LASER SURFACE ALLOYING OF A MILD STEEL FOR CORROSION RESISTANCE IMPROVEMENT

Abstract
Surface alloys were produced by laser melting of different predeposits into the outer part of a mild steel substrate. Three types of coatings were used : electroless nickel (containing phosphorous), electroless nickel in which chromium carbide particles were introduced during deposition and duplex coatings made of, first, a nickel layer (electroless or electrodeposited) and then, a chromium layer. The surface alloying treatments were performed using a cw high power CO₂ laser and the multiple track melting technique with 60 % overlap between each track. A single laser scan was used. The obtained surface alloys show excellent adherence without porosity or cracking. A detailed microstructural investigation revealed a fine dendritic microstructure. Phosphorous, when present, is segregated in the interdendritic areas. The corrosion resistance of the surface alloys was tested by electrochemical techniques in various neutral or acidified saline media deaerated or naturally aerated. With the first type of predeposit, surface alloys exhibit a corrosion resistance similar to that of electroless nickel but with a much higher adherence of the coating to the substrate. With the second type of predeposit, the increase in corrosion resistance is limited due to the low chromium concentration obtained in solid solution. On the contrary with the third type of predeposits (duplex layers), surface alloys exhibit a high corrosion resistance especially when electrolytic nickel coating was used.