CORROSION IN S O M E
RESISTANCE
AGGRESSIVE
OF
POROUS
TITANIUM
MEDIA
D. S. A r e n s b u r g e r , V. a n d I . M. F e d o r c h e n k o
S. P u g i n ,
Titanium exhibits low c o r r o s i o n r e s i s t a n c e in h y d r o c h l o r i c and sulfuric acid solutions. In numerous investigations, it has been d e m o n s t r a t e d that titanium r e s i s t s attack in these acids at acid concentrations of not m o r e than 5% at r o o m t e m p e r a t u r e . In m o r e concentrated acids, titanium rapidly dissolves. The m a j o r i t y of data on the c o r r o s i o n r e s i s t a n c e of titanium in these media, however, apply to nonporous s p e c i m e n s . Mauer and c o w o r k e r s [1] have r e p o r t e d the r e s u l t s of an investigation into the c o r r o s i o n behavior of porous titanium specimens in h y d r o c h l o r i c and nitric acids. As was to be expected, the c o r r o sion r a t e of porous specimens was found to be higher than that of nonporous specimens subjected to parallel t e s t s . However, allowing f o r the l a r g e specific s u r f a c e a r e a of the porous s p e c i m e n s , equal to 0.01 m2/g, the authors conclude that their c o r r o s i o n r e s i s t a n c e was higher than that of the corresponding nonporous s p e c i m e n s ; they attribute this finding to hindered t r a n s p o r t of corroding ions to the internal surface of a porous s p e c i m e n and, equally, to ineffective r e m o v a l of c o r r o s i o n products. In c o r r o s i o n tests on h i g h - p o r o s i t y a r t i c l e s , difficulties usually a r i s e in the choice of an appropriate c o r r o s i o n c r i t e r i o n . The method employed f o r nonporous articles, which consists in m e a s u r i n g the s p e c i m e n - w e i g h t loss due to c o r r o s i o n , is highly inaccurate in the case of porous articles owing to the difficulty of r e m o v i n g the c o r r o s i o n products f r o m the specimen. In the p r e s e n t - d a y p r a c t i c e of c o r r o s i o n tests on h i g h - p o r o s i t y s p e c i m e n s , the techniques of m e a s u r i n g the e l e c t r i c a l r e s i s t i v i t y or mechanical strength of specimens a r e commonly employed. Both these techniques a r e based on the fact that the state of the contacts between the powder p a r t i c l e s of which a porous m a t e r i a l is composed exerts a decisive influence on the p r o p e r t i e s of porous a r t i c l e s . Knowing the e l e c t r i c a l r e s i s t i v i t y of a porous specimen, it is possible to calculate the relative e l e c t r i c a l conductivity of the porous body in relation to that of the nonporous m a t e r i a l . The c h a r a c t e r of v a r i ation of the relative e l e c t r i c a l conductivity in the c o u r s e of c o r r o s i o n can then be utilized f o r computing the approximate operating life of the porous body in a c o r r o s i o n medium. To calculate the relative e l e c t r i cal conductivity of a porous body, it is n e c e s s a r y to know the e l e c t r i c a l r e s i s t i v i t y , in the nonporous state, of the m a t e r i a l constituting the porous specimen, and difficulties m a y often be encountered in this d e t e r mination. N e v e r t h e l e s s , c o m p a r e d with the weight-loss technique, as used in p a r t i c u l a r in [1], this p r o c e dure m u s t be r e g a r d e d as m o r e reliable in c o r r o s i o n tests on porous a r t i c l e s . Bearing in mind the limited amount of information on the c o r r o s i o n r e s i s t a n c e of porous titanium in h y d r o c h l o r i c acid and the complete lack of such information with r e s p e c t to sulfuric acid, as well as the foregoing discussion of the choice of a c o r r o s i o n - r e s i s t a n c e c r i t e r i o n for porous a r t i c l e s , the authors u n d e r took an investigation of the c o r r o s i o n r e s i s t a n c e of porous titanium specimens in h y d r o c h l o r i c and sulfuric acids and in caustic soda. In addition, tests were conducted on porous specimens subjected to nitriding and on specimens p r e p a r e d f r o m a titanium powder alloyed with palladium. EXPERIMENTAL
PROCEDURE
AND SCOPE
OF INVESTIGATION
Specimens 4 • 5 • 40 m m in size were p r e p a r e d for testing. The starting m a t e r i a l was a TIER-155 r e fined e l e c t r o l y t i c powder, with a - 1 . 0 + 0.25 m m p a r t i c l e size, produced by the Dneprovsk T i t a n i u m - M a g nesium Plant. Test specimens were sintered in thoroughly-dried argon at a t e m p e r a t u r e of ll00~ with a holding period of 2 h. The p o r o s i t y of the s i n t e r e d specimens was 30~ 1.5%. Some of the specimens p r e p a r e d in this m a n n e r were subjected to nitriding. This operation was p e r f o r m e d at a t e m p e r a t u r e of 850~ in a s t r e a m of nitrogen p r e v i o u s l y p a s s e d through a tube filled with copper shavings and preheated to a Institute of Materials Science, A c a d e m y of Sciences of the Ukrainian SSR. Translated f r o m P o r o s h kovaya Metallurgiya, No. 12 (72), pp. 59-64, December, 1968. Original article submitted October 3, 1967.
