Pitting corrosion and intergranular attack of austenitic Cr-Ni stainless steels in NaSCN

Pitting corrosion and intergranular attack of austenitic Cr-Ni stainless steels in NaSCN

Corrosion Science, 1970, eel. 10, pp. 607 to 615. Pergamon Press. Printed in Great Britain PITTING CORROSION A N D I N T E R G R A N U L A R A T r A...

1MB Sizes 0 Downloads 52 Views

Corrosion

Science, 1970, eel. 10, pp. 607 to 615. Pergamon Press. Printed in Great Britain

PITTING CORROSION A N D I N T E R G R A N U L A R A T r A C K OF AUSTENITIC Cr-Ni STAINLESS STEELS IN NaSCN* A . B. IJZERMANS A K Z O Research and Engineering N . V . Arnhem, The Netherlands Abstract--The susceptibility of 180"-10Ni-2Mo stainless steel to corrosion in 30%NaSCN solutions at 74°C has been investigated by means of potentiostatic and potentiokinetic polarization techniques. Solution heat-treated specimens of this steel show pitting corrosion in the potential range between -200 and + 200 mV (NILE). Heat treatment at 650°C raises the rate of pitting corrosion and leads to intergranular attack. In this case corrosion may be detected over almost the entire passive and transpassive regions. In the search for compositional effects, we found that M e shows the most pronounced effect in increasing the corrosion resistance, whereas C has the reverse effect. The stabilizing elements Ti and N b decrease the susceptibility to intergranular attack considerably, but lead to an increase in the rate of pitting corrosion. Experiments relating the corrosion to temperature and nature of the cation are d.escribed. The results are discussed in terms of structural heterogeneity and breakdown of passivity under the influence of anion adsorption. R~sum6---La tendance a la corrosion d'un acier inoxydable 18Cr-10Ni-2Mo dans des solutions contenant 30% de NaSCN ~t 74°C a 6t6 examinde ~ l'aide des m~thodes potentiocindtique et potentiostatique. Des ~hantillons de cet acier, trait~s par chauffage ~t 1250°C, se sent m o n t r ~ sujets ~t la corrosion par piqi~res entre -- 200 et + 200mV (ENH). U n traitement par chauffage ~ 650°C aecroit la vitesse de corrosion par piq~res et donne lieu a la corrosion intergranulalre. Dans ce cas, une action corrosive se constate sur le trajet quasi entier des domaines passif et transpassif. Dans l'examen des effets de composition de l'acier, le M e s'est montr6 le plus efficace a accroitre la r6sistance a la corrosion, alors que le carbone a un effet n~gatif. Les 61~ments stabilisants Ti et N b font diminuer la tendance/t la corrosion intergranulaire, tout en augmentant la vitesse/t la corrosion par piq6res. On a r~,alis6 quelques essais pour ~tudier la corrosion en fonction de la temperature et de la nature du cation. Dans la discussion des r~sultats on a consid6r(~ l'h~t~rog~n6it6 structurelle et la destruction de la passivit~ par adsorption d'anions. Zusammenfassung--Die Neigung zur Korrosion eines 18Cr-10Ni-2Mo nichtrostenden Stahls in 30%-igen NaSCN-L6sungen bei 74°C wurde mittels einer potentiokinetischen und einer potentiostatischen Methode untersucht. Es wurde festgestellt, dass 16sungsgegli~hte Proben dieses Stahls in einem Potentialgebiet zwischen -- 200 und + 200 mV~ Lochfrasskorrosion ausgesetzt sind. Bei 650°C gegliJhte Proben zeigten eine erh6hte Lochfrassanf/illigkeit und ausserdem wurde interkristalline Korrosion gefunden. In diesem Fall k6nnte fast im ganzen passiven und transpassiven Bereich ein Angriff nachgewiesen werden. Bei der Untersuchung des Einflusses der Zusammensetzung zeigte sich M e am effektivsten bei der UnterdrLickung der Korrosion, w/ihrend C einen negativen Effekt hatte. Die Neigung zur interkristallinen Korrosion wurde durch die Stabilisierungselemente Ti und Nb bedeutend herabgesetzt, aber die Lochfrassanf~Jligkeit steigerte sich. In einigen Versuchen wurde die Korrosion in Abh/ingigkeit yon Temperatur und Art des Kations studiert. Bei der Er6rterung der Versuchsergebnisse wurden die Strukturheterogenitit und die Zerst6rung der Passivit/it yon Anionen im Betracht genommen. *Manuscript received 24 November 1969. 607

