The improved passivation of iron induced by additions of tungsten

Corrosion Science, Vol. 3(/. No. 1, pp. 53-58. 199(I Printed in Great Britain

0010-938X/90 $3.00 + 0.00 © 1989 Pergamon Press plc

THE IMPROVED PASSIVATION OF IRON INDUCED ADDITIONS OF TUNGSTEN

BY

JIA C H E N a n d JIANN K U O W u D e p a r t m e n t of Materials Engineering, Tatung Institute of Technology, Taipei, Taiwan 10451, R . O . C .

Abstract--Electrochemical and conventional immersion tests on pure iron and various low tungsten steels in 1 N H 2 S Q under static and atmospheric conditions were performed. Similar experiments were also performed on pure iron passivated in 1 N H2SO ~ containing various concentrations of WO~ . A steel alloyed with 3.02 wt% tungsten in 1 N H2SO 4 containing various concentrations of C1- was also studied. The results showed that alloyed tungsten increases the passive region. The results of pure iron in 1 N H2SO 4 containing various concentrations of tungstate ion showed that the passivation current density decreases as tungstate ion increases; but it is limited at a critical concentration. The passive films of low tungsten steels do not offer a good resistance to CI ions. The corrosion products of low tungsten steels were examined by X-ray diffraction. W O 3 and W O 3 . 2 H , O were identified for main corrosion products. These results indicated that oxidized tungsten ion remained on the metal surface, by interaction with water and insoluble oxide WO3 formed, which can enhance the stability of the oxide layer and improve passivation. INTRODUCTION

THE PASSIVATION behavior of austenitic stainless steels can be improved in acid

solutions by alloying with tungsten, 1'2 especially in acid or chloride-containing solution. Tungsten, like molybdenum, 3 can also improve pitting resistance. The effects of alloy composition on the passivity of stainless steels has been extensively studied, l'e but the passivation behavior of tungsten in b,c.c. Fe, especially in low tungsten steels, has not been investigated. The purpose of this study is to investigate the effect of tungsten additions on the passive film of Fe in various aqueous solutions. The passivating inhibitor efficiency of a tungstate ion (WO]-) solution on pure iron was also evaluated. Two tests were utilized in this study. The first technique is the potentiostatic scan. The second method involves the measurement of weight loss and its conversion to a corrosion current by using Faraday's law. 4

EXPERIMENTAL METHOD Preparation of specimen and test solution The chemical compositions of pure iron and four low tungsten steels are listed in Table I. All specimens were cut from a round bar and machined to a diameter of 15 m m and 1.0 m m thickness, and they were heated at 950°C and furnace cooled in a v a c u u m furnace. The specimens then were polished with e m e r y paper with water up to 1000 grit, then washed with deionized water, and cleaned with acetone, quick dried by warm air blowing. A 1 N H2SO 4 solution was prepared for testing a series of pure iron and low tungsten steels. 1 N H2SO 4 containing 0.001, 0.01 and 0.1 M WO42- were used for pure iron test only. Alloy 4 (3.02 wt%) was also chosen to test 1 N H2SO 4 containing 0.1, 1 and 10 g 1- ~NaCI solutions.

Manuscript received 18 October 1988. 53

54

J. CHENand J. K. Wu TABLE 1.

CHEMICAL ANALYSIS OF PURE IRON AND LOW TUNGSTEN STEELS, WEIGHT PERCENT

Grade

C

Mn

P

S

Si

Mo

W

Alloy 1 Alloy 2 Alloy3 Alloy 4

0.049 0.036 0.030 0.024

0.11 0.09 0.07 0.09

0.012 0.015 0.017 0.020

0.027 0.025 0.027 0.024

0.07 0.06 0.05 0.05

0.028 0.033 0.027 0.036

0.52 1.28 2.06 3.02

* Analyses and material supplied by Bureau of Analysed Samples Ltd., Middlesbrough, England. Pure iron: 8 ppm C, 40 ppm S, 50 ppm Si, 30 ppm Cu, 5 ppm Ni, 30 ppm Cr, 60 ppin A1.

