Improvement of corrosion resistance of aluminium and its alloys, especially under operating conditions in aeronautic, nautic and automobile industries, is a necessity. Chromate conversion coatings were used with great success for decades but due to their cancerogenic and toxic activity their use is restricted by the RoHS Directive 2002/95/EC which took effect in 2006 in the EU. Due to this restriction, considerable efforts have been directed in the last decade towards the investigation of technologies alternative to chromate conversion coatings. Various approaches have been studied; however, a full alternative has not yet been put into market. In the last decade the European automobile industry has more than doubled the average amount of aluminium used in passenger cars and the trend in increasing. This implies an increased need for aluminium protection. Among conversion coatings lanthanide salts, especially cerium has been identified. Cerium is the most active and the only lanthanide, which can be oxidized in a stable salt with a higher oxidization state. The mechanism of surface protection includes the formation of a protective layer of hydroxide and/or oxide. This reaction is carried out on metal inclusions (Zn, Cu, Mg), which are electrochemically more negative compared to remaining surface of aluminium alloy, and act as a cathode site. Instead using individual inhibitors, a mixture of inhibitors may achieve an enhanced effect. Two methodologies were taken – non-accelerated immersion and accelerated conversion coatings. Another approach includes barrier hybrid coatings based on sol-gel methodology. The sol-gel process allows the introduction of organic molecules into an inorganic network, forming hybrid organic-inorganic coatings, the so-called ormosils also known as silanes. Prepared conversion and hybrid coatings were studied by electrochemical corrosion measurements in combination with various surface analytical methods.
B.04 Guest lecture
COBISS.SI-ID: 28879911