JOURNAL DE PHYSIQUE IV Colloque C7, supplBment au Journal de Physique 111, Volume 5, novembre 1995

Surprising Intergranular "Non-Corrosion" of a 304L Stainless Steel

Le Thi Quynh Anh and J. Le Coze

Ecole des Mines de Saint-Etienne, 42023 Saint-Etienne cedex 2, France

Abstract : A low chloride content solution, representative of an artificial saliva, was used to study the pitting and crevice resistance of a 304L wire used to fix teeth against each other in the mouth. On an industrial 304L alloy, corrosion inside deep pits showed a special character in which grain boundaries were not attacked : a honeycomb-like structure of the corroded surface was observed in which grain boundaries were the walls of the cells. This result was reproduced in a 1. 66 NaCl g/l solution, pH=7, on polished sections of ultra high purity base alloys, covered with a varnish layer in order to create a crevice-like situation. The electrochemical potential was imposed at values near passivity breakdown. The exposure times were 40 to 90 h at room temperature. UHP-alloys, representative of 304L steels, with subsequent additions of C, P and Mo were prepared and tested to determine the possible role of intergranular segregation or precipitation on honeycomb corrosion, in the as-quenched condition and after annealing (600°C, 30h). As a function of exposure time, different corrosion stages under the varnish layer were observed : crystallographic pitting, honeycomb corrosion and general dissolution.

1 - INTRODUCTION

304 L stainless steel wires (0 = 0.4 rnm) used to maintain teeth against each other inside the mouth are subject to several complex chemical and mechanical constraints. Due to the fabrication process, large inclusions containing Ca, C1, Si and other impurities can be found in the metal. These impurities are introduced by the lubricants used in the drawing process. During a technical study of the corrosion behaviour of such industrial wires in artificial saliva solutions, a particular pitting aspect at potentials near the end of the passivity range was observed : crystallographic and honeycomb-like corrosion were identified in the bottom of deep pits related to large inclusions (Fig. 1 and 2).

Such corrosion aspects, in which grain boundaries are in relief , have rarely been described, except by Hatwell (ref. 1) after corrosion of austenitic steels (25Cr20Ni and 304) and ferritic 30%Cr steels in oxalic solutions. Hatwell explained the honeycomb-like corrosion by a dechromization process related to intergranular carbide or sigma phase precipitation. Baroux (ref. 2) showed also grain boundary relief after electropolishing of aluminium bicrystals which he explained by intergranular energy considerations. Another honeycomb type of corrosion has been described for stainless steels in transpassive, highly oxidizing conditions characterized by intergranular grooves associated with intragranular pitting (ref. 3). This complex honeycomb structure consisting of intra- and inter-granular corrosion with a less corroded zone at the border of the grains, has been explained by Si intergranular segregation (ref. 4).

None of the proposed interpretations can be used here, in a low carbon stainless steel and low potential exposure conditions (NaC1 instead of nitric acid).

In order to analyse this phenomenon, pure stainless steels were prepared with subsequent additions of C , P, Mo, to separate the possible influences of the metallurgical state (composition and heat treatment effects) and of the electrochemical conditions, including the presence of large inclusions which can be dissolved during exposure to the corrosive medium.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:1995753

JOURNAL DE PHYSIQUE IV

2 - EXPERIMENTAL 2.1 - Materials.

The compositions of the alloys are given in table I. The 304L wire was used in an unknown "as delivered state" not given to the users (dentists). The pure base alloys were prepared from UHP- Metals (Fe, Ni and Cr) purified in our Laboratory (ref. 5). The alloys were induction melted and solidified in a pure argon atmosphere, in a boat-shaped silver crucible cooled by water. The introduction of doping elements needs precautions which have been described elsewhere (ref. 5). The ingots were forged into bars (@ = 12 mm) at 1100°C. Subsequent heat treatments were performed under argon : (i) quench annealing at 1100°C (lh) or (ii) quench annealing plus sensitization at 650°C (30h), in order to precipitate carbides and segregate phosphorus. The different alloys were recrystallized with equiaxe grains but the grain sizes were very different in the 304L wire (10 pm) and the UHP base alloys (100 p).

