The Role of Palladium in Enhancing Corrosion Resistance of Titanium
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The Role of Palladium in Enhancing Corrosion Resistance of Titanium By J. B. Cotton Research Department, Imperial Metal Industries (Kynoch) Limited, Birmingham high rate of corrosion is observed, and this is It is well known that the addition of then succeeded by a very significant reduction about 0.2 per cent palladium to in the corrosion rate. In the early work on titanium signijicantly increases its titanium-palladium alloys it was thought that resistance to corrosion in non-oxidising during this induction period preferential solu- acids. It has usually been assumed that tion of titanium took place, that when suffi- the palladium is present as elemental cient metallic palladium was available at the particles at the metal surface and that metal surface the anodic passivation mechan- these function electrochemically to pro- ism followed, and that this resulted in the vide anodic passivation of the titanium diminution of the corrosion rate. matrix. It has, however, now been There was, however, some doubt as to the demonstrated that even in low concentra- form in which titanium was present in tion palladium additions to titanium titanium-palladium alloys and further investi- farm Ti,Pd, and that before onset of gation has now shown that even at a con- passioation the palladium dissolves in centration as low as 0.15 wt per cent the the corrodent and is then reprecipitated palladium is present as Ti,Pd. Fig. I shows in elemental form from solution. This the presence of an elongated Ti,Pd phase in process is accompanied by the growth of the Ti-0.15 per cent Pd alloy at a magnifica- a protectivejilm of TiO,. In substantia- tion of 55,000. tion of this mechanism it can be shown Palladium present in the titanium matrix as that corrosion of unalloyed titanium in Ti,Pd clearly will not have the electro- raon-oxidising acids can be halted by the addition to the acid of a very small quantity of a soluble palladium salt. The increase in resistance to corrosion in non-oxidising acids conferred upon titanium by a small alloying addition of noble metal has been known for about seven years (I). Theory predicts that anodic passivation of the titanium matrix will be achieved once suffi- cient noble metal has concentrated at the sur- face to provide the requisite galvanic exchange current to elevate the titanium potential above a critical value. Protection is usually only achieved after an Fig. 1 The structure of a titanium-0.15 per cent palladium alloy showing elongated particles of initial induction period during which a fairly a Ti,Pdphase ( x 55,000) Platinum Metals Rev., 1967, 11, (2), 50-52 50 chemical properties attributable to metallic plete cessation of corrosion. If, however, the palladium and it is necessary to seek an acid is replenished at daily intervals, succes- alternative explanation for the mechanism by sive increments in the amount of corrosion are which the alloy functions. recorded after each replenishment, and there Corrosion tests in which coupons of is no permanent passivity, as shown in the titanium-0.15 per cent palladium are exposed graph of Fig. 2. to boiling 10 wt per cent sulphuric acid Analysis of the acid at the onsct of passivity under stagnant and replenished conditions, shows that at this point the bulk liquor con- provide an indication of the mechanism in- tains about 0.9 parts per million of palladium. volved. In stagnant conditions there is an This suggests that initially the Ti,Pd induction period of about 20 hours of phase as well as the matrix titanium suffers rapid corrosion, succeeded by almost com- dissolution, and that when a sufficient con- Platinum Metals Rev., 1967, 11, (2), 51 Fig. 4 Specimens of com- mercially pure titanium in boiling 10 per cent by weight sulphuric acid with palladium additions Top, left to right 0, 1, 5 p.p.m. palladium Bottom, left to right 10,20,50p.p.m.palladium centration of palladium ions has built up in At the 20 p.p.m. soluble palladium level solution metallic palladium is precipitated at there is visible evidence of the precipitation the metal surface. This could then bring into of metallic palladium at the titanium surface, play the anodic passivation mechanism sug- but at a concentration of 50 p.p.m. a dark gested by Stern and Bishop (I) and Cotton oxide film is formed, presumably containing (2, 3). The induction period of rapid cor- some palladium, and a continuing slow rosion is necessary in order to build up the weight increase is observed. required soluble palladium. There is con- A protective film of titanium oxide could firmation of this in that if the amount of result from oxidation of soluble titanium fol- alloying palladium in the alloy is increased lowed by a precipitation process. Thus when from 0.15 to 0.5 per cent, the induction period the titanium initially corrodes it passes into virtually disappears, presumably because the solution as Ti++‘and this will be oxidised by appropriate concentration of soluble palladium I’d+ I according to the reaction builds up much more rapidly. 2Ti++++ Pd++ -tzTi+++++ Pd, Further confirmation of this mechanism is obtained if unalloyed titanium is exposed to Ti++++is unstable and precipitates as TiO,. 10 wt per cent boiling sulphuric acid to This is a somewhat different concept of which palladium chloride is added to provide what is regarded as the normal anodic passiva- incremental amounts of soluble palladium at tion of metallic titanium that is presumed to levels from I to 50 p.p.m., the liquor being take place on the metal surface and it is changed daily. Results, illustrated graphically probable that both mechanisms are involved in Fig. 3, show that onset of passivity takes in the growth of protective films on titanium- place when the concentration of soluble palladium. palladium reaches about 10 p.p.m. This is a considerably higher palladium level than that detected in the bulk liquor resulting from References I Milton Stern and Claude B. Bishop, Trans. initial dissolution of the titanium-palladium A.S.M. 1960,52,239-256; see also Platinum alloy, but this could be accounted for by the Metals Rev., 1959, 3, 88-89 establishment of a concentration gradient in 2 J. B. Cotton, Chemistry & Industry, 1958, 68-69 and 492-493 a thin layer of stagnant liquor at the surface 3 J. B. Cotton, Werkstoffe undKorrosion, 1960, of the alloy specimens. 11, 152-155 Platinum Metals Rev., 1967, 11, (2), 52 .