Electrochemicalpotentiostatic measurementand immersion test The experimental arrangement used a three electrode system, a Nichia model G1001E potentiostat and a potential scanner. Scans were initiated by lowering the corrosion potential of the specimen to a preset value -0.8 V(SHE) and scanned to E = +1.8 V(SHE), at a rate of 0.1 mV s -1. All experiments were conducted at 25 + I°C under static and atmospheric conditions. The specimens were immersed in the same test solutions for 48 h, specimens were weighed and recorded after removing from test solutions. The solid corrosion products were also collected, dried and analysed by X-ray powder diffraction. E X P E R I M E N T A L RESULTS A N D D I S C U S S I O N Potentiostatic behavior and immersion test o f pure iron and low tungsten steels E v a n s diagrams for pure iron and four different low tungsten steels in 1 N H2SO 4 at 25°C are shown in Fig. 1. It can be seen that as the tungsten c o n t e n t of steels increases, the passivation region is extended, but the passivation current density and transpassive potential c h a n g e very slightly. T h e increase in tungsten content also results in a slight increase in corrosion potential. T h e weight loss of pure iron and four different low tungsten steels in 1 N H2SO 4 for 48 h is shown in Fig. 2. It can be seen that an increase of tungsten c o n t e n t in steel can i m p r o v e corrosion resistance. F r o m the polarization curves and immersion test u n d e r static conditions, it can be seen that alloyed tungsten in steels has a beneficial effect in improving the f o r m a t i o n of passive film in 1 N H2SO 4. T h e active-passive curves are very similar to the limiting current f o r m before passivation r e p o r t e d by V e r m a . 5 T h e alloyed W can e n h a n c e the adsorption of metal ions on the metal surface and f o r m a m o r e stable passive film in acid solutions u n d e r atmospheric conditions. X - r a y p o w d e r diffraction analysis for the collected corrosion p r o d u c t s of alloy 4 in 1 N H2SO 4 is shown in Fig. 3. It was f o u n d that the corrosion p r o d u c t s are mainly c o m p o s e d b o t h of WO3 and W O 3 . 2 H 2 0 , which is consistent with the possible p r o d u c t s of c o r r o d e d low tungsten steels f r o m Pourbaix's diagram. 1

Inhibitive properties o f tungstate ions In o r d e r to evaluate the inhibitive effect of WO4z- in solution, polarization studies were carried out with pure iron in 1 N H2SO4 containing with various c o n c e n t r a t i o n of N a 2 W O 4 . Figure 4 shows the polarization curves o b t a i n e d in 1 N HzSO 4 containing 0, 0.001, 0.01 and 0.1 M W O 2- at 25°C. T h e corrosion potential, as shown in Fig. 4, increases with addition of tungstate ion in solution, and there is a slight decrease in passive current density. T h e passivation region and transpassive potential is not c h a n g e d significantly.

Improved passivation of iron

55

~~

1.8 1.6

1.4 1.2

~.~

Pure~ron

~, I 0.8

w

All°Y 1

~io-~\ o

0.6

~. . . . .

Alloy 2 _ -

I

0.4

|

Alloy4

Alloy3

0,2

~,) ]

i 4 /

;,,~/ i~

--0.4

- . o~-o. . . .

1 J ItlIHI

i J tlJHll 10 2

10 3

i

J i ~,ltnl ~ '+~ ~,~,HIL t , *I~H 10 4 10 5

/b A / ' 2 rt) 2 )

FIG. 1. Potentiostatic polarization for pure iron and four low tungsten steels in 1 N H2SO4 solution. F o r c o m p a r i s o n , the results of i m m e r s i o n tests for p u r e F e in 1 N H2SO 4 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 t u n g s t a t e (WO42 ) at 25°C are shown in Fig. 5. T h e b e s t i n h i b i t o r efficiency was f o u n d at a c o n c e n t r a t i o n of 0.001 M W O e - for p u r e iron in 1 N H2SO4 solution. H i g h e r c o n c e n t r a t i o n s of W O ~ - d o not show b e t t e r inhibitive effect. A c c o r d i n g to R o s e n f e l d 6 a n d B a h a d u r , 7 M e O 4 - t y p e i n h i b i t o r s are

~5o

/

E

a-

/ /

ub

u9 Cb 20

o

•

~. - / 1 3

IC o o

6

1'2

1't

2'4

I~a-'S

T/~Ech~)

Fro. 2. Weight loss of pure Fe and four low tungsten steels immersed in 1 N H2SO~. [A] Pure iron. IV] Alloy 1. [IS]]Alloy 2. [O] Alloy 3. [+] Alloy 4.

