Pitting corrosion tantalum

Table 7.50 shows the pitting potentials of different metals and alloys observed in 0.1 M NaCl at 25°C. The particularly high values for titanium and tantalum provide an explanation for their excellent resistance to pitting corrosion in chloride-containing environments. 

Sulyma, C.M., Roy, D., 2010b. Voltammetric current oscillations due to general and pitting corrosion of tantalum Implications for electrochemical—mechanical planarization. Coiros. Sci. 52, 3086-3098. 

In hydrochloric acid at temperatures up to 100°C, the corrosion rate decreases with time and ferric iron concentration . The presence of air does not affect the general corrosion rate but in IOn acid it promotes pitting attack, which also arises in chloride-containing methanolic solutions in the absence of sufficient water to effect passivation . Alloying niobium with 2.5% or more of tantalum significantly decreases corrosion rates in hydrochloric acid . 

Most organic compoimds, including acids, alcohols, ketones, alkaloids, salts, and esters, have no effect on tantalum. Specific exceptions should be made for chemicals that may hydrolyze to free fluoride ion or contain (or liberate) sulfur trioxide or strong alkali. One other exception is worthy of note. Mixtures of anhydrous methanol with chlorine, bromine, or iodine cause a pit-type corrosion on tantalum at 65°C. ° This observation is imusual because tantalum is imattacked by either methanol, the halogens alone, or the reaction product, methyl halide, even at somewhat higher temperature. Furthermore, pit-type corrosion is rarely observed with tantalmn. It was concluded that the strong corrosive attack of mixtures of methanol and halogens on tantalum depended on the formation of a haloformic add intermediate. 

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