977
t e m p e r a t u r e of 800~ and two tubes filled with f u s e d - p o t a s h pellets to r i d it of oxygen and w a t e r - v a p o r contamination. Two nitriding durations - 30 rain and 8 h - w e r e employed. The 3 0 - m i n duration was chosen by analogy with the work r e p o r t e d by Grdina and c o w o r k e r s [2], Who c l a i m to have obtained a thin, h a r d e n e d s u r f a c e l a y e r in as little as 30 min by nitriding titanium using h i g h - f r e quency c u r r e n t heating in a nitrogen a t m o s p h e r e . The usual, r e c o m m e n d e d p a r a m e t e r s for the nitriding of titanium in a nitrogen a t m o s p h e r e , however, a r e a t e m p e r a t u r e of 800-850~ and a holding p e r i o d of 15-20 h [3]. P h o t o m i c r o g r a p h s of s i n t e r e d and n i trided s p e c i m e n s a r e i l l u s t r a t e d in F i g s . 1 and 2.
Fig. 1. M i c r o s t r u c t u r e of porous titanium, x 70. E l e c t r o l y t i c powder o f - 1 . 0 + 0.25 m m fraction; s i n t e r ing t e m p e r a t u r e 1100~ duration 2h.
F i g u r e 2 c l e a r l y shows the nitrided l a y e r . In view of its thickness and m i c r o h a r d n e s s , this l a y e r cannot be r e g a r d e d as being c o m p o s e d of titanium nitride. Instead, the m a t e r i a l of this l a y e r is m o r e likely to be a r i c h solid solution of nitrogen or t i tanium nitride in (~ titanium. In view of r e p o r t s of i n c r e a s e d c o r r o s i o n r e s i s t a n c e of t i tanium r e s u l t i n g f r o m the addition of s m a l l amounts of platinum and palladium [4], s p e c i m e n s containing 2, 0.2, 0.1, and 0.05 wt.% Pd w e r e a l s o p r e p a r e d . The s a l t PdC12, introduced into the powd e r in the f o r m of an aqueous solution, was employed for s p e c i m e n p r e p a r a t i o n ; the powder was then d r i e d on a w a t e r b a t h and heated in an a r g o n a t m o s p h e r e at 510~ to d e c o m p o s e the p a l l a dium chloride. The p r o c e s s was conducted at low a r g o n p r e s s u r e (0.5 m m Hg) for i h. The r e s u l t i n g powder was u s e d to p r e p a r e s p e c i m e n s by the a b o v e - d e s c r i b e d t e c h n i q u e . The c o r r o s i o n m e d i a investigated w e r e a 20% HC1 solution, a 40% H2SO4 solution, and a 30% caustic soda solution. All the solutions w e r e u s e d at r o o m t e m p e r a t u r e . Each s p e c i m e n was placed in a cylinder, which was then filled with 100 ml of a r e agent solution and c o v e r e d with a g l a s s . After 5, 30, 8 0 , 1 8 0 , 380, and 600 h, the s p e c i m e n s w e r e r e m o v e d , w a s h e d in cold rmming w a t e r , and d r i e d in a v a c u u m d e s i c c a t o r .
Fig. 2. M i e r o s t r u c t u r e of porous titanium, x 350. E l e c t r o l y t i c powder of - 1 . 0 +0..25 m m fraction; s i n t e r i n g t e m p e r a t u r e 1100~ duration 2 h; nitriding f o r 8 h at 850~ etched with 1.5% H F + 3.0% HNO3, balance w a t e r .