56

J. CHEN a n d J. K. W u 0.3

K :

WOs

:

H2WO,'H20

0.15 K

Y

10.00

26.00

42.00

58.00

74-00

90. O0

29 FIG. 3.

X - r a y diffraction p a t t e r n of c o r r o s i o n p r o d u c t s .

1.8 1.6 1.4

0.1M 1.2 ~__

1.0

. . . . . . . 0.01 M

0.8

,_ _ _ _

|

0.001 M

to

0.6 0.4 0.2 0.0

o o

o/ -0.2

o

o°/ ~

-0.4 -0.6 lO 2

lO 3 i (jJA/cm

FIG. 4.

lO 4

10 5

2 )

P o t e n t i o s t a t i c p o l a r i z a t i o n of p u r e Fe in 1 N H 2 S O 4 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 tions of WO42-.

Improved passivation of iron

57

40

j-

_i

so

/ E ~ 2O co o

1

A /

/

. . . . .

18

12

~

t~-

d

i

6

J J

2T4

-s ~418

TIME ( h r )

FIG. 5.

Weight loss of pure Fe immersed in 1 N H2SO 4 containing 0 to 0.1 M WO4 . [ i ] 0.0 M. [V] 0.001 M. [D] 0.01M. [0] 0.1M.

1.8

/~o

-

~,,lNocl

~

ooK

o "1 ~,i )q \

~

fr-I

o~F

: o.o_

j .

-02" 0 4 - o- c -

0.6

_

~

:

~

-

i

~

~

~ \ o .

~

~

10 2

10 3 i

FIG. 6.

J

%

(/J,A,/CN?2

10 4

10 5

)

Potentiostatic polarization of alloy 4 in 1 N HzSO4.

58

J. CnEN and J. K. Wu

e,i E

30

E

~ 20 o

..o

_A

~o 0 O

,

TIME(hr

FIG. 7. Weight loss of alloy 4 immersed in I N H2SO 4 containing 0 to 10 g/l NaCl. [U]]0.0 g/l. [O] 0.1 g/1. [&] 1.0 g/l. [~] 10.0 g/1. not involved in the formation of passive film, but only shift the electrochemical potential to more noble (self-passivator). The W O 2- will not be precipitated in the passivation process, although some residues of W O 2- can be found on the surface, s

The breakdown of passive films on low tungsten steels For investigating the b r e a k d o w n of passive film on alloyed tungsten steels, alloy 4 (3.02 w t % ) was selected for examination in 1 N H z S O 4 with various containing NaC1 at 25°C. Polarization curves are shown in Fig. 6. It can be seen that an increase of concentration of C1- produces a large current density increase. The transpassive potential also decreases with containing C1- ions. For comparison, the results of immersion tests for alloy 4 in 1 N H z S O 4 with solutions of H2SO4 containing varying amounts of NaC1 at 25°C are shown in Fig. 7. The passive films of low tungsten steel do not offer a good resistance to the C1- ion. It can be seen that an increase of concentration of CI- causes a higher corrosion rate. CONCLUSIONS (1) The passivation behavior of iron can be improved by alloying of W in iron or addition of W O 2- in solution, but if concentration of W O 2- is too high, the inhibitor efficiency will be lower. (2) X-ray powder diffraction study of corrosion products indicates that WO3 and WO3" 2 H 2 0 are the main composites. (3) The passive films of low tungsten do not offer a good resistance to Cl- ion.

1. 2. 3. 4. 5. 6. 7. 8.

REFERENCES N. Bu1, A. IRHZOand Y. LIMOUZ1N-MAIRE,Corrosion 39, 49l (1983). A. J. SEDRIKS, Corrosion 42, 376 (1986). A. GUENaOUR, J. FAUCHEUand A. BEN BACHIR, Corrosion 44,214 (1988). M. STERN and A. L. GEARY,J. electrochem. Soc. 104, 56 (1957). K. V E R M A , Metal Finishing 86, 85 (1988). I. L. ROSENFELD, Corrosion37, 371 (1981). A. BAHADUR, Corrosion Reviews 7, 96 (1987). V. S. SASTRIand R. H. PACKWOOD, Werkstoffe und Korrosion 38, 77 (1987).