Surface Technology, 6 (1977) 131 - 137 131 © Elsevier Sequoia S.A., Lausanne -- Printed in the Netherlands

PITTING OF PLAIN CARBON STEELS IN ACIDIC SOLUTION

VERONICA M. SILVA and LINDSAY F. G. WILLIAMS

Australian De fence Scientific Service, Department o f Defence, Materials Research Labo- ratories, P.O. Box 50, Ascot Vale 3032, Victoria (Australia)

(Received March 14, 1977)

Summary

The "end grain" pitting of plain carbon steels by acid pickling solutions is initiated by rapid dissolution of the sulphide inclusions. This is followed by simple dissolution of the metal matrix surrounding the original non-metallic inclusion. A pit in a ferrite grain typically has a rectangular cross section. Pitting of steels in an aqueous iodine/potassium iodide solution can occur by penetration of the solution along the non-metallic inclusion- steel interface, with subsequent dissolution of the steel.

Introduct ion

Although pitting of steels was known early this century to be associated with non-metallic segregates [1 - 4] , only recently has an extensive re- examination of the phenomenon been made [5 - 9]. Relatively recent tech- niques such as electron probe microanalysis (EPMA) have assisted in the identification of non-metallics [9, 10], in contrast to earlier qualitative techniques such as sulphur printing [11, 12]. The renewed interest in the pitting of plain carbon steels has sprung from two major failures, an ice-breaker and a bridge [7], in which the continuously cast steels employed were reported to be high in sulphide content . However, the investigations stimulated by these failures have almost exclusively centred on the pitting of steels in near-neutral solutions.

The "end grain" pitting of steel cartridge cases [13] led us to an inves- tigation of the mechanism of acid pitting of plain carbon steels. Some early work by Evans and Lewis and Evans [3, 4] showed that this pitting was associated with non-metallic segregates in both wrought iron and steel exposed to hydrochloric acid solutions. Stead [2] also suggested that pitting of steels in acidic solution was a result of reactions with sulphide segregates. In this investigation we have followed the pitting of a number of plain carbon steels in sulphuric acid pickling solutions using scanning electron microscopy (SEM) and EPMA of selected areas.

132

TABLE 1

Analysis of steels

Type of steel Percentage composition

C Si Mn P S Cu Cr

Mild 0.2 0.6 0.02 0.03 French armament 0.31 0.04 0.8 0.01 0.01 Medium carbon 0.48 0.04 0.58 0.02 0.017

0.06 0.05

E x p e r i m e n t a l

Tab le 1 shows analyses o f the th ree plain c a r b o n steel bars used in this inves t igat ion.

All t he steels were a s - m a n u f a c t u r e d and had a f e r r i t e -pea r l i t e s t ruc ture . The ma te r i a l was sec t ioned pe rpend icu l a r ly to the d i rec t ion o f d rawing and was pol i shed in the usual m anne r . I t was no t i ced dur ing the p r e p a r a t i o n of the samples t h a t t he non-meta l l i c inclusions in the steel were o f t en d is lodged b y the use o f u l t rason ic c leaning e m p l o y e d b e t w e e n each pol ishing opera- t ion , so th is p rac t i ce was d i scon t inued . The non-meta l l i c inclusions in the areas to be inves t iga ted were e x a m i n e d b y EPMA to iden t i fy the e l emen t s presen t . Af te r this , t he samples were given a f inal pol ish on 0 - 1 pm d i a m o n d to r e m o v e any c a r b o n depos i t ed in the EPMA and were degreased wi th e thano l . T h e steel samples were r insed in disti l led wa te r at 50 °C b e f o r e be ing i m m e r s e d in a p ickl ing so lu t ion con ta in ing 10% H2SO 4 at 50 °C. Af te r the a p p r o p r i a t e i m m e r s i o n t ime (ranging f r o m 1 to 30 min) the spec imens were r insed in dist i l led wa te r and qu ick ly i m m e r s e d in an e t h a n o l - a c e t o n e m i x t u r e con ta in ing a t r ace a m o u n t o f an inhib i t ing oil to p reven t rus t ing o f the surface . The samples were a i rdr ied and e x a m i n e d b y SEM. In one exper - imen t , a s ample was t r ea t ed wi th an a q u e o u s p o t a s s i u m iodide so lu t ion at r o o m t e m p e r a t u r e s a tu ra t ed wi th iodine.

