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The mechanism of laser cleaning corrosion and the analysis of process influencing factors

Superwave Laser Technology Co.,Ltd | Updated: Apr 02, 2018

With the increasing requirement of environmental protection laws and regulations in China, people's environmental protection and safety awareness are increasing, and the shortcomings of traditional cleaning industry are becoming more and more obvious. The cleaning features of laser cleaning, such as cleaner, non-contact, efficient and flexible and suitable for all kinds of materials, are considered to be the most reliable and effective solutions.

All kinds of metal surfaces react with chemical or electrochemical reactions to the surrounding media, causing the metal to corrode. Metal corrosion is a waste of resources, and its degree is very impressive. Corrosion of all kinds of metals is the most serious, and the annual steel loss caused by corrosion accounts for 1/5 ~ 1/4 of the total steel. At present, there are many researches on the corrosion of laser cleaning. Since the invention of the laser, the idea of laser cleaning has been proposed. In recent years, the research work on laser derusting has been carried out in China, and beneficial progress has been made. Dry laser cleaning efficiency is lower than the wet cleaning, but because of the wet cleaning is introduced into liquid membrane, causing the process stability is low, the industrial application of flexible enough, if mishandled, cause secondary corrosion factors, such as dry laser cleaning of the study is more important for promotion of industrialization.

This article from the perspective of material and laser coupling, explore process of laser cleaning of carbon steel rust removing effect, to guide practice, and beneficial exploration for the development direction of clean with laser.

2 laser cleaning corrosion mechanism.

From the view point of metal rust cleaning, modified layer and substrate belongs to homogeneous layer, the base layer elements contained in the membrane material, or the base layer of derivatives, namely all sorts of corrosion products or decay products. The main cleaning mechanism is the gasification mechanism, and the cleaning function is realized after the laser energy density reaches the ablation threshold of the rust layer. The specific removal methods are divided into direct gasification and recoil pressure.

The rust caused by iron and steel in the atmospheric environment is eroded by cracks, and a porous oxide coating is formed on the surface of the steel substrate, which can not be used as a closed protective effect. The infiltration of water, oxygen and other gases causes the matrix to be continuously corroded. However, based on the characteristics of steel rust layer, it is convenient to remove rust from laser.

Metal material corrosion essentially belongs to physical removal of laser cleaning, mainly through the rust layer evaporation, rust layer and substrate temperature gradient induced interface thermal stress, rust layer gap expansion of gas or liquid is heated force [lasercleaning], such as vaporization decomposition and interstitial phase rupture alternates, rust layer thickness decreases, finally realizes the effect of removing rust.

1) with the higher power density, is proportional to the increase of efficiency of rust layer, when the power density reaches a certain threshold (about 1.4 e+08 w/cm2), continue to improve power density, on the basis of corrosion clean, do not damage the base material of the power density range of about 7.5 e+08 e+07 w/cm2 ~ 1.5 w/cm2. 

3) when the power density and spot overlap ratio are determined, the size of the cleaning spot is independent of the number of cleaning times, that is, the single removal thickness is the same. Spot size is larger, can carry through raising the scanning velocity and fill the space, improve cleaning efficiency on the one hand, on the other hand, the cleaning speed to lower base material heat accumulation, base material surface oxidation condition improved, removing ability strong;

4) when the cleaning is carried out, the filling spacing should be reasonably determined according to the size of the cleaning spot, so as to avoid the appearance of uneven ground after cleaning;

5) when the spot overlap rate is low, there is a small unwashed area in the scanning area (spot shading), and then the rust layer remains; When the overlapping rate is high, the cumulative thermal effect of the light pulse is caused by the oxidation of the substrate.

The experiments showed that the corrosion was cleaned and the overlap rate of the base material was not significantly oxidized, which was about 20%~50% (the minimum overlap rate of the circular spot coverage area was 25%). When the power density is suitable, the comprehensive cleaning effect and the efficiency factor shall be formulated to make the spot overlap rate > 20%.