Non-negligible corrosion from vaporized boric acid on 304 stainless steel

•Sever corrosion of 304 SS in vaporized boric acid (V-BA) over solution were found.•Corrosion increases obviously in V-BA compared with in boric acid solution at 960 h.•Increasing B concentrations in solution accelerates corrosion apparently in V-BA.•Formation of droplet-orientated scabs on 304 SS in V-BA leads to more sever corrosion.•More adsorption of boron at scabs in V-BA than solution differs corrosion products. Based on the volatility and sublimation properties of boric acid, the corrosion behaviours of 304 stainless steel (SS) in boric acid-containing vapour over the boric acid aqueous solution and in the solution at high temperature were investigated. The results showed that the vaporized boric acid could cause or accelerate the formation of droplet-orientated scabs on the metal surfaces, which led to the formation of localized crust-like layers of corrosion products, whereas the boric acid in solution could only cause uniform corrosion with thin oxide layers formed on the metal surface. Prolonging the exposure duration or increasing the concentrations of boric acid in solution could accelerate the localized corrosion. The weight gains of 304 SS exposed to the vaporized boric acid reached 0.314 mg/cm2 at 180 °C for 960 h, while it was 0.055 mg/cm2 in the corresponding boric acid solution, being 5.7 times in weight changes. Furthermore, the possible corrosion pathway and formation mechanism of the corrosion products were proposed and discussed.