Table I - Composition of the alloys (Wt %)

B a s e : C r = 1 8 % - N i = 1 2 %

3WL Wire

No 1

NO2

NO3

NO4

Fig. 1 - Corroded aspect of a wire ($ = 0. 4 mm) in Fig. 2 - Honeycomb like structure on a wire in artificial artificial saliva at a potential near the end of saliva in the same conditions as fig. 1. the passivity range

2.2 - Electrochemical conditions.

C

0.032

0.034

0.030

0.034

0.032

Two types of solutions were used. The first experiments were done in an "artificial saliva" containing phosphates (0.4 g/l), carbonates (1.5 gP), chlorides (1.66 gn), KSCN (0.33 g/l) and urea (1.30 g/l) (ref. 6). This solution had been selected by the dentists to characterize the general behaviour (corrosion and corrosion-fatigue) of a wire inside the mouth (ref. 6). To study the honeycomb-like corrosion, a simplified 1.66 g/l NaCl solution was selected, after an electrochemical study of artificial saliva which showed that the other components of the saliva

P

0.026

0.030

0.030

S

0.001

Mo

0.3 1

0.3 1

0.29

Si

0.45

Mn

0.7 1

acting e. g. as inhibitors, have no primary effect on the honeycomb-structure appearance. The pH was fixed between 2 and 7, except in the "depassivation potential" study. The electrochemical potential was fixed at the end of the passivity plateau (Fig 3). The solution, in contact with ambient air, was not stirred .

2.3 - Sample preparation

The industrial wire samples were cleaned (in acetone) and introduced into the corrosive solution. At the air-solution interface, a layer of varnish protected the metal. For UHP-base alloys, the samples were polished (3 pm grade diamond), cleaned and covered with a varnish layer in order to give tight crevice conditions (Fig. 4). A difference can be noticed between the two types of samples : on the wire, the observation was made on the external surface of an industrial product in the as-delivered condition ; on UHP base alloys, the observation took place on a polished section.

Fig 3 - Current tension curve. Alloy NO1 in 1,66 g/l Fig. 4 - Sample mounting for crevice corrosion tests NaCl at pH7

10-4

10-5

10-6

p-

3 - RESULTS

Fig. 2, shows a honeycomb structure observed at the corroded surface of a 304L wire. The in- relief grain boundary network is observed in the corroded part of the wire shown in figure 1. Such images could suggest variations in composition near the grain boundaries related to solidification or produced by unspecified heat treatments.

I (AIcm2)

Varnish

Polymer

Fig. 5 - Honeycomb corrosion on alloy NO1 in as-quenched state.

-lo-' O id0 260 3d0 460 560

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On pure alloy No 1, after quenching, Fig 5 and 6 show the corrosion state under the varnish layer after 40h exposure at fixed potential (300mVlSCE) and pH = 2 in the bulk solution. The total exposure time was 40h. The liquid came from left to right, penetrating slowly between the metal surface and the varnish layer. On the same sample, developments in the crevice can thus be observed as a function of time, but the exact exposure time cannot be given for one precise place on the corroded surface.

(a) Fig 6 - Corrosion under the varnish layer

(b)

The parts which have just begun to be corroded show essentially crystallographic pitting inside the grains (Fig. 6b) and the parts in which corrosion is more developed show grain boundary relief (Fig. 5). The longest exposed parts of the surface are deeply corroded, and an irregular general corrosion is observed Fig. 6a).

In some cases it is difficult to decide if the grain boundaries are in relief or not but, even in this case, the main result is that the grain boundaries are never preferentially attacked.

The same observation can be made on each of the pure base alloys in the as-quenched condition, i.e. no influence of the added elements is visible on the honeycomb like corrosion. However, molybdenum (0.3 wt%) has an influence on pitting and on the different stages of corrosion : in our experimental conditions, the grain boundaries are mostly observed as "steps" from one grain to the other instead of intergranular wedges in alloys n02 and 4.

Fig. 7 - Intergranular corrosion after sensitization (650°C 30h)

After annealing at 650°C (30h) for intergranular carbide precipitation and phosphorus segregation (ref. 7,8), intergranular corrosion with deep grooves was observed (Fig. 7), as is usual on sensitized stainless steels.

