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The influence of heat treatment on the pitting corrosion of delta-52 steel in NaOH solutions
Surface Technology, 23 (1984) 283 - 290
283
THE I N F L U E N C E OF H E A T T R E A T M E N T ON THE PIT T IN G CORROSION OF DELTA-52 ST E E L IN NaOH SOLUTIONS s. M. ABD EL HALEEM Chemistry Department, Faculty of Science, Zagazig University, Zagazig (Egypt)
S. S. ABD EL REHIM and M. Sh. SHALABY Chemistry Department, Faculty of Science, Ain Shams University, Cairo (Egypt)
(Received November 9, 1983) Summary The influence of heat t r e a t m e n t on the pitting corrosion of Delta-52 steel, an Egyptian p r o d u c t , was investigated p o t e n t i o d y n a m i c a l l y in NaOH solutions containing NaC1 as a pitting corrodent. In chloride-free solutions, the active dissolution o f the hot-rolled steel increases with increases in b o t h the alkali c o n c e n t r a t i o n and the potential scanning rate. In the presence o f chloride ions, plots of the critical pitting potential against the chloride ion c o n c e n t r a t i o n are sigmoidal. The data reveal t ha t the hot-rolled specimens resist pitting corrosion b e t t e r than do the specimens which were hardened or annealed at 850 °C for 1 h. The susceptibility to pitting corrosion of the hardened specimens is also increased by tempering at 200 °C. The critical pitting potential Ep varies with the tempering hold time tm according to Ep = K
--
nt
m
where K and n are constants.
1. I n t r o d u c t i o n Although intensive studies have been focused on the effect of the environmental conditions on the corrosion of iron and steel, few studies have been devoted to examining the effect of heat t r e a t m e n t on their corrosion behaviour. H eyn and Bauer [1] showed t hat the corrosion rate of a carbon steel q u e n c h e d from high temperatures is relatively low in acidic media. T h e y attr ib ut e d this to the f or m at i on of a single martensite phase with carbon in the interstitial positions of a body-cent red tetragonal lattice o f iron atoms. A r a ndom distribution of the carbon atoms with the adjacent iron atoms limits their effectiveness as cathodes of local action cells. When the martensite is t e m p e r e d by heating at low temperatures and then air cooled, d eco mp o s i t i on to e iron carbide takes place. This two-phase structure sets up galvanic cells which accelerate corrosion. 0376-4583/84/$3.00
© Elsevier Sequoia/Printed in The Netherlands
284
P o t e n t i o d y n a m i c polarization t e c h n i q u e s have been used successfully to s t u d y the pitting c o r r o s i o n o f iron and steels [2 - 9]. In the presence of a pitting ion, the p o t e n t i a l does n o t reach t h a t of o x y g e n e v o l u t i o n , and the c u r r e n t rises s u d d e n l y at a certain lower p o t e n t i a l k n o w n as the critical pitting potential. This increase in c u r r e n t does n o t o c c u r i m m e d i a t e l y on a d d i t i o n o f t h e pitting ion, b u t a f t e r a certain time lapse. This time is r e f e r r e d to as the i n d u c t i o n time and is related to the p e n e t r a t i o n l l 0 , 11] or a d s o r p t i o n [12, 13] o f the pitting ion on the passive surface. In a c o n t i n u a t i o n o f o u r studies on the c o r r o s i o n b e h a v i o u r o f Delta-52 steel in a q u e o u s solutions [14, 15], it seems t o be interesting to investigate p o t e n t i o d y n a m i c a l l y the i n f l u e n c e o f heat t r e a t m e n t on the critical pitting p o t e n t i a l of this steel in NaOH solutions c o n t a i n i n g NaC1 as the pitting corrodent. 2. E x p e r i m e n t a l m e t h o d 2.1. Solutions S t o c k solutions o f NaOH or NaC1 were p r e p a r e d by dissolving the a p p r o p r i a t e weights in d o u b l y distilled water. T h e solutions were t h e n d i l u t e d as required. All reagents were analytical grade chemicals. 2.2. The electrodes T h e Delta-52 steel used in the p r e s e n t w o r k was p r o d u c e d in the electric arc f u r n a c e o f the Delta Steel Mill C o m p a n y , Cairo. Its c o m p o s i t i o n is given in Table 1. T h e ingots were h o t rolled (at 1200 - 1000 °C) and t h e n m a c h i n e d in the f o r m o f s h o r t rods ( h o t - r o l l e d electrodes}, each 27 m m in length and 10 m m in d i a m e t e r . T o investigate the i n f l u e n c e of heat t r e a t m e n t , the following p r o c e d u r e s were u n d e r t a k e n . T o p r e p a r e h a r d e n e d electrodes, h o t - r o l l e d specimens were r e h e a t e d gradually in an a u t o m a t i c a l l y c o n t r o l l e d m u f f l e f u r n a c e (15 °C min ~) f r o m r o o m t e m p e r a t u r e up t o 850 °C f o r 1 h and were t h e n q u e n c h e d in water at 20 °C. S o m e o f the h a r d e n e d e l e c t r o d e s were t e m p e r e d at 200 °C for various h o l d times and were t h e n c o o l e d in air. T o p r e p a r e annealed e l e c t r o d e s the h o t - r o l l e d e l e c t r o d e s were r e h e a t e d gradually in the a u t o m a t i c a l l y c o n t r o l l e d m u f f l e f u r n a c e (15 °C min 2) f r o m r o o m t e m p e r a t u r e up t o 850 °C f o r 1 h and were t h e n c o o l e d gradually in t h e same furnace. Each e l e c t r o d e was m o u n t e d in glass t u b i n g with Araldite in such a m a n n e r t h a t o n l y its cross-sectional area was e x p o s e d t o the e l e c t r o l y t e . TABLE 1 C o m p o s i t i o n o f Delta-52 steel Element A m o u n t (%)
C 0.28
Mn 0.68
Si 0.27
S 0.045
P 0.035
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2.3. The electrolytic cell T h e cell used was m a d e o f d o u b l e - w a l l e d P y r e x glass o f a b o u t 120 ml c a p a c i t y . T h e cell was k e p t at 25 + 1 °C b y circulating w a t e r within the d o u b l e wall. It was p r o v i d e d with an inlet a n d an o u t l e t to p e r m i t d e a e r a t i o n b y passing n i t r o g e n gas t h r o u g h t h e test solution f o r 1 h b e f o r e each run. T h e lid was f i t t e d to t h e cell b y a g r o u n d glass flange which h a d t h r e e sealable s o c k e t s i n t o w h i c h the e l e c t r o d e s a n d t h e salt bridge were inserted. 2.4. Measurements A p l a t i n u m c o u n t e r e l e c t r o d e was used, a n d all p o t e n t i a l s w e r e m e a sured w i t h r e f e r e n c e to a s a t u r a t e d c a l o m e l e l e c t r o d e (SCE). P o t e n t i o d y n a m i c p o l a r i z a t i o n m e a s u r e m e n t s were m a d e w i t h a W e n k i n g p o t e n t i o s c a n t y p e POS 73. T h e p o t e n t i a l versus c u r r e n t d e n s i t y curves w e r e r e c o r d e d on an A d v a n c e x - y r e c o r d e r o f t y p e A R 2 0 0 0 . T o s t a r t the p o l a r i z a t i o n , the p o t e n t i a l o f the w o r k i n g e l e c t r o d e was set to a m o r e p o s i t i v e value at the c h o s e n s w e e p rate.
