Mechanism of intergranular corrosion of 316L stainless steel in oxidizing acids

Scripta METALLURGICA

Vol. 14, pp. 1175-1179, 1980 Printed in the U.S.A.

Pergamon Press Ltd. All rights reserved

MECHANISM OF INTERGRANULAR CORROSIONOF 316L STAINLESS STEEL IN OXIDIZING ACIDS T. M. Devine, C. L. Briant, B. J. Drummond General Electric Corporate Research and Development Center P.O. Box 8, Schenectady, New York 12301 (Received May 7, 1980) (Revised September ii, 1980) The susceptibility of 316L stainless steel to intergranular corrosion attack in oxidizing acids following short time, intermediate temperature aging has been hypothesized to result from the intergranular precipitation of "pre-sigma" phase particles (1,2) or to an "invisible" phase (3).To further investigate the cause of this intergranular corrosion, specimens of 316L stainless steel were annealed at llO0°C/l hr., W.Q., aged at 6DO°C, 675°C, and 750°C for times of I/2-1OO hours and then tested for intergranular corrosion resistance in boiling acidified Cu-CuSO, solution (A262E test) (4) and boiling 65% HNO3 (A262C test) (4). The pitting corrosion susceptibility was evaluated galvanostatically and potentiodynamically in O. IN HCI at room temperature. Thin f o i l s of specimens were viewed in transmission electron microscopy. Additionally, surface segregation was studied by heating in an Auger spectrometer and observing the Auger electron spectra as a function of heat treatment time and temperature. The composition of the heat is listed in Table I. Figure l depicts the effect of the aging treatment on the cumulative weight loss following four 48-hour periods of immersion in boiling 65% HN03. In a l l cases the corrosion attack was intergranular. The attack was extremely severe in those samples aged at 675°C for longer than two hours. In fact, the sample aged at 675°C for l O0 hours completely disintegrated during the f i f t h 48-hour immersion period. This intense intergranular corrosion susceptib i l i t y following aging at 675°C is not the result of continuous grain boundary paths of severe chromium depletion such as accompanies M23C6 intergranular precipitation in higher carbon containing austenitic stainless steels, as aged specimens of 316L stainless steel were completely immune to intergranular corrosion in the A262E test. I t is unlikely that the intergranular corrosion in A262C in samples aged at 675°C results from the intergranular precipitation of o phase or some precursor of o, since the transmission electron microscopy studies did not reveal the presence of o even in samples aged for lO0 hours at 675:C. Furthermore, the precipitation of o, or a precursor of ~, should deplete the matrix immediately surrounding the precipitate of chromium and molybdenum. This should result in a decrease in pitting corrosion resistance. Yet potentiodynamic polarization tests in O.IN HCI indicate the specimens aged at 6DO°C, 675°C, and 750°C all exhibit the same pitting potent i a l , and galvanostatic tests in O. IN HCI indicate the specimens a l l possess the same p i t i n i t i a t i o n rate regardless of aging treatment. Preliminary Auger spectroscopy surface heating experiments suggest an alternative explanation for the enhanced intergranular corrosion attack in A262C in samples aged at 675°C. (Although direct studies of grain boundary segregation would be preferable, the d i f f i c u l t y of achieving intergranular fracture of this type in the Auger spectrometer led us to use surface heating.) As i l l u s t r a t e d in Figure 2, there is a marked effect of temperature on the amount of phosphorous which segregates to the free surface of specimens heated within the Auger spectrometer. Heating specimens for up to 42 hours at 600% resulted in a negligible amount of phosphorous segregating to the free surface. In marked contrast, there was a continuous increase with aging time in the phosphorous concentration measured at the free surface of a specimen aged at 675%. In the specimen heated at 750°C, there was an abrupt increase in phosphorous concentration which appeared to reach a saturation value within l hour. On the basis of the surface segregation experiments we suggest that aging at 675°C results in grain boundary segregation of phosphorous. The l a t t e r results in accelerated intergranular corrosion in the A262C test. Aging at 600°C does not result in surface segregation of phosphorous nor presumably to grain boundary segregation. While aging at 750°C does result in phosphorous surface segregation, we suggest that i t does not result in extensive phosphorous grain boundary segregation. I f the binding energy of the solute to the free surface is s u f f i c i e n t l y greater than that to the

1175 0036-9748/80/111175-05502.00/0 Copyright (c) 1980 Pergamon Press Ltd.