608

A.B. DZFJ~tANS INTRODUCTION

Tim MANUFACTtraEof acrylic fibres according to the wet-spinning process comprises the following steps :1 polymerization of acrylonitrile with other monomers, dissolving the polymer in a suitable solvent, and extruding the solution through orifices into a coagulation bath to form the filaments. A concentrated aqueous solution of N a S C N is often used as a solvent for the polymer, and a diluted solution as a coagulant. In an installation for the recovery of N a S C N solutions we have found that an 18Cr-10Ni-2Mo stainless steel failed by pitting corrosion and intergranular attack in ~ 30%NaSCN at 74°C. According to Dechema Werkstoff-Tabelle, 2 stainless steels may be considered to be completely resistant to hot K S C N solutions; in boiling 25%NH4SCN solutions 18Cr-8Ni stainless steels have a corrosion rate of about 0.4 mm/y, but the Mocontaining steels are considerably more resistant. Cihal and Prazak s mention the addition of 0.01%KSCN to 1N H~SO4 to increase the c.d. of passivation of Cr-Ni stainless steels, but definite information on the corrosion of stainless steels in thiocyanate solutions could not be obtained from the literature. Therefore, efforts were made to obtain information by using potentiokinetic and potentiostatic polarization techniques. EXPERIMENTAL The stainless steels used as specimens are listed in Table 1 (surface area of about 5 cm2); in some comparative measurements, Pt wire electrodes were used (surface area of about 1 cm~). The steel specimens were subjected to one of the following heat treatments: (a) Solution heat treatment at 1250°C for 15 min followed by quenching in water; metallographic inspection after this treatment showed that small amounts of ~-ferrite were present on the grain boundaries of steels 2, 7, 8 and 9 (Table 1). (b) Solution treatment at I050°C for 15 min followed by quenching in water. (c) Treatment (a) followed by sensitizing at 650°C for 5 h and cooling in air. In this heat treatment precipitation of carbides occurred at the grain boundaries, and ~5-ferrite in steels 2, 7, 8 and 9 was partly transformed into ~-phase. Heat treatments (a)-(c) are referred to in Table 3. TABLE1.

ANALYSIS OF THE STAINLESS STEELS EXAMINED ( % )

Steel

Cr

Ni

Mo

C

1 2 3 4 5 6 7 8 9 10 11 12

17"4 17"9 17'6 18"1 17"6 18"9 16"9 17"5 17"8 17"4 24"6 26"0

10-8 12-2 11"3 10-9 12"2 15"7 13"9 11"4 10"9 19-9 19"6 24"2

-2"4 2"1 2"3 2"8 3"4 4"3 2-2 2'3 2"5 -2'3

0'06 0.06 0'04 0'025 0.06 0'03 0"06 0'07 0'08 0"04 0'10 0"05

Some other elements -------Nb Ti Cu 2'5, Nb Ti

Austenitic Cr-Ni stainless steels in NaSCN

609

Next, the specimens were polished to 3/0 emery paper, degreased with acetone and rinsed with distilled water before they were put into the polarization cell. 4 Most of the experiments were made in stirred 30 % N a S C N prepared from the pure substance and distilled water. Solutions of K S C N and NH4SCN having the equivalent concentration of S C N - (36% and 28%, respectively) were also used. The solutions were deaerated with prepurified N~ with an O3 content < 1 ppm. The minor amounts of by-products, such as HaS and SO2, which were detected in the N a S C N solution from the recovery installation, did not have any effect on the experimental results. In most tests the solutions were thermostatically maintained at 74°C, which was the temperature at which the corrosion was found in practice. The p H values of the solutions at 20°C and 74°C are given in Table 2. TABLE 2. p H VALUES OF THIOCYANATE SOLUTION.S

