Pulse laser cleaning of iron rust: Ablation, phase transformation, and recorrosion behaviors

Laser cleaning is an emerging technique that has advantages of high energy performance and low chemical-waste emissions, thus minimizing environmental impact. We investigated pulse laser derusting of steel substrates severely corroded by iron rust (β-FeOOH) containing high concentrations of Cl− ions as well as corrosion resistance of the resulting surface. Analyzing residual rust at different laser spot overlap rates and the same laser fluence revealed that laser energy density is a key parameter controlling removal of rust and Cl− ions. Under the same energy density but different laser fluences, the ablation rate strongly depended on laser fluence. The threshold fluence for rust removal was about 5 J/cm2, above which ablation of β-FeOOH proceeded continuously and left behind Fe3O4 and/or FeO grains. Below the threshold fluence, β-FeOOH became dehydrated and transformed into γ-Fe2O3, which built up on the rust layer underneath. In contrast, the Cl content in the residual rust monotonically decreased without saturation, which indicated that Cl− desorption proceeded independently of ablation of the rust layer. A few months after the cleaning, atmospheric recorrosion of the samples were examined. If the cleaning had left behind residual Fe3O4 and FeO, there was significant recorrosion recovering β-FeOOH, where Cl− ions segregated at the surface of these iron oxides facilitated absorption of moisture. The threshold surface chloride density where recorrosion did not proceed coincided with the generally accepted value of 50 mg/m2 to warrant painting on it. In contrast, a surface covered by a Cl−-depleted γ-Fe2O3 passivation layer showed no recorrosion.