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The pitting behavior of titanium and titanium alloys, pure nickel, aluminum 6061, and stainless steel Type 304 has been studied using electrochemical techniques in methanol in the presence of sulfates or chlorides. An important factor is the water content of these solutions. In anhydrous solutions of methanol and hydrochloric acid (HCl), passivation cannot occur for titanium and high corrosion rates are observed. Addition of water at a concentration as low as 0.6 percent leads to passivation. The passive region is limited to noble potentials by the pitting potential which, up to 2.5 percent water, increases linearly with water content and is more or less independent of water content for higher concentrations. Similar observations are made for methanol containing sulfuric acid (H2SO4). Aluminum 6061 suffers from severe pitting when polarized anodically in methanol with 0.1 or 1.0 N H2SO4. Pitting was observed also for nickel after anodic polarization experiments in 2 N H2SO4 in methyl alcohol (CH3OH). Iron, on the other hand, could not be passivated and pitting, therefore, did not occur. Pitting occurred for stainless steel Type 304 in 0.1 N H2SO4 in CH3OH at rather noble potentials in the transpassive region. These results indicate that the solvent can have a large influence on the pitting susceptibility of metals. Titanium is very resistant in aqueous media, but shows pitting behavior similar to aluminum in organic media. It is also found that sulfates which do not cause pitting in aqueous media, where they even might be used as inhibitors, cause severe pitting in organic media for certain materials. Perchlorates have also been found to induce pitting in methanol. Phosphates lead to passivation by formation of a film of high electronic resistance. While pitting occurs on titanium in methanol containing 0.1 N H2SO4, no localized attack was observed under the same conditions in ethanol, ethylene glycol, or isopropanol.
corrosion, chlorides, sulfates, pitting, passivation, perchlorates, phosphates, methanol
Member of technical staff, Science Center, Rockwell International, Thousand Oaks, Calif.