3. Results a n d discussion
3.1. Influence o f the N aO H concentration T h e curves o f Fig. 1 r e p r e s e n t the p o t e n t i o d y n a m i c p o l a r i z a t i o n o f the h o t - r o l l e d e l e c t r o d e in N a O H s o l u t i o n s o f various c o n c e n t r a t i o n s ranging f r o m 0.01 to 2 M. T h e curves were s w e p t b e t w e e n - - 1 2 5 0 and 750 m V (SCE) at a scan r a t e o f 2.5 m V s -1. T h e a n o d i c p o r t i o n s o f the curves are c h a r a c t e r i z e d b y an arrest b e f o r e a d i s s o l u t i o n c u r r e n t p e a k is r e a c h e d . T h e 100~
60( E
a,
2oc
-20C
-600 -I000
-500 E , mV
0
500
(SCE)
Fig. 1. T h e r e l a t i o n b e t w e e n t h e c u r r e n t d e n s i t y j ( p A c m - 2 ) a n d t h e a n o d i c p o t e n t i a l E ( m V ) ( S C E ) f o r t h e h o t - r o l l e d D e l t a - 5 2 s t e e l f o r v a r i o u s c o n c e n t r a t i o n s o f N a O H (p = 2.5 m V s - l ) : c u r v e 1, C = 1 x 1 0 - z M ; c u r v e 2, C = 1 x 10 - 1 M; c u r v e 3, C = 5 x 1 0 1 M; c u r v e 4, C = t M; c u r v e 5, C = 2 M.
286
presence o f this arrest may be due to the oxidation of hydrogen which is adsorbed on the surface and/ or absorbed in the metal lattice. The dissolution current peak is certainly due t o iron electrode processes. When the thermodynamic stability range is taken into account, the reaction taking place in the potential range may be [ 16 ] Fe + 2OH
.
' Fe(OH)2(ads) + 2e
B e y o n d the dissolution peak the electrode becomes passive and the current density decreases. The cur r ent flowing in the passive region is governed by the rate of chemical attack of the passive film by the electrolyte [14, 17]. Increases in the potential merely increase the thickness of the passive film, maintaining the field strength required to continue the necessary cation transport, by high field conduct ance, for the film growth and dissolution. When the potential imposed on the electrode becomes more positive to make the reaction 4OH- ~
02 + 2H20 + 4e
t h e r m o d y n a m i c a l l y (or kinetically) easier than the earlier high field ionic conduction, the current density increases suddenly and oxygen evolution becomes the p r e d o m i n a n t reaction [17]. Inspection o f the curves of Fig. 1 reveals t hat the alkali concent rat i on has a marked influence on the anodic behaviour of the electrode. With increasing alkali c o n c e n t r a t i o n , the height of the current peak increases and at the same time both the zero c ur r e nt potential and the potential corresponding to the cur r ent peak shift in the more negative direction. I000
6 O0
E "~ 7 O0
- 200
-600 -1000
500 E , mY
0
500
(SCE)
Fig. 2. T h e r e l a t i o n b e t w e e n t h e c u r r e n t d e n s i t y j ( p A c m --2) a n d t h e a n o d i c p o t e n t i a l E ( m V ) ( S C E ) f o r t h e h o t - r o l l e d D e l t a - 5 2 s t e e l in 1 × 10 2 M N a O H at v a r i o u s s w e e p r a t e s : curve 1, p = 1.0 m V s 1; c u r v e 2, v = 2.5 m V s - 1 ; c u r v e 3, p = 5.0 m V s ] ; c u r v e 4, v = 15.0 m V s 1 ; c u r v e 5, v = 4"0.0 m V s 1
287
3.2. Influence o f the sweep rate T h e curves o f Fig. 2 illustrate the i n f l u e n c e o f the s w e e p rate p ( m V s -1) b e t w e e n - - 1 2 5 0 and 7 5 0 m V (SCE) on the p o t e n t i o d y n a m i c p o l a r i z a t i o n o f the h o t - r o l l e d steel in 0,01 M N a O H . It is clear that t h e current densities c o r r e s p o n d i n g to b o t h the active and the passive regions increase with increasing s w e e p rate. H o w e v e r , an increase in the s w e e p rate has n o significant e f f e c t o n either the z e r o current or the p e a k p o t e n t i a l . 3.3. Susceptibility o f the Delta-52 steel to pitting corrosion in alkali solution Figure 3 s h o w s the a n o d i c p o l a r i z a t i o n o f the h o t - r o l l e d D e l t a - 5 2 steel in 0.01 M N a O H s o l u t i o n with t h e a d d i t i o n o f various c o n c e n t r a t i o n s o f NaC1 as a pitting c o r r o d e n t . Similar e x p e r i m e n t s w e r e carried o u t in 0.1 M
1000
t
600
E <[
^ 2O0
-20(
-60C
/[!
