An electrochemical model for hot-salt stress-corrosion of titanium alloys
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An e l e c t r o c h e m i c a l m o d e l of h o t - s a l t s t r e s s - c o r r o s i o n c r a c k i n g of t i t a n i u m a l l o y s is p r o p o s e d to c o r r e l a t e the d i v e r s e o b s e r v a t i o n s m a d e c o n c e r n i n g this p r o c e s s . A c c o r d i n g to the m o d e l , an o x y g e n - c o n c e n t r a t i o n c e l l e x i s t s b e t w e e n the e x p o s e d s u r f a c e of the c o r r o s i o n s p e c i m e n and s h i e l d e d a r e a s , such a s under s a l t c r y s t a l s and at c r a c k t i p s . Oxygen is r e d u c e d at the e x p o s e d cathode r e g i o n s , while s u b s t r a t e d i s s o l u t i o n o c c u r s at the s h i e l d e d a n o d e s with the f o r m a t i o n of c o m p l e x h a l i d e s . The h y d r o l y s i s of t h e s e h a l i d e s is the s o u r c e of h y d r o g e n that s u b s e q u e n t l y e m b r i t t l e s the s u b s t r a t e . The m o d e l is c o n s i s t e n t with o b s e r v a t i o n s m a d e c o n c e r n i n g the d i s t r i b u t i o n of v a r i o u s ions and a b s o r b e d h y d r o g e n in the v i c i n ity of s t r e s s - c o r r o s i o n c r a c k s . The m o d e l i s a l s o c o n s i s t e n t with the e f f e c t s of s a l t c o m p o s i t i o n and v a r i o u s s u r f a c e a d d i t i v e s on the s u s c e p t i b i l i t y to s t r e s s - c o r r o s i o n c r a c k i n g a s well a s the e f f e c t s of a l l o y i n g and c o a t i n g s .
T I T A N I U M alloys commonly used in compressor components of turbine aircraft engines have been found in numerous laboratory experiments to be susceptible to hot-salt stress-corroslon cracking. Service failures of such compressor components have been reported, but none could be conclusively related to stress-corrosion cracking, t To resolve this discrepancy, extensive investigations of the mechanical and atmospheric conditions that lead to cracking of commercial alloys have been conducted.2's Apparently, the limiting conditions that lead to cracking have not been exceeded in present engines under normal operating conditions. However, because future engines will probably be operating at higher temperatures and stresses for prolonged periods of time, it is imperative that the processes responsible for stress-corrosion cracking in titanium alloys be identified. The observation by previous investigators2'a that the suhstrate is embrittled as a result of the s t r e s s - c o r r o sion process precludes simple dissolution as the failure mode and instead suggests that an embrittling species is generated by the corrosion process. It is now generally accepted that such is the case and that hydrogen embrittlement is the direct cause of cracking. Disagreement exists,~-s however, as to the mechanism of hydrogen generation. Stress-corrosion cracking models can be divided into two classes. Models in the first class require direct salt-atmosphere-substrate interactions for the formation of the embrittling species, of which the Rldeout Pyroh
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