H . R. C O P S O N
The International N ickel Company, Inc., Bayonne, N. J.
G A L V A N IC C O R R O SIO N tests were made on steel coupled to nickel in tap water. With a 3 to 1 area ratio of nickel to steel, galvanic corrosion of the steel was appreciable but not excessive.
Treating the water with 3 0 0 parts per million of sodium chromate practically inhibited corrosion, provided the steel was rubbed occasionally. Undisturbed steel was liable to pit. Treating the water with lime to pH 11 cut down the total corrosion but localized it, with the result that the maximum rate of penetration was increased.
N
IC K E L -C L A D steel h ad been specified as the m aterial for constructing a canal lock, a t a tim e when nickel was becoming scarce owing to w artim e restrictions. I t was considered expedient to complete th e lock w ith bo th plain carbon steel and nickel-clad steel. T he gate was to be m ade of painted carbon steel, and th e slots in which it would move were to be lined with nickel-clad steel. Included in th e assembly which would operate in th e slots would be unprotected steel tracks and rollers. Con
ditions would be such th a t th e area of unprotected steel would be ab o u t one th ird th e area of nickel in its im m ediate vicinity. I t was feared t h a t appreciable galvanic corrosion of th e steel m ight occur. L aboratory tests were u ndertaken to determ ine the prob
able ex ten t of th e galvanic effects and to investigate m eans of m inim izing them . I t was known th a t th e canal w ater in the slots would be sta g n an t and w ould be replaced a t infrequent in
tervals, such as once a m onth. Therefore th e use of a chromate inhibitor or of lim e to raise th e p H of th e w ater seemed practical.
B ayonne ta p w ater is sim ilar enough to th e canal w ater to be suitable for th e tests. T able I gives an analysis of th e ta p water.
CO ND ITIO N S
T he following couples were studied a t 30° C .:
1 and 2: steel-B ayonne ta p w ater (pH 8)-nickel
3 and 4: ste el-tap w ater + 300 p.p.m . N a2CrOi (pH 8) - nickel
5 and 6 : ste el-tap w ater + lime (pH ll)-n ic k e l
T he procedure described by Wesley (4) was followed closely.
S eparate 4-liter glass jars were used for each couple, and nor
mally corroding uncoupled specimens were included in each jar.
T he electrodes were shorted through external one-ohm resistances.
T he steel specimens were 1 X 5 X 0.060 inch w ith long tab s Vs inch wide. T he nickel specimens were 3 X 5 X 0.031 inch with similar tabs. T he tab s served for electrical connections. The immersed p a rt of th e tab was protected w ith wax. T his m ade the area ratios 3 to 1.
F or th e first 36 days, for the sake of close control of conditions, the solutions were satu rated w ith air and th e specimens moved in a vertical circular p a th a t a ra te of 16 feet per m inute (3). T he
Table I. Analysis of Bayonne Tap Water (Parts per M illion)
O2 c o n s u m ed 2 . 4 S o a p h a r d n e s s as C aC Os 2 5 . 0
N 2 as f r e e a m m o n ia 0 . 0 2 Iron 0 . 0 8
N 2 as a lb u m in o id a m m o n ia 0 . 0 5 C a lc iu m 7 . 8
N 2 as n itr ite s 0 . 0 0 M a n g a n e s e 0 . 0 5
N 2 a s n itr a te s 0 . 0 3 M a g n e s iu m 1 . 4 5
C hlo rides 3 . 4 Silica 2 . 9
S ulfa tes 1 2 . 4 T o t a l d is so lv e d s o lid s 4 7 . 3
A lk a lin it y 5 0 . 7
I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 37, No. 8
RESULTS O F PO TENTIA L M EA SUR EM EN T
T IM E , DAYS
a ir for tJie lime solutions passed through 30% potassium hydrox- ■ ide. E ven so, th e p H of th e lime solution dropped gradually w ith th e p recip itatio n of calcium carbonate, and daily additions of lim e w ater were required. F o r th e next 20 days, to m ore closely approach th e conditions a t th e canal lock, th e tests were ru n w ith
o u t aeration or m otion. T h e tests lasted 56 days.