I
i
-1000
I
-500 0 E, my (5CE]
I 500
I
Fig. 3. T h e relation b e t w e e n the current d e n s i t y j (AtA c m 2) and the a n o d i c p o t e n t i a l E ( m V ) ( S C E ) for the hot-rolled D e l t a - 5 2 steel i m m e r s e d in 1 × 1 0 - 2 M N a O H s o l u t i o n c o n t a i n i n g various c o n c e n t r a t i o n s o f NaC1 (p = 2 . 5 m V s - l ) : c u r v e 1, 0; c u r v e 2, 1 × 10 - 3 M; c u r v e 3, 2 × 10 - 3 M ; c u r v e 4, 5 × 10 - 3 M; c u r v e 5, 8 × 10 - 3 M ; c u r v e 6, 1 × 10 - 2 M; c u r v e 7, 5 × 10 2 M ; c u r v e 8, 6 × 1 0 - 2 M.
5OO
~
~.00
- 400 ,
-38o
p
-26o
i
1
-,oo
o
LOg CCIFig. 4. The relation between the pitting corrosion potential Ep ( m V ) ( SCE) and the logarithm of the chloride ion concentration (p = 2.5 m V s - 1 ) : cur ve 1, hot-rolled steel in I x 10 -1 M N a O H ; c u r v e 2, hot-rolled steel in 1 × 10 - 2 M N a O H ; cur ve 3, hardened steel in 1 × 10 - 2 M N a O H .
288
NaOH. The addition of increasing amounts of chloride ion up to a certain value, which depends on the alkali c onc e nt r at i on, has no significant effec't on the polarization curves recorded in the chloride-free solution. It appears t h a t these lower concentrations of chloride ion cannot affect the dissolution kinetics of the passive film. With increasing chloride ion concentration, the current density in the passive region remains more or less constant over a certain potential range before it rises suddenly at a certain definite potential t e r m e d the critical pitting potential Ep, indicating the initiation of pitting attack. The relation between Ep and log Ccl- is given in Fig. 4. Plots of Ep against log Cc~- are sigmoidal. Similar results have been report ed previously [9, 181. The data reveal t ha t the critical pitting potential shifts in the negative direction with increasing chloride ion c oncent rat i on and in the more positive direction with increasing alkali concentration.
3.4. Influence o f heat treatment The influence of a hardening process on the pitting corrosion of the hot-rolled steel was examined. Figure 5 represents the anodic polarization of specimens hardened at 850 °C for 1 h and then quenched in water at 20 °C. The experiments were p e r f o r m e d between - - 1 2 5 0 and 750 mV (SCE) in 0.01 M NaOH and in the presence of various concentrations of chloride ion using a sweep rate of 2.5 mV s 1, The relation between E , and log Cci is shown in Fig. 4, curve 3. Under the prevailing experimental conditions, it is clear that the hardening process shifts the critical pitting potential to more negative values, denoting a decreased resistance for pitting attack. This could be explained as a result of sequential structure changes of hot-rolled steel under the effect of the hardening process. It is known t hat the main structure of h y p o e u t e c t o i d steel (Delta-52) after hardening is transformed into martensite and retained austenite [19].
500
jl 6 5/ /" E <~ 0
/
o
;
o
-50C
~'t -I000
500
I
0 E j mY (SCE)
I
]
500
I
Fig. 5. T h e r e l a t i o n b e t w e e n t h e c u r r e n t d e n s i t y j (/2A c m : ) a n d t h e a n o d i c p o t e n t i a l E ( m V ) ( S C E ) f o r t h e h a r d e n e d D e l t a - 5 2 s t e e l i m m e r s e d in I X 10 2 M N a O H s o l u t i o n c o n t a i n i n g v a r i o u s c o n c e n t r a t i o n s o f c h l o r i d e i o n (p = 2 . 5 m V s 1): c u r v e 1, 0; c u r v e 2, 2.5 x 10 -3 M; c u r v e 3, 5 x 10 3 M ; c u r v e 4, 7 . 5 × 10 3 M ; c u r v e 5, 1 × 10 2 M ; c u r v e 6 , 5 × 10 2 M ; c u r v e 7, 7.5 × 10 2 M.
