Corrosion behavior of AZ31 magnesium alloy in the chloride solution containing ammonium nitrate

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.