A t 14, 28, an d 42 days th e solutions were changed, and all specim ens in jars 1, 3, an d 5 were rubbed. T he rubbing was in
ten d ed to sim ulate th e effect of m ovem ent of th e steel p a rts in th e slo t of th e canal lock. A nalysis of th e chrom ate in ja r 4 a t 14 days show ed th a t little chrom ate was lost.
A bout every o th er d ay th e cu rren t flowing was m easured by p o ten tial readings across th e one-ohm resistance. T h en th e circuits w ere opened for one h our an d th e open-circuit p o ten tials m easured. A bout once a week th e p o ten tials of all th e specim ens were m easured ag ain st a sa tu ra te d calomel cell.
Figures 1 to 4 are plots of p o ten tial m easurem ents a g a in s t tim e . T h e potentials of th e norm ally corroding specimens are n o t show n, b u t th ey did n o t differ g reatly from th e poten
tials of th e electrodes.
F igure 1 shows th a t, in general, th e nickel p o ten tials were unaffected by th e chrom ate an d lim e additions.
T hey were close to zero on th e sa tu rated calom el scale. O n th e contrary, th e steel p o ten tials were g reatly af
fected (Figure 2). In general, th ey were ab o u t —0.7 volt in pu re ta p w ater. A ddition of lim e knocked th em down to —0.4 to —0.5 volt.
A ddition of chrom ate m ade th e steel even m ore noble, so t h a t in couple 3 th e steel p o ten tial was as noble as t h a t of nickel.
T h e open-circuit p o ten tials in Fig
ure 3 are th e difference betw een the corresponding electrode potentials in Figures 1 and 2. T h e currents in Figure 4 b e ar some relation -to th e open-circuit potentials. T h ey are greatest in ta p w ater, practically zero in th e chrom ate solutions, and in term ed iate in th e lim e solu
tions.
Stopping th e m otion an d aeration a t 36 days h a d little effect on th e po
tentials. Also, changing th e solutions a t 14, 28, and 42 days h a d little effect, b u t th e rubbing action h a d consider
able effect. I n B ayonne ta p w ater clean steel ten d ed to be passive, b u t th e steel soon ru ste d an d became less noble. T h e ru s t w as loose so th a t rubbing rem oved it an d tem porarily restored th e passivity. T he effect is a p p are n t in th e current an d p o ten tial curves for couple 1.
In th e chrom ate solutions th e steel specimens were passive a t first; b u t if undisturbed, p ittin g developed and th e p o ten tials becam e less noble.
R ubbing p rev en ted p its from devel
oping an d th e steel rem ained pas
sive.
T h e effect is clearly shown by th e curves for couples 3 a n d 4 in Figures 2 an d 3.
A fter 56 days th e tests were stopped, an d th e specim ens were inspected, cleaned, and weighed. In ta p w ater th e steel was uni
form ly coated w ith loose ru s t w hich was reddish on th e outside and black next to th e steel, and th e nickel was coated w ith a thin, loose, reddish ru s t deposit. I n th e chrom ate solutions all th e specimens app eared unaffected except th e u n ru b b ed steel which h ad scaly V 4-inch-diam eter ru s t sp o ts w ith p its u nder
n eath . I n th e lim e solutions large p ortions of th e steel surface re
m ained passive, b u t o th er large areas w ere severely corroded and coated w ith an ad h eren t d a rk brow n scale. T h e nickel specim ens were coated w ith a h a rd w hite deposit, presum ably calcium car
bonate.
T able I I show s w eight losses an d p it d epths. T hese su b sta n tia te th e visual observations. T h e nickel specim ens were scarcely affected. T h e n orm al corrosion ra te of steel in ta p w ater was
I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 723