289
500 1000 5 l"
500
3
0
30C
t~
10C
o <[
-m
0
- 5 O0 -1000
500 E,
mv
0 (SCE)
-100
I
I
I
I
I
2
3
~-
5
6
I
f 8h
Fig. 6. The relation between the current density j (pA cm -2) and the anodic potential E (mV) (SCE) for the hardened Delta-52 steel tempered at 200 °C for various times (1 × 10 2 M NaOH; 1 x 10 -2 M NaC1; v = 2.5 mV s-l): curve 1, 0;curve 2, 1 h;curve 3, 2 h; curve 4, 4 h;curve 5, 6 h; curve 6, 8 h. Fig. 7. The relation between the pitting corrosion potential Ep (mV) (SCE) and the duration of tempering: curve 1, 1 X 10 2 M NaOH + 1 / 10 -2 MNaCl;curve 2, 1 x 10 -1 MNaOH+ 1 x 10 I MNaCI. The e f f e c t o f t e m p e r i n g the h a r d e n e d steel on the critical p i t t i n g c o r r o s i o n was tested. The curves o f Fig. 6 s h o w the a n o d i c p o l a r i z a t i o n f o r the h a r d e n e d specimens after t e m p e r i n g at 200 °C f o r various h o l d times in a s o l u t i o n c o n t a i n i n g 0.01 M N a O H and 0.01 M NaC1 at a sweep rate o f 2.5 m V s 1 b e t w e e n - - 1 2 5 0 and 750 m V ($CE). Similar e x p e r i m e n t s were p e r f o r m e d in a s o l u t i o n c o n t a i n i n g 0.1 M N a O H a n d 0.1 M NaC1. I n s p e c t i o n o f the curves o b t a i n e d reveals, t h a t t e m p e r i n g the h a r d e n e d specimens shifts the critical p i t t i n g p o t e n t i a l in the m o r e negative d i r e c t i o n , and the value o f the shift increases with increases in the t e m p e r i n g hold time. The depend e n c e o f t h e p i t t i n g p o t e n t i a l Ep on the t e m p e r i n g hold time t m is given in Fig. 7. Straight lines were o b t a i n e d w h i c h satisfy the relation Ep
= K
- - nt m
w h e r e K and n are c o n s t a n t s w h i c h a p p e a r to d e p e n d on the solution comp o s i t i o n and the t e m p e r i n g h o l d time. T h e decrease in the resistance t o w a r d s p i t t i n g a t t a c k with increases in the h o l d time c o u l d be a t t r i b u t e d t o phase t r a n s f o r m a t i o n s o c c u r r i n g in the t e m p e r i n g process. It is f o u n d t h a t air t e m p e r i n g o f the steel specimens causes the d e c o m p o s i t i o n o f the m a r t e n s i t e t o e iron carbides w h i c h act as c a t h o d i c sites w h i c h accelerate the c o r r o s i o n reaction. With f u r t h e r increases in t h e t e m p e r i n g h o l d time, s o m e finely divided c e m e n t i t e particles ( F % C ) were f o r m e d b y c o n t i n u o u s d e c o m p o s i t i o n o f the e iron carbides. The c e m e n t i t e particles increase the areas o f the c a t h o d i c sites [ 19].
290
'E 500
% <
o
5OO ~000
500 [, mV
0 (SC[)
500
Fig. 8. The relation between the current density j (pA cm 2) and the anodic potential E (mV) (SCE) (p= 2.5 mV s 1): curve 1, annealed Delta-52 steel in 1 x 10 2 M NaOH + 1 x 10-2 M NaCl; curve 2, hardened Delta-52 steel in 1 x 10 2 M NaOH + I x 10 2 M NaCl;curve 3, annealed Delta-52 steel in 1 x 10 I M NaOH + I x 10 I M N a C l ; c u r v e 4 , h a r d e n e d D e l t a - 5 2 s t e e l i n 1 × 10 1 M N a O H + 1 x 1 0 1 MNaCl.
The influence of annealing the hot-rolled steel on the critical pitting c o r r o s i o n p o t e n t i a l w a s e x a m i n e d a n d s o m e o f t h e r e s u l t s are g i v e n in Fig. 8. T h e d a t a r e v e a l t h a t a n n e a l i n g t h e s p e c i m e n s at 8 5 0 °C f o r ] h s h i f t s t h e c r i t i c a l p o t e n t i a l m a r k e d l y t o a m o r e n e g a t i v e v a l u e . S u c h an e f f e c t c o u l d be a t t r i b u t e d t o p h a s e t r a n s f o r m a t i o n s o c c u r r i n g in t h e a n n e a l i n g p r o c e s s s i n c e a t w o - p h a s e s t r u c t u r e o f f e r r i t e a n d p e a r l i t e is p r o d u c e d [ 1 9 ] .
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
E. Heyn and O. Bauer, J. Iron S t e e l Inst., L o n d o n , 79 (1909) 109. F. Tetsuo, B o s h o k u Gijutsu, 25 (7) (1976) 453. R. Stefec and F. Franz, Corros. Sci., 18 (2) (1978) 161. J. Tousek, Werkst. Korros., 28 (9) (1977) 619. H. Amon, J. Disam, J. Osterwald and H. H. Shulte, Werkst. Korros., 30 (10) (1979) 690. D. N. Roy, Indian J. Technol., 17 (8) (1979) 364. M. J. Czachor, Werkst. Korros., 31 (8) (1980) 606. R. Scheidegger and R. O. Mueller, Werkst. Korros., 31 (5) (1980) 387; Chem. A b stracts, 93 (1980), no. 83373t. S. S. Abd E1 Rehim, S. M. Abd E1 Haleem, S. M. Abd-E1 Wahaab and M. Sh. Shalaby, Surf. Technol., 19 (1983) 261. S. C. Britton and U. R. Evans, J. Chem. Soc., (1930) 1773. U. R. Evans, Metallic Corrosion, Passivity and P r o t e c t i o n , Arnold, London, 2nd edn., 1946, p. 21. B. Ershler, Discuss. Faraday Soe., 1 (1947) 269. B. Kabanov, R. Burstein and A. Frumkin, Discuss. Faraday Soc., ! (1947) 259. S. M. Abd El Haleem, S. S. Abd E1Rehim, M. Sh. Shalaby and A. M. Azzam, Werksl. Korros., 2 7 (1976) 630. S. M. Abd El Haleem, S. S. Abd El Rehim and M. Sh. Shalaby, A c t a Chim. Acad. Sci. Hung., 99 (1979) 173. D. Geanan, A. A. E1 Miligy and W. J. Lorenz, J. A p p l . E l e c t r o c h e m . , 4 (1974) 337. A. M. Shams E1 Din, F. M. Abd E1Wahab and S. M. Abd El Haleem, Werkst. Korros., 2 4 (1973) 389. S. M. Abd El Haleem, Werkst. Korros., 30 (1979) 631. M. Sh. Shalaby, Ph.D. Thesis, Ain Shams University, Cairo, 1980.
