate
substitutes will be necessary to assure, to the extent possible, that they
do not present a similar predicament.
B. CORROSION EFFECTS OF NTA WITH SPECIAL REFERENCE TO ITS
USE AS A BUILDER TO REPLACE POLYPHOSPRATES IN DETERGENT
FORMULATIONS
87
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INTRODUCTION
Nitrilotriacetic acid (NTA) originated in Germany during the 30's and was
first marketed under the name "Trilon A".-'- Its use as a sequestering
agent, particularly in detergent formulations, was not economic until
recently. During the 60's, more economical methods of manufacture were ,, _
developed resulting in both a reduction in cost and greater availability. '
In recent years, the use of NTA as a detergent builder has expanded rapidly.
As a detergent builder, best results are obtained when it is used to re-
place part but not all of the polyphosphate in the detergent mix. However,
it appears more difficult to process NTA into detergent formulations.-*
CHEMISTRY OF AMINO POLYCARBOXYLIC ACIDS
Of the amino polycarboxylic acids, EDTA (ethylene diamine tetracetic acid)
is the best known and most widely used. >"~° The related product NTA
(nitrilotriacetic acid) has only recently become widely known.2 The
chemical structure of the trisodium salt of NTA is shown in Figure 1.
NTA has a capability similar to EDTA to sequester metal ions. Details of
the chelating chemistry of NTA is rather limited; however, one can obtain
an approximate idea of its chemical performance by studying the behavior
of EDTA.
The sodium salts of EDTA and NTA are highly soluble. For the trisodium
salt of NTA, saturation in water is reached atabout 40 percent by weight.1
Similarly the solubility of the tetrasodium salt of EDTA is about 51 per-
cent by weight. 10 As might be expected, each of these sodium salts, which
is the salt of a weak acid, has a strong alkaline reaction.
EDTA will sequester almost every polyvalent metal in the periodic table.
Usually the molecular ratio of the metal ion chela ted to EDTA is one
to one. Similarly NTA will complex or chelate most, polyvalent cations.2
The metal is bound into the chelate structure by both ionic and covalent
bonds forming an extremely stable complex. This effectively deactivates
the metal and eliminates the detrimental effects often caused by metal
ions in aqueous systems.
The sequestering activity is affected by the pH since there is competition
between hydrogen ions and metal ions. At high pH, say 10 and above, the
complexing of metal ions is most effective.
In hard waters, NTA (or EDTA) will, in effect, soften the water by the
removal of calcium and magnesium ions from solution. This action prevents
the formation of insoluble soaps and precipitates. The alkaline earth
complexes are highly stable. The reaction of NTA and EDTA in sequestering
calcium is shown in Figure 2.2 On a weight basis, NTA will chelate almost
50 percent more calcium ions than will EDTA. However, the stability con-
stant for any given NTA-sequestered cation is invariably lower than that
for the same EDTA-sequestered cation. This may require an excess above the
stoichiometric amount of NTA to insure completion of the sequestering
action in some applications.2
88
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CH2—C—ONo
/ 0
/ /
N—CH2—C—ONo
\ 0
\ /
CH2—C—ONa
.CH,COONo ,CH2COOV
NoOOCCH2-N +Co**-*NoOOCCH2-N -/Co + 2Nof
CH2COONo ^w.rnn/
C.V.- 389mg.Co COj/g m.
or 2-57 parts NTANa3:1 part CaCOz
NoOOCCHZv ,CH2COONa NoOOCCH2x
VNCH2CH2N' *Co+* N
NaOOCCHg "CH2COONa H2C \t ,' CH2
COO-Ca'-OOC
C.V.=263mgCaC03/gm.
or 3.80 parts EDTANa^ I port CaCOj
CH2COONa
FIGURE I. STRUCTURE OF THE TRISODIUM
SALT OF NTA (TRISODIUM
NITRILO TRIACETATE (9)
FIGURE 2. CHELATING CAPACITY OF THE SODIUM
SALT OF NTA COMPARED WITH THE
SODIUM SALT OF EDTA (2)
400
350
300
250
200
150
100
50
SEQUESTRATION OF CALCIUM
AT 149 °F
(O.I%Na2C204 indicator)
mg-CoCOj/gm.