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grain boundary, as would appear reasonable, then McLean's (5) model of equilibrium grain boundary segregation would predict a temperature TM below which the equilibrium grain boundary segregation would reach a maximumand above which i t would rapidly decrease to zero with increasing temperature. Becauseof the higher binding energy of the solute to the surface, the surface segregation would remain at i t s high value and not begin to decrease until temperatures rose significantly above TM. Evidence for such a difference between surface and grain boundary segregation can be found in the literature as summarized in Figure 3. While Yen et al s found no change in the concentration of phosphorus which segregated to the free surface of a 3330-type low all,oy steel heated between 450° and 550°C, Mulford et al ~ found a continuous decrease in phosphorus grain boundary concentration with increasing temperature in an identical steel. The decrease in surface and grain boundary segregation of phosphorus with temperatures below 675°C could be due to several factors• First, the decrease in d i f f u s i v i t y with decreasing temperature and second the phosphorus could be tied up by the molybdenum, either by forming Mo-P clusters, or by forming MoxPywhich then decompose at ~75°C. Such Mo-P interactions have been postulated by Yu and McMahon to occur in low alloy steels. 8 Further support for the idea of grain boundary segregation of phosphorus in 316L is provided by the work of Josh and Stein9 who found phosphorus grain boundary segregation in 304 stainless steel when heat treated at 550°C. That phosphorus surface segregation (and presumably grain boundary segregation) does not occur below 675°C in 316L is hypothesized to be due to the presence of the molybdenum. That phosphorus grain boundary segregation can lead to enhanced corrosion in the A262C test is suggested by the work of Armijo et al I° who found that phosphorus doped samples of 304 stainless steel suffered intense grain • 11 boundary attack. Tedmonand Verm]lyea havespeculated that impurities such as phosphorus segregated at grain boundaries might locally increase the electronic conductivity of the oxide causing an increase in the corrosion at the grain boundaries• In summary, the pitting tests and the transmission electron microscopy results do not support the hypothesis that the enhanced intergranular corrosion attack in A262C of 316L stainless steel aged for longer than two hours at 675°C is the result of o-phase. Preliminary Auger spectroscopy findings suggest that the enhanced intergranular corrosion attack in A262C of 316L stainless steel aged at 675°C results from the equilibrium grain boundary segregation of phosphorus. References I.

D. Warren, Corrosion, 15, 213t (1959).

2.

M. A. Streicher, Corrosion, 20, 57t (1964).

3.

R. F. Steigerwald, Corrosion, 33, 338 (1977).

4.

ASTMDesignation A262-70.

5.

Grain Boundaries in Metals, D. McLean, Oxford University Press, London, 1957.

6.

W. R. Graham and A. C. Yen, Met. Trans. A., gA, 1461 (1978).

7.

R. A. Mulford, C. J. McMahon,"J r . , D. P. Pope and H. C. Feng, Met. Trans. A, 7A, I183 (1976).

Bo

J. Yu and C. J. McMahon, J r . , Met Trans. A~ l l 4 , 277 (1980).

9.

A. Joshi and D. F. Stein, Corrosion~ 28, 321 (1972).

10.

J. S. Armijo, Corrosion 24, 24 (1968).

1!1.

C. S. Tedmon and D. A. Vermilyea, Met. Trans. l , I076 (1970). Acknowledgements

Miss A, M. Ritter performed the transmission electron microscopy. Mr. E. C. Nagy assisted in the heat treating• Table l Composition of 31---i-6-[Stainless Steel 17.13% Cr

12.86% Ni

2.17% Mo

.014% C

.025% S

.028%P

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