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THE I N F L U E N C E OF H E A T T R E A T M E N T ON THE PIT T IN G CORROSION OF DELTA-52 ST E E L IN NaOH SOLUTIONS s. M. ABD EL HALEEM Chemistry Department, Faculty of Science, Zagazig University, Zagazig (Egypt)
S. S. ABD EL REHIM and M. Sh. SHALABY Chemistry Department, Faculty of Science, Ain Shams University, Cairo (Egypt)
(Received November 9, 1983) Summary The influence of heat t r e a t m e n t on the pitting corrosion of Delta-52 steel, an Egyptian p r o d u c t , was investigated p o t e n t i o d y n a m i c a l l y in NaOH solutions containing NaC1 as a pitting corrodent. In chloride-free solutions, the active dissolution o f the hot-rolled steel increases with increases in b o t h the alkali c o n c e n t r a t i o n and the potential scanning rate. In the presence o f chloride ions, plots of the critical pitting potential against the chloride ion c o n c e n t r a t i o n are sigmoidal. The data reveal t ha t the hot-rolled specimens resist pitting corrosion b e t t e r than do the specimens which were hardened or annealed at 850 °C for 1 h. The susceptibility to pitting corrosion of the hardened specimens is also increased by tempering at 200 °C. The critical pitting potential Ep varies with the tempering hold time tm according to Ep = K
--
nt
m
where K and n are constants.
1. I n t r o d u c t i o n Although intensive studies have been focused on the effect of the environmental conditions on the corrosion of iron and steel, few studies have been devoted to examining the effect of heat t r e a t m e n t on their corrosion behaviour. H eyn and Bauer [1] showed t hat the corrosion rate of a carbon steel q u e n c h e d from high temperatures is relatively low in acidic media. T h e y attr ib ut e d this to the f or m at i on of a single martensite phase with carbon in the interstitial positions of a body-cent red tetragonal lattice o f iron atoms. A r a ndom distribution of the carbon atoms with the adjacent iron atoms limits their effectiveness as cathodes of local action cells. When the martensite is t e m p e r e d by heating at low temperatures and then air cooled, d eco mp o s i t i on to e iron carbide takes place. This two-phase structure sets up galvanic cells which accelerate corrosion. 0376-4583/84/$3.00
© Elsevier Sequoia/Printed in The Netherlands
284
P o t e n t i o d y n a m i c polarization t e c h n i q u e s have been used successfully to s t u d y the pitting c o r r o s i o n o f iron and steels [2 - 9]. In the presence of a pitting ion, the p o t e n t i a l does n o t reach t h a t of o x y g e n e v o l u t i o n , and the c u r r e n t rises s u d d e n l y at a certain lower p o t e n t i a l k n o w n as the critical pitting potential. This increase in c u r r e n t does n o t o c c u r i m m e d i a t e l y on a d d i t i o n o f t h e pitting ion, b u t a f t e r a certain time lapse. This time is r e f e r r e d to as the i n d u c t i o n time and is related to the p e n e t r a t i o n l l 0 , 11] or a d s o r p t i o n [12, 13] o f the pitting ion on the passive surface. In a c o n t i n u a t i o n o f o u r studies on the c o r r o s i o n b e h a v i o u r o f Delta-52 steel in a q u e o u s solutions [14, 15], it seems t o be interesting to investigate p o t e n t i o d y n a m i c a l l y the i n f l u e n c e o f heat t r e a t m e n t on the critical pitting p o t e n t i a l of this steel in NaOH solutions c o n t a i n i n g NaC1 as the pitting corrodent. 2. E x p e r i m e n t a l m e t h o d 2.1. Solutions S t o c k solutions o f NaOH or NaC1 were p r e p a r e d by dissolving the a p p r o p r i a t e weights in d o u b l y distilled water. T h e solutions were t h e n d i l u t e d as required. All reagents were analytical grade chemicals. 2.2. The electrodes T h e Delta-52 steel used in the p r e s e n t w o r k was p r o d u c e d in the electric arc f u r n a c e o f the Delta Steel Mill C o m p a n y , Cairo. Its c o m p o s i t i o n is given in Table 1. T h e ingots were h o t rolled (at 1200 - 1000 °C) and t h e n m a c h i n e d in the f o r m o f s h o r t rods ( h o t - r o l l e d electrodes}, each 27 m m in length and 10 m m in d i a m e t e r . T o investigate the i n f l u e n c e of heat t r e a t m e n t , the following p r o c e d u r e s were u n d e r t a k e n . T o p r e p a r e h a r d e n e d electrodes, h o t - r o l l e d specimens were r e h e a t e d gradually in an a u t o m a t i c a l l y c o n t r o l l e d m u f f l e f u r n a c e (15 °C min ~) f r o m r o o m t e m p e r a t u r e up t o 850 °C f o r 1 h and were t h e n q u e n c h e d in water at 20 °C. S o m e o f the h a r d e n e d e l e c t r o d e s were t e m p e r e d at 200 °C for various h o l d times and were t h e n c o o l e d in air. T o p r e p a r e annealed e l e c t r o d e s the h o t - r o l l e d e l e c t r o d e s were r e h e a t e d gradually in the a u t o m a t i c a l l y c o n t r o l l e d m u f f l e f u r n a c e (15 °C min 2) f r o m r o o m t e m p e r a t u r e up t o 850 °C f o r 1 h and were t h e n c o o l e d gradually in t h e same furnace. Each e l e c t r o d e was m o u n t e d in glass t u b i n g with Araldite in such a m a n n e r t h a t o n l y its cross-sectional area was e x p o s e d t o the e l e c t r o l y t e . TABLE 1 C o m p o s i t i o n o f Delta-52 steel Element A m o u n t (%)
C 0.28
Mn 0.68
Si 0.27
S 0.045
P 0.035
285
2.3. The electrolytic cell T h e cell used was m a d e o f d o u b l e - w a l l e d P y r e x glass o f a b o u t 120 ml c a p a c i t y . T h e cell was k e p t at 25 + 1 °C b y circulating w a t e r within the d o u b l e wall. It was p r o v i d e d with an inlet a n d an o u t l e t to p e r m i t d e a e r a t i o n b y passing n i t r o g e n gas t h r o u g h t h e test solution f o r 1 h b e f o r e each run. T h e lid was f i t t e d to t h e cell b y a g r o u n d glass flange which h a d t h r e e sealable s o c k e t s i n t o w h i c h the e l e c t r o d e s a n d t h e salt bridge were inserted. 2.4. Measurements A p l a t i n u m c o u n t e r e l e c t r o d e was used, a n d all p o t e n t i a l s w e r e m e a sured w i t h r e f e r e n c e to a s a t u r a t e d c a l o m e l e l e c t r o d e (SCE). P o t e n t i o d y n a m i c p o l a r i z a t i o n m e a s u r e m e n t s were m a d e w i t h a W e n k i n g p o t e n t i o s c a n t y p e POS 73. T h e p o t e n t i a l versus c u r r e n t d e n s i t y curves w e r e r e c o r d e d on an A d v a n c e x - y r e c o r d e r o f t y p e A R 2 0 0 0 . T o s t a r t the p o l a r i z a t i o n , the p o t e n t i a l o f the w o r k i n g e l e c t r o d e was set to a m o r e p o s i t i v e value at the c h o s e n s w e e p rate.
