Stainless steels are exceptionally corrosion resistant to many corrosive environments. As a group, however, they possess the weakness of corroding by pitting in aerated solutions of chloride ions. This tendency can be overcome by addition of alkali.
Specimens of 18-8 were subject to an accelerated test at 90 degrees C in 4 per cent NaCl to which increasing amounts of NaOH were added. Without NaOH, an average of 14 pits formed within 24 hours on specimens measuring 5 x 1 x ⅛ inch, and corresponding average weight loss was 4.4 mdd. No pitting occurred if the solution contained more than 0.8 percent NaOH. The average weight loss in this concentration of alkali was 1 mdd.
In similar tests conducted at room temperature for 14 months, no pitting occurred in 4 percent NaCl containing at least 0.4 percent NaOH and weight loss was inappreciable. In these tests, it appeared that a more dilute alkali would also have been effective, although such tests have not yet been carried out. The mechanism of inhibition is discussed.
