Corrosion behavior of AZ31 magnesium alloy in the chloride solution containing ammonium nitrate. (10th July 2018) - Cui, Zhongyu
Abstract: Corrosion behavior of AZ31 magnesium alloy in NH4 NO3 -containing chloride solution was investigated by immersion tests, hydrogen evolution collection, electrochemical measurements, and microscopic observation. Addition of NH4 NO3 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 NH4 NO3, attributed to the preferential occurrence of NO3 − reduction. Loose and cracked corrosion products, which are mainly composed of Mg2 (OH)3 Cl·4H2 O and magnesium hydroxy carbonates, form in the NH4 NO3 -containing environment. When the NH4 NO3 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 NH4 NO3 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. Highlights: Corrosion behavior of AZ31 alloy in the Cl-NH4 + -NO3 - solution system is studied. Cathodic reaction is accelerated due to the reduction of NO3 − supported by NH4 + . NO3 − reduction preferentially occurs and decreases the hydrogen evolution volume. Mg2 (OH)3 Cl·4H2 O and magnesium hydroxy carbonates dominate the corrosion products. Autocatalytic pitting corrosion occurs at specific Cl − and NH4 NO3 concentrations.