On the influence of ion-induced surface modification on localized corrosion of aluminium

Modern surface modification techniques offer means for changing properties of materials related to physical or chemical reactions at or near the surface. Thus the corrosion behaviour of metals can be affected by applying this method. The influence of Mg and Cr ion implantation and coatings with Zr or Si thin films on the corrosion properties of aluminium and an aluminium alloy was studied. Corrosion measurements were performed under pitting and stress corrosion cracking conditions. The pitting measurements took place in aerated aqueous solution of 2 wt.% NaCl at 25 degrees C. The resistance against stress corrosion cracking was measured using a method according to ASTM G44, i.e. alternating immersion in an aqueous solution containing 3.5 wt.% NaCl under a constant strain of 100 MPa. The results showed that ion-induced surface modifications can lead to a better corrosion resistance of aluminium. Pitting could be reduced by Cr implantation as well as thin coatings of chromium and zirconium. The initiation of stress corrosion cracking was suppressed for a prolonged period of 890 h with an ion beam mixed Zr coating, in comparison with 39 h for the untreated sample. The results of the corrosion measurements in combination with the other studies lead to the assumption that under the given conditions pitting is not a prerequisite for stress corrosion cracking of the investigated Al alloy.