Substance

Concentration in water (%)

T = 20°C

pH T = 74°C

NaSCN KSCN NI-I4SCN

30 36 28

8"3 8.5 4"4

7.5 7"6 3"7

A Tacussel potentiostat (Type P R T 2000) was used for the polarization measurements. The potentiokinetic measurements were made by varying the potential between - - 750 and + 800 mV (NHE) with a traverse rate of 5 mV/min and recording the current continuously. In the potentiostatie method the specimen was maintained at a constant potential for 20 h before the current was recorded, a new specimen in a fresh solution being taken at each value of the potential. In both techniques the test electrodes were previously activated at - - 750 mV for 15 rain. The potentials were measured with the aid of a SCE and are given relative to the normal hydrogen electrode (NHE). RESULTS AND DISCUSSION Potentiokinetic c.d. vs. potential curves Figure 1 shows the i/E curves obtained with a solution-treated (at 1250°C) and a sensitized specimen of an 18Cr-10Ni-2Mo stainless steel (No. 2 in Table 1) in 3 0 % N a S C N at 74°C. For comparison the curve obtained with Pt is included. The cathodic parts of the curves do not differ much. Extrapolation of the cathodic curve for Pt to zero current yields a potential between - - 400 and - - 350 mV. This indicates a p H of about 6, which agrees only approximately with the p H value of 7-5 measured in the N a S C N solution at 74°C (Table 2). In the anodie parts of the curves a sharp increase in current may be seen, starting at about + 300 mV on Pt and at + 400 mV on the steel specimens. This increase was accompanied with the formation of a yellow product soluble in the solution at c.ds. below 1 mA/cm 2. At higher c.ds., however, a yellow deposit was formed on the electrode, as a result of which some flattening in c.d. started at about + 650 mV. In the potential region above + 650 mV the colour of the deposit changed into orange-red. Visible evolution

610

A . B . DzEgsO~s

lO ~

~o ~ c J_

1o2

\ lO1

I / I I -

400

0

II

I

I

+400

[ + 800

Potentialt mV (nhe)

FIG. 1. Potentiokinetic curves in 30%NaSCN at 74°C. 1: Pt. 2: 18Cr-10Ni-2Mo stainless steel (No. 2 in Table 1) after heat treatment: 1250°C × 15 min/W. 3: same stainless steel after heat treatment: 1250°C × 15 min/W + 650°C × 5 h/A. Full lines: anodic curves; broken lines: cathodic curves.

of oxygen occurred only at high overpotentiais; on steel it occurred at -k- 2200 mV at ~ 80 mA/cm 2. Since the increase in anodic current at about 400 mV occurs both on steel and on Pt, it must be ascribed to the oxidation of a component in the solution. It is possible that S C N - may be oxidized to thiocyanogen (SCN)2, a yellow compound analogous to Br2 and I2 molecules. It is known that (SCN)2 may polymerize to a red mass, 5 which probably explains the change in colour of the deposit on the electrode. As art oxidizing agent (SCN)2 lies between Br~ and I~. 5 Accordingly, the standard oxidation potential for the system ( S C N ) J S C N - should be between B r J B r - and Is/I-, i.e. between 540 and 1050 mV, and this was estimated by Latimer e to be 770 mV. In 3 0 % N a S C N the oxidation potential will be considerably below this value, especially when hardly any (SCN)~ is present. Even in that case a value of about 400 mV, which may be read from the curves, does not correspond well with an oxidation potential of 770 mV. The curves obtained on the steel specimens show current peaks at q- 100 mV, which do not occur on Pt. On the sensitized specimen, which gave the higher c.d. in