Successive e x a m i n a t i o n s in the SEM were done using the same field o f view as chosen in t he EPMA so t h a t t he progress ive e f fec t o f the pickl ing so lu t ion on a su lphide inclus ion and the su r round ing ma t r i x could be s tudied .

Resul ts and d iscuss ion

The resul t o f successively subjec t ing a t yp i ca l mild steel ba r t o a 10% H~SO4 so lu t ion can be seen in Fig. 1. The pol i shed surface (Fig. l ( a ) ) r ap id ly d e v e l o p e d f ine pi ts assoc ia ted wi th t he non-meta l l i c inclusions in the steel. As the sur face s tud ied was a cross sec t ion o f a mild steel ba r con ta in ing e longa ted non-meta l l i c inclusions, the p i t t ing c o n t i n u e d to progress to some apprec iab le d e p t h and some pi ts u l t ima te ly t e r m i n a t e d dur ing the course o f

133

Fig. 1. Scann ing e l ec t ron mic rographs o f a mi ld steel surface a f t e r (a) 0 s, (b) 60 s, (c) 300 s, (d) 600 s, (e) 900 s and (f) 1500 s i m m e r s i o n in a 10% H2SO 4 so lu t ion at 50 °C, showing the d e v e l o p m e n t o f end grain pi t t ing.

134

the experiment. One interesting feature which is particularly predominant in Fig. 1 is the rectangular cross section of the pits formed in ferrite grains. In a single grain any two rectangular pits were aligned with each other and with any faceting of the surface. At grain boundaries, the pits were complicated in cross section but still retained the orientation effects of the two separate grains. This type of pitting was a clear indication that dissolu- t ion of the matrix was an important factor in the development of the attack by the pickling solution.

Similar results were obtained with the two other steels examined, as shown in Fig. 2. The French steel was attacked along the sulphide contain- ing non-metallic inclusions, which were generally in the ferrite grains. Heat t rea tment of this steel to produce a martensitic structure did have some effect on the shape of the pits but pitting still occurred at sulphide inclu- sions. The medium carbon steel in Figs. 2(e) and 2(f) had sulphide inclu- sions in both the ferrite and pearlite grains. In the pearlite grains the shape of the pits followed that of the original inclusion, with undissolved carbides protruding into the pit.

The initiation step in the acid pitting of these steels can be seen in Fig. 3. An inclusion containing manganese and sulphur, Fig. 3(a), was dissolved after a mere 5 s exposure to the acid pickling solution as seen in Fig. 3(b). By plating the steel with nickel and carefully cross sectioning, these pits were shown to extend to the end of the inclusions, i .e. the complete sulphide inclusion stringer was dissolved in the first 5 s of the pickling process. When duplex inclusions occurred, the sulphide-containing portion of the non- metallic segregate was similarly dissolved, leaving the insoluble portion in a cavity as shown in Fig. 3(c).

In the French steel the sulphide stringer inclusions were in the ferrite but closely associated with the banded pearlite. After prolonged pickling, this gave the misleading impression under the optical microscope that the pitting may have been associated with the pearlite (see Fig. 3(e)).

The existence of gaps around sulphide inclusions has been postulated by Baker and Charles [15]. If these do exist, they would facilitate the dis- solution of the inclusion by permitting the acidic solution to penetrate down the edge of the inclusion more rapidly than might normally be expected. To test the possibility of solution penetrating along such gaps, we decided to s tudy a corrosion reaction which did not dissolve the inclusion and which resulted in soluble iron corrosion products. An I2/KI solution was chosen, therefore, to examine the reaction of the French steel around the sulphide inclusions. This solution has been used in the past for removing oxide films from steel [14]. After a 15 s immersion, the pitting had occurred around most inclusions as shown in Fig. 3(d). Careful examination showed intermit- tent penetration of the solution around most inclusions, which resulted in pits as shown in Fig. 3(d). Moreover, in Fig. 3(d) there appeared to be two main areas in which penetration of the solution had occurred and, as would be expected, the bases of the two pitted segments were at two different levels below the remaining surface. These results are consistent with the