NTANoi
STPP
TSPP
5 6 7 8 9 10 I! 12
pH
FIGURE 3. A COMPARISON OF THE SEQUES-
TRATION OF CALCIUM BY NTA
AND BY POLYPHOSPHATES AT
DIFFERENT pH LEVELS (2)
© 0.75gm STPP
© 0.45 gm NTANo3H20
<3> 0.60 gm NTANa3H20
in 100 ml H20
water hordness=l50 ppm
12345
HCI, milliequivalents
FIGURE 4. BUFFER CAPACITY OF NTA
VERSUS TRISODIUM
POLYPHOSPHATE (STPP) (2)
89
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NTA VERSUS POLYPHOSPHATES IN DETERGENTS
Polyphosphates, as well as NTA, are more effective in sequestering calcium
at the higher pH levels. At 140 F and at a pH of 10 and above, it can be
seen from Figure 3 that the trisodium salt of NTA is roughly twice as
effective as STPP and three times as effective as the tetrasodium pyro-
phosphate (TSPP) in sequestering calcium. 2 of concern in its effect on
the corrosion environment is NTA's buffering capacity. As can be seen in
Figure 4, it requires more acid to lower the pH from say 11 to 8, with NTA
in solution than is the case with STPP.2 Thus the sequestering capability
of NTA in alkaline solution is less readily disturbed.
In the typical detergent formula, NTA can be used to replace part or all
of the polyphosphate. NTA achieves a higher brightness level on fabrics
than does STPP at about one-half the quantity of builder.2 Other benefits
of NTA as a builder in detergent formulation include (a) lower mineralization
of cotton fabric washed in hard water compared to results when using STPP
as a builder, (b) good foam stabilization, (c) improved rinsability of
detergent components from fabrics and hard surfaces because of the greater
solubility of NTA versus STPP, (d) stabilization of minor ingredients such
as bleaches, brightners, and sanitizing a gents. 2>3>5
The important differences between the amino acid chelating agents; e.g.,
NTA and the polyphosphates,
(1) The amino acids can combine with a wider range of polyvalent metal
ions than the polyphosphates.
(2) The polyphosphates tend to revert to the simple phosphate form as
the result of excessive acid or alkaline conditions. High temperatures
also increase this tendency. For detergents normally packaged in powdered
form, this instability is usually not a factor but for concentrated liquid
detergents, formulators often elect to omit the polyphosphate because of
stability problems.
(3) Polyphosphates exhibit a so-called threshold effect, a characteristic
not present in amino acid compounds. A few parts per million of poly-
phosphate will keep much larger molar quantities of chemical compounds,
which would otherwise tend to precipitate, in solution.
Because the effects of NTA and STPP, as builders, are not identical, the
most effective detergent formulations incorporate a combination of both
products. 2>3 A molar ratio of sodium tripolyphosphate to sodium nitrilo-
triacetate of 3:1 to 1:3 is found to give good results.^
90
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CORROSION REACTIONS OF NTA AND POLYPHOSPHATES
Polyphosphates have been used in water systems for scale control and for
inhibition of steel corrosion. Protection appears to be the result of the
deposition of a film at the cathodic sites.12
Under the alkaline conditions and concentrations used in detergent solutions,
polyphosphates are found to be corrosive to alloys of aluminum, zinc, and
to copper-nickel-zinc alloys (German silvers).^ When NTA is substituted
for polyphosphate, the same metals are affected.
Factors Affecting Corrosivity
NTA solutions can be expected to be corrosive to metals because of the
following factors:
(1) NTA ties up the alkali-metal ions, particularly calcium, into soluble
complexes. This prevents the formation of protective scales, such as
calcium carbonate, in effect, allows corrosion to proceed.
(2) As metal ions from corrosion reactions enter solution they are se-
questered by the NTA. This prevents the formation of protective films of
corrosion products. Existing films may be sequestered by the NTA and
thus removed. In addition, the low concentration of unsequestered metal
ions in solution stimulates the corrosion reaction by preventing the
attainment of equilibrium.
(3) NTA penetrates a variety of protective films formed by corrosion
inhibitors.4
Many of these reactions are similar to those of the polyphosphates. Phos-
phates complex calcium ions (Item 1 above) and also sequester metal ions
from corrosion reactions (Item 2).13 On the other hand, the presence of
low levels of polyphosphate, unlike NTA, will often increase the effective-
ness of an inhibitor.
Use of Inhibitors
One problem with high levels of polyphosphates in detergent solutions has•
been the tendency toward corrosion of the "white metals". The necessity for
the inhibition of phosphate-bearing detergents is well established.4 For
example, aluminum corrosion can be inhibited by adding silicates.^>14
Without the silicates, the polyphosphates will maintain a fresh surface
for initial attack. The proper silicate, if added, will be chemisorbed on
the metal and prevent further sequestration. It is found that aluminum
corrosion is effectively inhibited by sodium silicate having a ratio of
Si02 : Na20 of about 1.6.^,14 The inhibition of corrosion of copper-
nickel-zinc alloys in phosphate containing detergents can be accomplished
by benzotriazole. 4
3
Neither of the above inhibitors is effective toward zinc or "Zamak" with
either polyphosphate or NTA as builders. The corrosion rate can be inhibited
to an acceptable range by the addition of an alkyl phosphonate according
to a British pa tent.4
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Table 1. Corrosion of Zamak 3^a) by Detergent(b) Containing Sodium
Nitrilotriacetate (NTA) With and Without Inhibitor(c)
After 6 Hours in Agitated Solutions at 140 F.4
Loss in Mg/Sq Dm/Day
Detergent Concentration
0.27.