3. Results a n d discussion
3.1. Influence o f the N aO H concentration T h e curves o f Fig. 1 r e p r e s e n t the p o t e n t i o d y n a m i c p o l a r i z a t i o n o f the h o t - r o l l e d e l e c t r o d e in N a O H s o l u t i o n s o f various c o n c e n t r a t i o n s ranging f r o m 0.01 to 2 M. T h e curves were s w e p t b e t w e e n - - 1 2 5 0 and 750 m V (SCE) at a scan r a t e o f 2.5 m V s -1. T h e a n o d i c p o r t i o n s o f the curves are c h a r a c t e r i z e d b y an arrest b e f o r e a d i s s o l u t i o n c u r r e n t p e a k is r e a c h e d . T h e 100~
60( E
a,
2oc
-20C
-600 -I000
-500 E , mV
0
500
(SCE)
Fig. 1. T h e r e l a t i o n b e t w e e n t h e c u r r e n t d e n s i t y j ( p A c m - 2 ) a n d t h e a n o d i c p o t e n t i a l E ( m V ) ( S C E ) f o r t h e h o t - r o l l e d D e l t a - 5 2 s t e e l f o r v a r i o u s c o n c e n t r a t i o n s o f N a O H (p = 2.5 m V s - l ) : c u r v e 1, C = 1 x 1 0 - z M ; c u r v e 2, C = 1 x 10 - 1 M; c u r v e 3, C = 5 x 1 0 1 M; c u r v e 4, C = t M; c u r v e 5, C = 2 M.
286
presence o f this arrest may be due to the oxidation of hydrogen which is adsorbed on the surface and/ or absorbed in the metal lattice. The dissolution current peak is certainly due t o iron electrode processes. When the thermodynamic stability range is taken into account, the reaction taking place in the potential range may be [ 16 ] Fe + 2OH
.
' Fe(OH)2(ads) + 2e
B e y o n d the dissolution peak the electrode becomes passive and the current density decreases. The cur r ent flowing in the passive region is governed by the rate of chemical attack of the passive film by the electrolyte [14, 17]. Increases in the potential merely increase the thickness of the passive film, maintaining the field strength required to continue the necessary cation transport, by high field conduct ance, for the film growth and dissolution. When the potential imposed on the electrode becomes more positive to make the reaction 4OH- ~
02 + 2H20 + 4e
t h e r m o d y n a m i c a l l y (or kinetically) easier than the earlier high field ionic conduction, the current density increases suddenly and oxygen evolution becomes the p r e d o m i n a n t reaction [17]. Inspection o f the curves of Fig. 1 reveals t hat the alkali concent rat i on has a marked influence on the anodic behaviour of the electrode. With increasing alkali c o n c e n t r a t i o n , the height of the current peak increases and at the same time both the zero c ur r e nt potential and the potential corresponding to the cur r ent peak shift in the more negative direction. I000
6 O0
E "~ 7 O0
- 200
-600 -1000
500 E , mY
0
500
(SCE)
Fig. 2. T h e r e l a t i o n b e t w e e n t h e c u r r e n t d e n s i t y j ( p A c m --2) a n d t h e a n o d i c p o t e n t i a l E ( m V ) ( S C E ) f o r t h e h o t - r o l l e d D e l t a - 5 2 s t e e l in 1 × 10 2 M N a O H at v a r i o u s s w e e p r a t e s : curve 1, p = 1.0 m V s 1; c u r v e 2, v = 2.5 m V s - 1 ; c u r v e 3, p = 5.0 m V s ] ; c u r v e 4, v = 15.0 m V s 1 ; c u r v e 5, v = 4"0.0 m V s 1
287
3.2. Influence o f the sweep rate T h e curves o f Fig. 2 illustrate the i n f l u e n c e o f the s w e e p rate p ( m V s -1) b e t w e e n - - 1 2 5 0 and 7 5 0 m V (SCE) on the p o t e n t i o d y n a m i c p o l a r i z a t i o n o f the h o t - r o l l e d steel in 0,01 M N a O H . It is clear that t h e current densities c o r r e s p o n d i n g to b o t h the active and the passive regions increase with increasing s w e e p rate. H o w e v e r , an increase in the s w e e p rate has n o significant e f f e c t o n either the z e r o current or the p e a k p o t e n t i a l . 3.3. Susceptibility o f the Delta-52 steel to pitting corrosion in alkali solution Figure 3 s h o w s the a n o d i c p o l a r i z a t i o n o f the h o t - r o l l e d D e l t a - 5 2 steel in 0.01 M N a O H s o l u t i o n with t h e a d d i t i o n o f various c o n c e n t r a t i o n s o f NaC1 as a pitting c o r r o d e n t . Similar e x p e r i m e n t s w e r e carried o u t in 0.1 M
1000
t
600
E <[
^ 2O0
-20(
-60C
/[!