It is weU known that p a s s i v a t i o n of titanium in h y d r o c h l o r i c and sulfuric acid solutions p r o c e e d s the m o r e r e a d i l y the higher the Ti t§ ion concentration in these solutions. It has been shown [7], f o r e x a m p l e , that in 40% sulfuric acid solution at 20~ at Ti 4+ ion concentration of only 0.006 mole is sufficient to p a s s i v a t e titanium. In view of this, such t e s t s should be p e r f o r m e d under conditions of continuous a c i d - s o l u t i o n r e n e w a l . In spite of this, h o w e v e r , even in the a b s e n c e of acid renewal, p a s s i v a t i o n of the s p e c i m e n m a t e r i a l s was not attained in o u r t e s t s , and dissolution continued even when the solutions w e r e s t r o n g l y colored b y c o r r o sion products.
The e l e c t r i c a l r e s i s t a n c e of the s p e c i m e n s , m e a s u r e d by m e a n s of an MD-6 bridge, was used to c o m pute the e l e c t r i c a l r e s i s t i v i t y p~ of the porous s p e c i m e n s , ppWaS converted, with the aid of V. V. O d e l e v s k i i ' s f o r m u l a Pb =pp (1-1.5 0),Pro the e l e c t r i c a l resistivityp7o of the s p e c i m e n s in the nonporous condition. E r r o r in the e l e c t r i c a l - r e s i s t a n c e m e a s u r e m e n t s was * 100 ~ , giving a s c a t t e r of Pb values of * 5 ]~2-cm. T e s t s in Caustic Soda. The s p e c i m e n s f r o m t h e p u r e titanium powder and those nitrided f o r 30 m i n exhibited complete c o r r o s i o n r e s i s t a n c e o v e r the whole p e r i o d of 600 h. The s p e c i m e n s u r f a c e a f t e r testing r e m a i n e d bright, the e l e c t r i c a l r e s i s t i v i t y of the s p e c i m e n s did not i n c r e a s e during the tests, and the color of the solution was unaltered. Specimens of the other m a t e r i a l s w e r e not tested in caustic soda.
978
38O
I/
32O ? 28O
~, 2oo
120
I7iI
80
i
4O 2
4
# ~ 10
20
,40
60 80 100
Time,h
400 6006 )/000
200
Fig. 3. Influence of t e s t duration on c o r r o s i o n r e s i s t a n c e of p o r o u s titanium in h y d r o c h l o r i c acid. 1) p u r e titanium; 2) t i tanium nitrided f o r 8 h; 3) titanium nitrided f o r 0.5 h. Titanium containing: 4) 2.0% Pd; 5) 0.2% Pd; 6) 0.1% Pd; 7) 0.05% Pd.
"
2~0 ::s
li
I
200
~160
o_
.~ 80
2
,~
6810
fO
IIII
40 6080/00
I I tilfllt 600800/000'
200
400
Time, h
Fig. 4. Influence of t e s t duration on c o r r o s i o n r e s i s t a n c e of p o r o u s titanium in sulfuric acid: 1) p u r e titanium; 2) titanium nitrided f o r 8 h; 3} titanium nitrided f o r 0.5 h. Titanium c o n mining: 4} 2.0% l~d; 5) 0.2% Pd; 6} 0.1% Pd; 7) 0.05% Pd. T e s t s in H y d r o c h l o r i c Acid. In the h y d r o c h l o r i c - a c i d solution, the s p e c i m e n s f r o m the p u r e powder and the s p e c i m e n s nitrided f o r 30 min w e r e found to have low c o r r o s i o n r e s i s t a n c e (Fig. 3). The e l e c t r i c a l r e s i s t i v i t y of the s p e c i m e n s began to r i s e a f t e r a holding period of as little as 5 h, a f t e r 30 h the solution turned m a u v e , and a f t e r 80 h it b e c a m e totally n o n t r a n s p a r e n t . Deep c o r r o s i o n pits a p p e a r e d on the s p e c i m e n s u r f a c e . The s p e c i m e n s f r o m the p u r e powder c o m p l e t e l y d i s i n t e g r a t e d a f t e r 220 h and those nitrided f o r 30 min, a f t e r 350 h. The s p e c i m e n s nitrided f o r 8 h r e s i s t e d attack b y the acid f o r up to 300 h, a f t e r which t h e i r e l e c t r i cal r e s i s t i v i t y g r a d u a l l y i n c r e a s e d and the acid solution began to show slight