0.5%
Type of Water
HardW
Distilled
Inhibited
39
238
Uninhibited
355
688
(a) Nominal composition, 4 Al, 0.04 Mg, remainder Zn.
(b) See Table 2, for composition.
(c) Inhibitor is "0.3 percent of random octadecyl phosphonic acid prepared
by adding phosphorous acid to a random mixture of octadecenes using
gamma radiation as a source of radicals".
(d) Hardness = 8.4 grains per imperial gallon or 120 ppm.
See Table 1 for composition of Zamak.
Examples of Corrosion Behavior
The composition of a detergent containing NTA is given in Table 2.
Table 2. Composition of a Spray-Dried Granular Detergent Containing
NTA in Percent by Weight^
Percent
Sodium straight chain alkyl benzene sulphonate having an average 13.3
chain length of 13 carbon atoms
Sodium tripolyphosphate (STPP) 41.4
Sodium nitrilotriacetate (NTA) 9.6
Sodium silicate having a SiO£ : Na^O weight ration of 1.6 10.0
Marine oil fatty acids 0.5
Tallow fatty acids 1.5
Sodium carboxymethyIcellulose 0.33
Sodium sulfate 11.26
Water 11.00
Miscellaneous ingredients including perfume, optical brighteners, L It
and pigments
Note that the ratio of STPP to NTA is about 4:1. The corrosion rates given
in Table 1 are for Zamak 3 in agitated detergent solutions at 140 F for 6
hours. The hard water (equivalent to 120 parts per million by weight)
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contained a 0.2 percent concentration of detergent by weight. The "random
octadecyl phosphonic acid" inhibitor, at 0.3 percent strength, reduced the
corrosion rate to about 1/9 of that of the uninhibited solution. The in-
hibition in the distilled water solution with 0.5 percent detergent was
only partially effective.
In another experiment, aluminum and Zamak 3 were found to be corroded
by 0.35 percent detergent solutions, using the same two waters as in the
Zamak 3 tests above. The detergent composition was:
Sodium tetrapropylene benzene sulfonate 20%
Sodium tripolyphosphate 50%
Sodium sulfate 25%
Inhibitor 5%
In this experiment, the exposure was for 3 hours at 140 F at a pH of 9.5.
Adding 5 percent phosphonic corrosion inhibitor, e.g., 10 nonadecyl phos-
phonic acid or pentadecyl benzyl phosphonic acid resulted in inhibition
not only for -aluminum but also for Zamak 3.
When NTA is substituted for the polyphosphate, the corrosion rates also are
unacceptable. However, the attack on both metals is effectively inhibited
by phosphonic corrosion inhibitors.
DISCUSSION
Much of the literature on NTA discusses analytical techniques, problems of
sewerage treatment, efficiency as a detergent builder, and packaging
problems. Corrosion data are almost nonexistent in the open literature.
One gathers from a study of detergents in general that there is an almost
endless variety on the market. Many of these detergent formulations tend to
be corrosive, and manufacturers of laundry and washing machines have found
it necessary to construct their units of corrosion-resistant materials. >14-1
The introduction of NTA for part of the polyphosphate does not appear to have
altered the situation as far as domestic washing machines are concerned.
A recent suggestion is to replace part of the polyphosphate builder-' by
increasing the amount of surface-active agent to partially compensate for
the change. It is not easy to predict how these alterations in formulation
will reflect the corrosivity of detergent solutions to white metals such as
zinc and aluminum. Builders such as sodium carbonate may be required to
maintain alkalinity.
No indication was found in the literature that NTA-bearing, spent-detergent
solutions are more corrosive to sewer lines and to sewerage plants than is
the case for phosphate-based detergents.
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REFERENCES
1. Sisley, J. P., and Wood, P. J., Encyclopedia of Surface-Active Agents,
Vol. _! (1952) Chemical Pub. Co., N. Y.
2. Pollard, R. B., "Amino Acid Chelating Agents in Detergent Applications",
Soap and Chemical Specialities, 42 (9), 58-62, 130-5 (1966).
3. Singer, J. J., "Chelating Agents in Detergents", Coap and Chemical
Specialties, 37 (10), 49-51, 125-6 (1961).
4. Zimmerer, R. E., "Detergent Composition", British Patent No. 1,131,738
(October 23, 1968).