Austenitic Cr-Ni stainless steels in NaSCN

611

the peak, small pits and grooves were found between 0 and W 200 mV, and on the solution-treated specimen initiation of pitting was found to occur in the same potential range. The current peaks thus clearly point to a breakdown of the.passive state. In this respect SCN- behaves like the halides, in particular CI-. 7 The fact, however, that breakdown only occurs in a small potential range around q- 100 mV is not characteristic of a pseudo-halide ion. C.d. vs. potential curves were also measured in KSCN and NHaSCN solutions at 74°C, using the same test electrodes (steel 2, Table 1). In the KSCN solutions,which had almost the same pH as the NaSCN solutions, the curves did not differ much from those given in Fig. 1. The only marked difference was that the peaks at 100 mV reached values of 0-2 and 1.1 mA/cm 2 for the solution-treated and sensitized specimens. In the NH4SCN solutions, with pH = 3-7 at 74°C, the position of the cathodic curves had moved to potentials about 300 mV higher than the cathodic curves of Fig. 1, but the anodic curves retained their position. This indicates that the position of the peak at -1- 100 mV and the anodic current above q- 400 mV are not affected by pH. As for the latter, this is in accordance with the oxidation of SCN- to (SCN)z. The peak heights in NH4SCN were higher than in NaSCN, i.e. 0.5 mA/cm ~ and 1.5 mA/cm z for the solution-treated and sensitized specimens of steel 2. C.d. vs. potential curves obtained on sensitized specimens of steel 2 in 30 %NaSCN at temperatures between 20 and 74°C showed that the temperature had virtually no influence on the position of the peak. Peak heights, however, increased somewhat with decreasing temperature. From the polarization measurements made with sensitized specimens of the steels 1, 2, 5, 8 and 9 in 30%NaSCN at 74°C it appeared again that the position of the peak, as contrasted to its height, did not change significantly with the composition of the steel. This is a remarkable observation, since it is well known that the critical potential of pitting formation changes considerably with alloy composition. 7 On the other hand, the change of the critical potential is smaller according as the concentration of the solution is higher, v,a which may be the explanation of the stable position of the current peak. Potentiostatic c.d. vs. potential curves The potentiostatic measurements performed on specimens of steel 2 (Table 1) in 30%NaSCN at 74°C are given in Fig. 2. The i/E curves in this figure are quite similar to the potentiokinetic curves of Fig. 1, with the exception of a small cathodic c.d. ( < 10 ~A/cm ~) which appeared between -- 200 and -- 400 mV. This c.d. is as yet unexplained. It can neither be ascribed to O~ reduction (solutions were deaerated) nor to H + reduction (at p H 7 only possible at E < -- 400 mV). The full symbols in Fig. 2 represent the rates of corrosion which were determined from the weight loss of the specimens. On the solution heat-treated specimens, corrosion rates > 0.02 mm/y are all found between -- 200 and -~ 200 mV; they reach a maximum at d- 100 mV. Outside this potential range the corrosion rates are ~ 0.01 mm/y, which was the limit of detection in the measurements. Pits could be observed in the corroded solution-treated specimens, which confirms the conclusion from the potentiokinetic curves (Fig. 1) that breakdown of the passive state may occur in the range around + 100 inV. The polarization curve is similar to the one described by Brauns and Schwenk a for

612

A.B. I J ~ s

18Cr-10Ni stainless steel in 1N HzSO4 containing 1 mole/l NaCI and 0.5 mole/l NaNOs where pitting was found only between q- 150 and -t- 500 mV; in the solution

A

104

lO2

=L,.

E

0

E w

"~ 103

101 '~

'E o u

102

101

10-1

-+1 0

+ 300

+ 600

+900 ---

41200

Potential. mV (nhe)

_1o 1

FIG. 2. Potentiostatiecurves for 18Cr-10Ni-2Mo stainlesssteel ('No. 2 in Table 1) in 30%NaSCN at 74°C. A&, after heat treatment: 1250"C × 15 min]W, o o , after heat treatment: 1250°C × 15 min/W + 650°C × 5 h/A. Open symbols: e.ds. (connected by drawn lines), full symbols: rates of corrosion.