I
i
-1000
I
-500 0 E, my (5CE]
I 500
I
Fig. 3. T h e relation b e t w e e n the current d e n s i t y j (AtA c m 2) and the a n o d i c p o t e n t i a l E ( m V ) ( S C E ) for the hot-rolled D e l t a - 5 2 steel i m m e r s e d in 1 × 1 0 - 2 M N a O H s o l u t i o n c o n t a i n i n g various c o n c e n t r a t i o n s o f NaC1 (p = 2 . 5 m V s - l ) : c u r v e 1, 0; c u r v e 2, 1 × 10 - 3 M; c u r v e 3, 2 × 10 - 3 M ; c u r v e 4, 5 × 10 - 3 M; c u r v e 5, 8 × 10 - 3 M ; c u r v e 6, 1 × 10 - 2 M; c u r v e 7, 5 × 10 2 M ; c u r v e 8, 6 × 1 0 - 2 M.
5OO
~
~.00
- 400 ,
-38o
p
-26o
i
1
-,oo
o
LOg CCIFig. 4. The relation between the pitting corrosion potential Ep ( m V ) ( SCE) and the logarithm of the chloride ion concentration (p = 2.5 m V s - 1 ) : cur ve 1, hot-rolled steel in I x 10 -1 M N a O H ; c u r v e 2, hot-rolled steel in 1 × 10 - 2 M N a O H ; cur ve 3, hardened steel in 1 × 10 - 2 M N a O H .
288
NaOH. The addition of increasing amounts of chloride ion up to a certain value, which depends on the alkali c onc e nt r at i on, has no significant effec't on the polarization curves recorded in the chloride-free solution. It appears t h a t these lower concentrations of chloride ion cannot affect the dissolution kinetics of the passive film. With increasing chloride ion concentration, the current density in the passive region remains more or less constant over a certain potential range before it rises suddenly at a certain definite potential t e r m e d the critical pitting potential Ep, indicating the initiation of pitting attack. The relation between Ep and log Ccl- is given in Fig. 4. Plots of Ep against log Cc~- are sigmoidal. Similar results have been report ed previously [9, 181. The data reveal t ha t the critical pitting potential shifts in the negative direction with increasing chloride ion c oncent rat i on and in the more positive direction with increasing alkali concentration.
3.4. Influence o f heat treatment The influence of a hardening process on the pitting corrosion of the hot-rolled steel was examined. Figure 5 represents the anodic polarization of specimens hardened at 850 °C for 1 h and then quenched in water at 20 °C. The experiments were p e r f o r m e d between - - 1 2 5 0 and 750 mV (SCE) in 0.01 M NaOH and in the presence of various concentrations of chloride ion using a sweep rate of 2.5 mV s 1, The relation between E , and log Cci is shown in Fig. 4, curve 3. Under the prevailing experimental conditions, it is clear that the hardening process shifts the critical pitting potential to more negative values, denoting a decreased resistance for pitting attack. This could be explained as a result of sequential structure changes of hot-rolled steel under the effect of the hardening process. It is known t hat the main structure of h y p o e u t e c t o i d steel (Delta-52) after hardening is transformed into martensite and retained austenite [19].
500
jl 6 5/ /" E <~ 0
/
o
;
o
-50C
~'t -I000
500
I
0 E j mY (SCE)
I
]
500
I
Fig. 5. T h e r e l a t i o n b e t w e e n t h e c u r r e n t d e n s i t y j (/2A c m : ) a n d t h e a n o d i c p o t e n t i a l E ( m V ) ( S C E ) f o r t h e h a r d e n e d D e l t a - 5 2 s t e e l i m m e r s e d in I X 10 2 M N a O H s o l u t i o n c o n t a i n i n g v a r i o u s c o n c e n t r a t i o n s o f c h l o r i d e i o n (p = 2 . 5 m V s 1): c u r v e 1, 0; c u r v e 2, 2.5 x 10 -3 M; c u r v e 3, 5 x 10 3 M ; c u r v e 4, 7 . 5 × 10 3 M ; c u r v e 5, 1 × 10 2 M ; c u r v e 6 , 5 × 10 2 M ; c u r v e 7, 7.5 × 10 2 M.
