Abstract
Corrosion behavior of AZ31 magnesium alloy in NH4NO3-containing chloride solution was investigated by immersion tests, hydrogen evolution collection, electrochemical measurements, and microscopic observation. Addition of NH4NO3 accelerates corrosion due to the deterioration of Mg(OH)2 by NH4+ and the promotion of cathodic NO3− reaction, which is supported by the buffering effect of NH4+. Hydrogen evolution is suppressed with addition of high concentrations of NH4NO3, attributed to the preferential occurrence of NO3− reduction. Loose and cracked corrosion products, which are mainly composed of Mg2(OH)3Cl·4H2O and magnesium hydroxy carbonates, form in the NH4NO3-containing environment. When the NH4NO3 concentration is between 0.005 and 0.03 M, autocatalytic pitting corrosion occurs due to the synergistic effects of Cl−, NH4+, and NO3− at the specific concentration range. The H+ (supplied by NH4+ hydrolysis), Cl− in combination with the depolarization effect of NO3− support the autocatalytic growth and eventually perforate the alloy. With the addition of NH4NO3 higher than 0.05 M, corrosion pits with shallow-dish shape form initially and then evolves into general corrosion, because general corrosion is promoted by the decrease of solution pH, suppression of Mg(OH)2 precipitation, and enhancement of cathodic reactions.
Published Version
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