without NaNO3 pitting was observed in the entire passive region above q- 150 mV. It has been postulated that the inhibition of pitting by NO~ may be ascribed to the greater adsorbability of NO~ ions at higher potentials, which leads to a displacement ment of the passivity-destroying CI- ions from the metal surface. ? However, the differences in adsorbability between C1- and NO~ throughout the potential range of passivity, which are usually explained by the differences in the polarizability of the anions, still have a weak theoretical foundation. The inhibition of pitting corrosion in 30%NaSCN at potentials above + 200 mV might be explained by an adsorption

FIG. 3. 18Cr-10Ni-2Mo stainless steel (No. 2 in Table 1) after heat treatment 1250°C x 15 min/W + 650°C x 5 h/A, potentiostatically corroded at 74°C. Vilella etch, x 150. a: + 1 0 0 m V ; b : + 600mV, c: + 1250mV.

Austenitic CY-Ni stainless steels in NaSCN

613

displacement from the metal surface of the aggressive S C N - by the passivating oxygen from O H - ions or water molecules. The difficulty remains, however, how to account for the evidently greater adsorbability of S C N - at about - - 1.00 mV, where pitting corrosion starts. The sensitized specimens showed a measurable rate of corrosion in a wider potential range (Fig. 2). F r o m microscopic observation it appeared that the susceptibility to pitting corrosion had considerably increased, particularly between - - 100 and + 200 mV; at higher potentials up to + 600 mV pitting could also be demonstrated (Fig. 3b). Furthermore it appeared that intergranular attack had taken place. The intensity of this type of corrosion also changed markedly with potential. In the range between - - I00 and + 200 mV a rather pronounced intergranular attack was found (Fig. 3a) which changed at higher potentials in the passive and transpassive regions into a weak corrosion (Figs. 3b and 3c). Undoubtedly, the susceptibility to intergranular attack is related to the precipitation of carbides at the grain boundaries under the influence of the heat treatment at 650°C. x° The increased tendency to pitting corrosion may also be ascribed to structural heterogeneity caused by this anneal, n

TAnLE 3.

30%NaSCN

Steel 1

2 3 4 5

RJ~SULTS OF POTENTIOSTATIC COI~OSION TEST IN

AT 74°C (POTENTIAL KEPT AT ~t. I00 m V FOR 20 h)

Corrosion rates (re_m/y) After heat After heat treatment (a) treatment (c) 4.2 7.0 3.7 2.4 0.32

1.2

0.9 0.30 0.17 0.06

(0.42)* (0.20)

< 0.01 < 0.01 < 0.01 0.11

0.44 0.12 0.05

0"03 6

7 8 9 10 11 12

< 0"01

< 0.01 0.05 < 0.01 0.40 0-32 0.26 0.51 < 0.01 < 0.01 0.05 0.03 < 0.01 < 0.01

<

< (0.06) (O.O2) (0.12) (0.27) (0.42) (0.25) < < <

0.76 0"51 0"01 0'01 0.21 0.23 2.0 4.6 3-1 3.9 0.07 0.01 0.34 0.72 0.01 0.01

*Values between brackets represent corrosion rates of specimens subjected to heat treatment (b).

614

A.B. IJZmLMANS

Compositional and structural effects Solution heat-treated (at 1250°C) and sensitized specimens of the stainless steels given in Table 1 were subjected to a potentiostatic corrosion test in 3 0 % N a S C N at 74°C. The potential was kept at + 100 mV, where a peak was found in the polarization curves. As appears from the potentiokinetic curves the position of this peak does not change with the composition of the steel. For comparison some experiments were made with specimens heat-treated at 1050°C. The corrosion rates calculated from the weight loss after a test period of 20 h are given in Table 3. On the corroded solution-treated specimens only pitting corrosion was found. The inclusions of 6-ferrite, which were present in small amounts in some of the specimens, did not affect the corrosion rate, as may be seen by comparing the results after heat-treating at 1250°C and 1050°C (steels 2, 7, 8 and 9 in Table 3). Except for the completely resistant steels 6 and 12, Table 3 shows that higher corrosion rates have been measured on the specimens sensitized at 650°C. Evidently, this may be ascribed to the structural heterogeneity introduced by the anneal. F r o m a microscopic inspection it appeared not only that the susceptibility to pitting corrosion was increased under the influence of this heat treatment, but also that intergranular corrosion had occurred on the non-stabilized steels 1-5, 7 and 11. 10 ~



A

E - 101

+

.I o

1

,o,

i No +

×

\ I

I

I

0

20

50

I

¥\

74 100 ~--~ Temperature, oC

FIG. 4. Corrosion rates of 18Cr-10Ni-2Mo stainless steel (No. 3 in Table l) in NH~SCN (28%), KSCN (36%) and NaSCN (30%) solutions at different temperatures. Corrosion rates were measured in a potentiostatic test at + 100 mV for 20 h.