4 - DISCUSSION 4 .1 - Main results of this study

In the as-quenched state, all the UHP-base alloys (copies of a 304L) show the same behaviour : crystallographic pitting starts from the centre of the grains and extends toward the grain boundaries. The connection of two pits produces relief. This relief can be a grain boundary wedge, or in some cases (large grain size), intragranular wedges or summits where dense crystallographic planes intersect.

(a) (b) Fig. 8 - Schematic evolution of crystallographic pitting (a) resulting in intergranular wedges (b).

Fig. 9 - Determination of depassivation pH (alloy NO1)

4.2 - Tentative Explanation A simple explanation of such an idiosyncratic aspect of corrosion is proposed in figure 8.

Crystallographic pitting initiates at the tip of the crevice under the varnish layer. Pits start in the grains, without special relation with the grain boundaries. During further lateral growth and deepening, the pH at the tip of the crevice becomes more and more acid as compared with the pH of the bulk solution. Inside one pit, the pH is also lower than at the opening of the pit. Then the crystallographic growth of the pit cannot continue and a more spherical shape develops, revealing more conventional corrosion aspects.

It is possible, then, to observe grain boundary relief only at the first stages of growth of the crystallographic pits. After a very short time, only individual pits are visible and after a long time,

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irregularly corroded surfaces appear. The grain boundary relief features constitute a transitory state between individual pitting and general corrosion. They can be observed because grain boundaries have no special role in our experimental conditions (neither precipitation nor segregation).

4.3 - Conditions in the crevice

The true conditions inside the crevice are not known but the pH was established by measuring the "depassivation pH" of the metal (fig 9). A series of I-E curves in 1. 66gP NaCl solutions at different pH was drawn on bare samples (NO1 alloy). It can be seen that the depassivation pH is about 1.5, a classical result in more usually studied (30gP-NaC1) solutions. However, imposing pH=1.5 on a bare surface is not sufficient to initiate crystallographic or honeycomb corrosion. On a specimen exposed in an aerated corrosive solution (pH<1.5, E=passive breakdown, NaCl=1,66 wt%, during 40h), a corrosive layer was observed, without crystallographic pitting or a honeycomb-like aspect. This means that strongly de-aerated conditions, such as can be found at the tip of a deep crevice, are necessary to develop crystallographic pitting and honeycomb structures.

5 - CONCLUSIONS : - The honeycomb-like corrosion of quench annealed 304L steeI in low NaCl concentration

solutions, has nothing to do with segregation or intergranular energy. It results from crystallographic pitting starting in the grains with no particular initiation on the grain boundaries.

- Honeycomb-like corrosion is a transitory state between early stage crystallographic pitting and general corrosion.

- The electrochemical conditions for honeycomb-like appearance (and crystallographic pitting) are those of low pH and low oxygen content in the solution, as can be found at the tip of a deep crevice.

- This observation was possible by using a low chloride (1,66gP) solution instead of the concentrated (30gP or 9gP) more habitual solutions.

- The explanation of Hatwell (ref. 1) does not tally here : chromium depletion by precipitation cannot be invoked.

Acknowledgements : The authors are grateful to R. Moya and F. Avondo for technical support.

References

(1) - H. Hatwell : Mem Sci Rev Met. , LIII nOIO, p 729-749 (1956)

(2) - B. Baroux, C.Goux : Mem Sci. Rev.Met, LXVIII, N05, 351-356 (1971).

(3) - J. Stolarz, J. Le Coze, A.Dtsestret, 9th European Congress on Corrosion, Utrecht, vol.1 FU-227 (1989).

(4) - J. Stolaa, J.Le Coze, Colloque de Physique C1,5 1,641-645 (1990).

(5) - J. Le Coze, R. Tardy, M. Biscondi, A. Kobylanski : ((Preparation of high purity metals and metallurgical studies performed at the Ecole des Mines from 1970. A review >>. Proceedings UHPM-94, Ed. K.Abiko, K. Hirokawa, S.Takaki, The Japan Institute of Metals (1995).

(6) - J. Brugirard, cc Etude du comportement Clectrochimique des mitaux et alliages dentaires >, ed Julien Pelat, Paris (1970).

(7) - V. Cihal, in Handbook of Stainless Steels , ed D. Peckner, I.M. Bemstein, McGraw-Hill(1977).

(8) - P.Dumoulin, M. Foucault, M.Palmier, M.Wayman, M.Biscondi, M.Guttmann, Metal Science 1-15 (1980