coloring by c o r r o s i o n p r o d u c t s . After holding f o r 60 h, the e x t e r n a l a p p e a r a n c e of s p e c i m e n s was unchanged. The s p e c i m e n s with 0.05% p a l l a d i u m behaved in a r a t h e r s i m i l a r m a n n e r . The s p e c i m e n s containing 2, 0.2, and 0.1% p a l l a d i u m r e t a i n e d t h e i r r e s i s t a n c e to the acid solution a f t e r holding f o r 600 h. T h e i r e l e c t r i c a l r e s i s t i v i t y did not r i s e , the color of the solution showed no change, and the s p e c i m e n s u r f a c e r e m a i n e d bright. P a l l a d i u m added to titanium is known to shift the potential of the l a t t e r in the positive direction, as a r e s u l t of which a p a s s i v e s t a t e of the m e t a l is attained even in the p r e s e n c e of active chlorine ions. An i m p o r t a n t p a r t is played h e r e by the palladium concentration in the s u r f a c e s p e c i m e n l a y e r .
979
An investigation [5] into the m e c h a n i s m of the p r o t e c t i v e effect e x e r t e d by palladium in boiling 5% h y d r o c h l o r i c acid solution e s t a b l i s h e d that, during the initial p e r i o d of c o r r o s i o n of T i - P d alloys containing as little as 0.1 wt.% palladium, a substantial accumulation of the l a t t e r o c c u r s on the s u r f a c e . The amount of palladium on the s u r f a c e i n c r e a s e s up to 75 t i m e s c o m p a r e d with the initial palladium content, as a r e sult of which about 20% of the s p e c i m e n s u r f a c e b e c o m e s coated with palladium a f t e r initial e x p o s u r e to boiling 5% H C1 f o r 30 rain. Taking this into consideration, it would be expected that m i n o r palladium a d d i tions introduced into a titanium powder f r o m a PdC12 solution should be m o r e effective than the alloying of a titanium powder with palladium. Mauer and c o w o r k e r s [1] c l a i m that the addition of 0.2-0.8% palladium to porous titanium s p e c i m e n s does not r a i s e the c o r r o s i o n r e s i s t a n c e of the l a t t e r in t e s t s in 5% h y d r o c h l o r i c acid at a t e m p e r a t u r e of 20~ The r e s u l t s of o u r c o r r o s i o n t e s t s indicate that M a u e r r e a c h e d this f a l s e conclusion because, in his investigation, palladium was added to a titanium powder by m e c h a n i c a l mixing. T e s t s in Sulfurc Acid. The c o r r o s i o n r e s i s t a n c e of the s p e c i m e n s f r o m the p u r e titanium powder and of the s p e c i m e n s nitrided f o r 8 h and 30 m i n in the sulfuric acid solution was low and a p p r o x i m a t e l y equal (Fig. 4). No significant difference could be detected in the b e h a v i o r of the s p e c i m e n s nitrided f o r 8 h and 30 rain. After 100 h, the solution began to show coloring and the s p e c i m e n s u r f a c e b e c a m e d a r k e r . After 380 h, the s p e c i m e n s u r f a c e exhibited signs of intense attack. This r e s u l t is of c o n s i d e r a b l e i n t e r e s t in view of the fact that, in an e a r l i e r investigation [6] into the c o r r o s i o n dissolution of titanium in sulfuric acid, it was d e m o n s t r a t e d that s p e c i m e n s nitrided f o r 5 h at a t e m p e r a t u r e of 800~ p r e s e r v e d complete c o r r o s i o n r e s i s t a n c e in 40% sulfuric acid at 25~ Such a m a r k e d difference in the c o r r o s i o n r e s i s t a n c e of porous and nonporous s p e c i m e n s can p r o b a b l y be attributed to the effect of c r e v i c e c o r r o s i o n in t e s t s on p o r o u s s p e c i m e n s . The s p e c i m e n s f r o m the palladized