Corrosion Behavior of Aluminium Alloys : Potential Application as Sacrificial Anodes

Sammanfattning

Cathodic protection is one of the most used techniques to protect steel from corrosion. Protection is achieved by applying a coating with a more active metal forming a galvanic cell. In modern steel industry, Zn is usually applied as the sacrificial anode to protect high strength steel from corrosion in different environments, however, its presence has also shown to increase the risk of hydrogen embrittlement in the materials. To investigate possible solutions, several Al alloys were proposed and tested for the assessment of future applications as sacrificial anode materials. The OCP test and PDP scan were performed in 3.5 wt.% NaCl solutions for 5 different grades of Al alloys, pure Zn, pure Aland plain carbon steel. The initiation of pitting was evaluated and quantified by advanced optical metrology system after immersion of the 5 Al alloys, pure Al and plain carbon steel in 3.5 wt.-% NaCl for 1 h and 24 h. The presence of additional elements including 2 wt.-% Fe, 15 wt.-% Zn, 0.2 wt.-% Bi, 0.2 wt.-% Sn and0.5 wt.-% MM (60wt%Ce-20wt%La-20wt%Nd) were shown to result in lower OCP for Al used as a sacrificial anode. Considering uniform pitting distribution as a desirable criterion for sacrificial anode-based coatings, the combination of alloying element Bi-Sn-MM showed a significant effect increasing the pitting number and uniformity of pitting distribution in the Al matrix. The single addition of 2 wt.-% Fe did not provide ideal pitting distribution for sacrificial anode applications, whereas introducing 15 wt.-% Zn in an Al-2 wt.-% Fe alloy greatly improved the uniformity of the pitting distribution.