Austenitie Cr-Ni stainless steels in NaSCN

615

In general, the higher-alloyed steels with the higher Cr and Ni content exhibit the higher corrosion resistance, but more pronounced are the effects of M e and C. It may be seen in Table 3 that an increase in M e content has a positive effect on the corrosion resistance (compare the steels I, 2, 5 and 7 and the steels 4 and 6). Evidently, this is related to the capability of this element to increase the stability of the passive state, lz On the other hand, an increase in C content leads to a considerable increase of the corrosion rate, both on solution-treated and on sensitized steels (nos. 2, 3 and 4). Evidently, the ~-phase which is present in very small amounts within the inclusions of 8-ferrite after sensitization, does not exert a detectable influence on corrosion. This, for instance, may be seen by comparing the results with the steels 2, 5 and 7, where only No. 7 contains some ~-phase. The corrosion rate continuously decreases in this order, and, therefore, may completely be explained from the content in Me. Effects o f cation and temperature

The effects were studied on sensitized specimens of steel 3 (Table 1) by means of the potentiostatic corrosion test at q- 100 mV. The results are given in Fig. 4, from which it may be seen that the rates of corrosion depend on temperature in a remarkable way. Raising the temperature in the C N S - solutions from 0°C to 20°C gives rise to an increase of the corrosion rate from 0.1-1 to 15--40 mm/y. At higher temperatures the rates of corrosion decrease with the cation in the order NH4 +, K +, N a +. A possible effect o f p H (see Table 2) must be excluded, since the same rate of corrosion was found in a N a S C N solution at 74°C acidified to p H 3.7 and in the solution to which no acid had been added. The decrease in corrosion rate with increasing temperature suggests a desorption of the passive layer-rupturing ions. The specific effect of the cation, however, is a further complication which cannot be easily fitted into an adsorption theory. KEFER.ENCES I. H. F. MARK, N. G. GAYLORDand N. H. BIKALES(Eds.), Encylopaedia of Polymer Science and Technology. Vol. I, p. 346. Intcrscience, New York (1964). 2. E. R.ABALDand H. BRETSCH~EXDER(Eds.), Dechema Werkstoff-Tabelle, III Ed, Dcchema, Frankfurt a. Main, Nos. 819 and 113 (1959). 3. M. P ~ and V. CmAL, Cortes. Sci. 2, 71 (1962). 4. A. B. IJZE~ANS and A. J. VANde I~O~T, Corros. Sci. 8, 679 (1968). 5. E. S. GOULD,Inorganic Reactions and Structure, p. 293. Holt, Rinehart and Winston, New York (1955). 6. W. M. LATIMER,Oxidation Potentials, p. 138. Prentice-Hall, New York (1952). 7. YA. M KOLOTYS~rN,Corrosion 19, 261t (1963). 8. According to data by A. B~.LrMELand E. VFa~HLrrENmentioned in the paper by G. I'~RBSLEeand and W. SCHWFa~, Werkstoffe Korros., Mannheim 18, 685 (1967). 9. E. BRAtrNsand W. ScHw~nc, Werkstoffe Korros., Mannheim 12, 73 (1961). 10. H-J. SCHOLLr~,P. SCHWA.~a3and W. SCHWm,~K,Arch. EisenhiittWes. 12, 853 (1962). 11. N. D. TOMASHOV,G. P. CHmU,~OVAand O. N. M~COVA, Corrosion 20, 166t (1964). 12. M. A. STRHCHErt, J. Electrochem. Soc. 103, 375 (1956).