Passivation process in quasi-continuous laser derusting with intermediate pulse width and line-scanning method.

Floating rust composed of particles and aggregates is the primary product of iron or steel corrosion. Because the floating rust has a porous structure and small thickness, part of the irradiating laser energy can be transmitted through the rust layer and be absorbed by the iron substrate. The adherent force between the floating rust and the metal substrate is weak. In this paper we carried out a series of experiments on this specific rust type to achieve laser derusting and passivating simultaneously. We used a line-scanning method (50% overlapping ratio between adjacent laser spots) to get the nearly average uniform distribution of laser fluence in a large cleaning area. The laser irradiation can directly heat a metal surface to cause thermo-elastic vibration to shake off the rust layer and to cause oxidization to form a protective layer. The most important factor of laser passivating is that the iron surface must be heated to the melting point of iron but not much higher. During this short melting period, on the one hand the iron surface could be oxidized completely; on the other hand the melting of the iron surface could make uniform the oxygen concentration and temperature in the molten iron bath.