powder with 2 and 0.2% palladium w e r e found to r e s i s t attack by sulfuric acid f o r 600 h. T h e i r s u r f a c e r e m a i n e d bright and t h e i r e l e c t r i c a l r e s i s t i v i t y did not r i s e . Additions of 0.1 and 0.05% palladium, h o w e v e r , p r o v e d to be too s m a l l to p a s s i v a t o the s p e c i m e n s . F o r the s p e c i m e n s with 0.05% palladium, the e l e c t r i c a l r e s i s t i v i t y a f t e r e x p o s u r e f o r 600 h was twice the original value. The s p e c i m e n s containing 0.1% palladium c o r r o d e d s o m e w h a t l e s s rapidly, exhibiting a m a r k e d r i s e in e l e c t r i c a l r e s i s t i v i t y a f t e r holding f o r 600 h. After the e x p o s u r e of the s p e c i m e n s containing 0.1 and 0.05% palladium to the acid solution f o r 600 h, the solution was found to be intensely colored b y c o r r o s i o n p r o d u c t s and the s p e c i m e n s u r f a c e showed signs of strong attack. CONCLUSIONS 1. Titanium s p e c i m e n s , of 30% p o r o s i t y , p r e p a r e d f r o m an e l e c t r o l y t i c powder of the - 1 + 0.25 m m f r a c t i o n exhibit low c o r r o s i o n r e s i s t a n c e in 40% H2SO4 and 20% H C1 solutions at r o o m t e m p e r a t u r e . Specim e n s nitrided at 850~ f o r 30 min d i f f e r only little in this r e s p e c t f r o m s p e c i m e n s which have not been nitrided. 2. The nitriding of such s p e c i m e n s f o r 8 h at 850~ substantially i n c r e a s e s their r e s i s t a n c e to c o r r o sion by 20% HC1. After 600 h of testing, the e l e c t r i c a l r e s i s t i v i t y of the s p e c i m e n s r i s e s only negligibly and their s u r f a c e r e m a i n s b r i g h t . The c o r r o s i o n r e s i s t a n c e of such s p e c i m e n s in 40% H2SO 4 is e s s e n t i a l l y no different f r o m that of s p e c i m e n s f r o m p u r e titanium. 3. The s u r f a c e alloying of titanium powder with 2, 0.2, and 0.1% palladium, p e r f o r m e d f r o m a PdC12 solution, p a s s i v a t e s s p e c i m e n s p r e p a r e d f r o m such powder in 20% H C1, as a r e s u l t of which they r e s i s t c o r r o s i o n in this acid at r o o m t e m p e r a t u r e and holding p e r i o d s of up to 600 h. Specimens containing 0.05% palladium begin to c o r r o d e a f t e r 400 h of holding in this acid. In 40% H2SO4, p a s s i v i t y of s p e c i m e n s is a t tained by introducing 2 o r 0.2 wt.% palladium into titanium powder. D e c r e a s i n g the p a l l a d i u m addition to 0.1 o r or 0.05 wt.% l o w e r s the c o r r o s i o n r e s i s t a n c e of titanium s p e c i m e n s . 4. Specimens of all the p o r o u s titanium m a t e r i a l s investigated a r e totally immune to c o r r o s i o n in 30% caustic soda solutions. LITERATURE 1. 2.
980
CITED
K. Mauer, H. Grewe, and H. Weigand, DEW Tech. B e r . , 6, No. 3, 115 (1966). Yu. V. Grdina, L. T. Gordeeva, and L. G. Timonina, Izv. Vysshikh Uchebn. Zavedenii, Chernaya Met., No. 6, 128 (1962).
.
4. 5o
6. 7.
V. N. Eremenko, Titanium and Its Alloys [in Russian], Izd-vo AN UKr.SSR, Kiev (1960), p. 212. N. D. Tomashov and R. M. Al'tovskii, Corrosion and Protection of Titanium [in Russian], Moscow (1963), p. 125. No D. Tomashov, M. N. Shu2yapnikov, and Yu, M. Ivanov, Zashchita Metal., 1, 122 (1965). N. D. Tomashov, R. M. Al'tovskii, et aL, in: Corrosion and Protection of Constructional Materials [in Russian], edited by N. D. Tomashov, Mashgiz (1961), p. 151. V. V. Andreeva and E. A. Yakovleva, in: Corrosion and Protection of Constructional Alloys [in Russian], edited by A. I. Golubev, Izd-vo nNauka," Moscow (1966}.
981