Droplet corrosion of steel with simulated atmospheric dust from mixtures of ammonium salts

The corrosion of steel with 100 μl droplets of 0.171 M solutions of various ammonium salts and their mixtures, simulating atmospheric dust, is compared to corrosion with NaCl. The key methods are in-situ optical monitoring in a self-built air-conditioned observation chamber and under a microscope with humidity-control, as well as the determination of corrosion products and their crystallite sizes ex-situ using XRD. The cations, Na + or NH 4 + influence the width of the cathodic zone of the droplet and the occurrence of secondary spreading through the pH value of the resulting hydroxide solution in the cathodic zone. Anions influence the resulting type of corrosion products, assumingly by incorporation in the intermediate product green rust. With Cl − anions the detected corrosion product was γ-FeOOH, with SO 4 2− anions additionally α-FeOOH forms. Immediately after droplet application NO 3 − has a passivating effect, but as soon as active corrosion begins, NO 3 − actually accelerates it. The rapid corrosion with NO 3 − was assumed to be the reason for smaller crystallite sizes of the main corrosion product γ-FeOOH. The mixture of the three anions Cl − , SO 4 2− and NO 3 − , as found in atmospheric dust, leads to a synergistic effect. The aggressive anions Cl − and SO 4 2− suppress the passivating effect of NO 3 − anions leading to accelerated corrosion and the formation of a thick layer of magnetite, which do not form with single salt solutions. • Droplet corrosion on steel with ammonium salts differs strongly from that with NaCl. • Cations influence the size of the cathode and the occurrence of secondary spreading. • Anions can influence which corrosion products are formed. • Nitrate shows passivating and accelerating effects on droplet corrosion of steel. • Mixtures of nitrate, sulphate and chloride lead to a dense magnetite layer.