289
500 1000 5 l"
500
3
0
30C
t~
10C
o <[
-m
0
- 5 O0 -1000
500 E,
mv
0 (SCE)
-100
I
I
I
I
I
2
3
~-
5
6
I
f 8h
Fig. 6. The relation between the current density j (pA cm -2) and the anodic potential E (mV) (SCE) for the hardened Delta-52 steel tempered at 200 °C for various times (1 × 10 2 M NaOH; 1 x 10 -2 M NaC1; v = 2.5 mV s-l): curve 1, 0;curve 2, 1 h;curve 3, 2 h; curve 4, 4 h;curve 5, 6 h; curve 6, 8 h. Fig. 7. The relation between the pitting corrosion potential Ep (mV) (SCE) and the duration of tempering: curve 1, 1 X 10 2 M NaOH + 1 / 10 -2 MNaCl;curve 2, 1 x 10 -1 MNaOH+ 1 x 10 I MNaCI. The e f f e c t o f t e m p e r i n g the h a r d e n e d steel on the critical p i t t i n g c o r r o s i o n was tested. The curves o f Fig. 6 s h o w the a n o d i c p o l a r i z a t i o n f o r the h a r d e n e d specimens after t e m p e r i n g at 200 °C f o r various h o l d times in a s o l u t i o n c o n t a i n i n g 0.01 M N a O H and 0.01 M NaC1 at a sweep rate o f 2.5 m V s 1 b e t w e e n - - 1 2 5 0 and 750 m V ($CE). Similar e x p e r i m e n t s were p e r f o r m e d in a s o l u t i o n c o n t a i n i n g 0.1 M N a O H a n d 0.1 M NaC1. I n s p e c t i o n o f the curves o b t a i n e d reveals, t h a t t e m p e r i n g the h a r d e n e d specimens shifts the critical p i t t i n g p o t e n t i a l in the m o r e negative d i r e c t i o n , and the value o f the shift increases with increases in the t e m p e r i n g hold time. The depend e n c e o f t h e p i t t i n g p o t e n t i a l Ep on the t e m p e r i n g hold time t m is given in Fig. 7. Straight lines were o b t a i n e d w h i c h satisfy the relation Ep
= K
- - nt m
w h e r e K and n are c o n s t a n t s w h i c h a p p e a r to d e p e n d on the solution comp o s i t i o n and the t e m p e r i n g h o l d time. T h e decrease in the resistance t o w a r d s p i t t i n g a t t a c k with increases in the h o l d time c o u l d be a t t r i b u t e d t o phase t r a n s f o r m a t i o n s o c c u r r i n g in the t e m p e r i n g process. It is f o u n d t h a t air t e m p e r i n g o f the steel specimens causes the d e c o m p o s i t i o n o f the m a r t e n s i t e t o e iron carbides w h i c h act as c a t h o d i c sites w h i c h accelerate the c o r r o s i o n reaction. With f u r t h e r increases in t h e t e m p e r i n g h o l d time, s o m e finely divided c e m e n t i t e particles ( F % C ) were f o r m e d b y c o n t i n u o u s d e c o m p o s i t i o n o f the e iron carbides. The c e m e n t i t e particles increase the areas o f the c a t h o d i c sites [ 19].
290
'E 500
% <
o
5OO ~000
500 [, mV
0 (SC[)
500
Fig. 8. The relation between the current density j (pA cm 2) and the anodic potential E (mV) (SCE) (p= 2.5 mV s 1): curve 1, annealed Delta-52 steel in 1 x 10 2 M NaOH + 1 x 10-2 M NaCl; curve 2, hardened Delta-52 steel in 1 x 10 2 M NaOH + I x 10 2 M NaCl;curve 3, annealed Delta-52 steel in 1 x 10 I M NaOH + I x 10 I M N a C l ; c u r v e 4 , h a r d e n e d D e l t a - 5 2 s t e e l i n 1 × 10 1 M N a O H + 1 x 1 0 1 MNaCl.
The influence of annealing the hot-rolled steel on the critical pitting c o r r o s i o n p o t e n t i a l w a s e x a m i n e d a n d s o m e o f t h e r e s u l t s are g i v e n in Fig. 8. T h e d a t a r e v e a l t h a t a n n e a l i n g t h e s p e c i m e n s at 8 5 0 °C f o r ] h s h i f t s t h e c r i t i c a l p o t e n t i a l m a r k e d l y t o a m o r e n e g a t i v e v a l u e . S u c h an e f f e c t c o u l d be a t t r i b u t e d t o p h a s e t r a n s f o r m a t i o n s o c c u r r i n g in t h e a n n e a l i n g p r o c e s s s i n c e a t w o - p h a s e s t r u c t u r e o f f e r r i t e a n d p e a r l i t e is p r o d u c e d [ 1 9 ] .
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
E. Heyn and O. Bauer, J. Iron S t e e l Inst., L o n d o n , 79 (1909) 109. F. Tetsuo, B o s h o k u Gijutsu, 25 (7) (1976) 453. R. Stefec and F. Franz, Corros. Sci., 18 (2) (1978) 161. J. Tousek, Werkst. Korros., 28 (9) (1977) 619. H. Amon, J. Disam, J. Osterwald and H. H. Shulte, Werkst. Korros., 30 (10) (1979) 690. D. N. Roy, Indian J. Technol., 17 (8) (1979) 364. M. J. Czachor, Werkst. Korros., 31 (8) (1980) 606. R. Scheidegger and R. O. Mueller, Werkst. Korros., 31 (5) (1980) 387; Chem. A b stracts, 93 (1980), no. 83373t. S. S. Abd E1 Rehim, S. M. Abd E1 Haleem, S. M. Abd-E1 Wahaab and M. Sh. Shalaby, Surf. Technol., 19 (1983) 261. S. C. Britton and U. R. Evans, J. Chem. Soc., (1930) 1773. U. R. Evans, Metallic Corrosion, Passivity and P r o t e c t i o n , Arnold, London, 2nd edn., 1946, p. 21. B. Ershler, Discuss. Faraday Soe., 1 (1947) 269. B. Kabanov, R. Burstein and A. Frumkin, Discuss. Faraday Soc., ! (1947) 259. S. M. Abd El Haleem, S. S. Abd E1Rehim, M. Sh. Shalaby and A. M. Azzam, Werksl. Korros., 2 7 (1976) 630. S. M. Abd El Haleem, S. S. Abd El Rehim and M. Sh. Shalaby, A c t a Chim. Acad. Sci. Hung., 99 (1979) 173. D. Geanan, A. A. E1 Miligy and W. J. Lorenz, J. A p p l . E l e c t r o c h e m . , 4 (1974) 337. A. M. Shams E1 Din, F. M. Abd E1Wahab and S. M. Abd El Haleem, Werkst. Korros., 2 4 (1973) 389. S. M. Abd El Haleem, Werkst. Korros., 30 (1979) 631. M. Sh. Shalaby, Ph.D. Thesis, Ain Shams University, Cairo, 1980.