135

Fig. 2. Scann ing e l ec t ron mic rog raphs of (a) - (d) F r e n c h and (e) and (f) m e d i u m c a r b o n steel surfaces a f te r (a) 0 s, (b) 300 s, (c) 1200 s, (d) 1800 s, (e) 0 s and (f) 120 s immers ion in a 10% H2SO 4 so lu t ion at 50 °C. T he sample in (e) was l ight ly e t c h e d wi th nital .

136

L = ~ k ~ ~ L

4.

• 201Jm

Fig. 3. Scanning e lec t ron micrographs of the pi t t ing o f (a) med ium carbon steel before , and (b) and (c) after , 5 s immers ion in a 10% H2SO 4 solut ion at 50 °C; (d) F rench steel af ter a 15 s immers ion in an I2/KI so lu t ion at r o o m t empera tu re ; and (e) an optical micrograph of a cross sec t ion of the a r m a m e n t steel af ter 900 s immers ion in a 10% H2SO 4 solut ion at 50 °C.

137

pos tu la te o f some degree o f mismatch be tween the sulphide inclusion and the steel. The mechan i sm of pi t t ing in this I2 /KI solut ion appears to be similar to tha t p roposed by Wranglen [6] for steel in aerated salt solutions. The effects in the aqueous 12/KI so lu t ion also indicate tha t the rapid disso- lu t ion o f sulphide inclusions in acid solut ions m a y be assisted by gaps a round the inclusions.

In summary , these results clearly demons t r a t e tha t the end grain pi t t ing o f plain ca rbon steels in acid pickling solut ions occurs by the rapid dissolu- t ion o f the sulphide inclusions, fo l lowed by pickling o f the steel in the resul- t an t holes. The sulphide ion released m a y have some accelerat ing effect on the subsequent rate o f d issolut ion o f the steel su r rounding the inclusion bu t this would no t af fec t the general mechanism. Stead [2] originally pos tu la ted this t y p e o f mechan i sm bu t did no t show explicit visual evidence; the cur ren t results suppor t his findings.

References

1 J. W. Cobb, J. Iron Steel Inst., London, 83 (1911) 170. 2 J. E. Stead, J. Iron Steel Inst., London, 88 (1916) 74. 3 U. R. Evans, J. Soc. Chem. Ind., London, 47 (1928) 62. 4 K. G. Lewis and U. R. Evans, Rep. Corr. Comm. Iron Steel Inst., 2 (1934) 209. 5 M. Janik-Czacher, A. Szummer and Z. Szklarska-Smialowska, Br. Corros. J., 7 (1972)

90. 6 G. Wranglen, Corros. Sci., 9 (1969) 585; 14 (1974) 331. 7 G. Wranglen, Int. Conf. Localized Corrosion, Williamsburg, Va., U.S.A., 6 - 10 De-

cember, 1971, National Association of Corrosion Engineers, Houston, Texas, 1974. 8 G. Eklund, Scand. J. Metall., 1 (1972) 331. 9 Z. Szklarska-Smialowska, Corrosion (Houston), 28 (1972) 388.

10 R. Kiessling and N. Lange, Non-metallic Inclusions in Steel, Part II, Iron and Steel Inst., Publication 100, London, 1966.

11 W. H. Hatfield, J. Iron Steel Inst., London, 113 (1926) 168. 12 L. Northcott, Metallurgist (London, 1925 - 41) 10 (1935) 17. 13 L. F. G. Williams and D. J. Newton, Mater. Res. Lab., MRL Rep. 632, 1975. 14 U. R. Evans, J. Chem. Soc., (1927) 1024; Nature (London) 120 (1927) 584. 15 T. J. Baker and J. A. Charles, J. Iron Steel Inst., London, 680 (1972) 702.