5. Kastra, R. D., "Formulating Detergents With Less Phosphate", Soap and
Chem. Specialties, 47 (2), 36-42, 54-6, 107 (1971).
6. Smith, R. L., The Sequestration of Metals, Chapter V, MacMillan Co. (1959),
7. Dwyer, F. P., and Mellor, D. P., Chelating Agents and Metal Chelates,
Chapter 7, Academic Press, New York, N. Y. (1964).
8. Martell, A. E., "Complexing Agents", Kirk-Othmer Encyclopedia of Chemical
Technology, 2nd Edition, p 1-24 in Vol. 6, John Wiley (1965).
9. Shumate, K. S., Thompson, J. E., Brookhart, J. D., and Dean, C. L.,
"NTA Removal by Activated Sludge—Field Study", J. Water Poll. Control
Fed., 42 (4), 631-640, (1970).
10. The Merck Index, p 428, Merck & Co. (1960).
11. McGilvery, J. D., "STPP for Modern Detergents", Soap and Chemical
Specialties, 40 (12), 241-3, 254-7 (1964).
12. Hatch, G. B., "Influence of Inhibitors on the Differential Attack of
Steel", Corrosion, .21 (6), 179-87 (1965).
13. Shen, C. Y., "Properties of Detergent Phosphates and Their Effects on
Detergent Processing", J.Am. Oil Chem. Soc., 45, 510-16 (1968).
14. Lange, K. R., "Recent Studies in the Roles of Silicate Builders in
Detergent Formulations", J. Am. Oil Chem. Soc., 45, 487-92 (1968).
15. Chandler, R. H., "Corrosion Resistance of Vitreous Enamel", Corrosion
Prevention and Control, 8 (8), 41-2 (1961).
16. Stupel, H., and Koch, F., "Die Korrosion von Zinc durch Neuzeitliche
Waschmittel" [The Corrosion of Zinc by Modern Detergents], Werkstoffe
und Korrosion, 10 (1), 33-39 (I960).
17. Mosle, H. G., Wolf, W., and Bode, W., "Untersuchung uber den Einfluss
von Haushalts Wasch Mitteln auf Metalle und von Metallen auf Haushalts-
Waschmittel" [Investigation of the Effect of Domestic Detergents on
Metals and of Metals on Detergents], Werkstoffe und Korrosion, 15 (2),
130-4; (3), 221-227 (1964).
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Subject
(1) Accession Number (2) Field & Group
SELECTED WATER RESOURCES ABSTRACTS
Input Transaction Form
(5) Organization
Battelle Memorial Institute, Columbus Laboratories
(6) Title
Corrosion Potential of NTA in Detergent Formulations
(10) Author(s)
Moreland, Peter J.
Boyd, Walter K.
Lutz, Garson A.
(16) Project Designation
EPA, WQO Contract No. 14-12-943
(21) Note
(22) Citation
(23) Descriptors (Starred First) - corrosion, eutrophication, detergents,
pollution, phosphates, plumbing, metals, stainless steels
(25) Identifiers (Starred First)
NTA, STPP, laundering equipment, polarization studies
(27) Abstract
Laboratory studies were conducted to determine the corrosion potential of
nitrilotriacetic acid (NTA) as a substitute for sodium tripolyphosphate (STPP)
in detergents. Coupon-weight loss and linear polarization studies were employed
to investigate the corrosion of typical materials of construction which might
be subject to exposure to NTA in normal use in laundering.
Detergent formulations were used which were representative of heavy-duty
granular detergents. Solutions of 0.06, 0.12, Snd 0.18 weight percent using
15- and 150-ppm water hardness and temperatures of 130 and 160 F represented
laundering conditions used by the average housewife
NTA-based detergents were more corrosive by a factor between 1 and 7 to
the materials 1100 Aluminum, 260 Brass, electrolytic copper, die-cast zinc,
1020 carbon steel and chemical lead. Corrosion was generally greatest in NTA
and STPP solutions with soft water. In both soft and hard waters, corrosivity
increased with increase of detergent concentration.
Types 304 and 420 stainless steel and 201 Nickel were very corrosion re-
sistant (0.01 to 0.15 mil per year), 260 Brass, electrolytic copper and 1100
-Aluminum were moderately resistant (0.2 to 3 mils per year) and die-cast zinc,
1020 carbon steel and chemical lead poorly corrosion resistant at rates of 2
to 60 mils per year. Cast iron showed extreme corrosion in NTA solutions with
corrosion rates between 30 and 120 mils per year. NTA detergents could increase
metal ion pickup at a sewage plant by a factor between 1 and 7.
Abstractor
Moreland, Peter J.
Institution
Battelle Memorial Institute, Columbus Laboratories
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