Pickling of metals
Etching rate of aluminium and its effect
Etching rate - why? Only degreasing do not create a qualitative good basis for an excellent surface! The etching process is necessary to remove the natural oxide layer on aluminium, to activate the surface for following chemical coating layers as well as for generating an equal good adhesive base.Microcrystalline deformation layer
Only appear on aluminium and its alloys and is a result of strong heat input through rolling, heating or extrusion processes. Components are metal oxides and intermetallic phases.The risk is that the chemical active metal oxides and intermetallic phases only wait to act corrosive.
Pickling
Important factors, that influence the pickling process, are solved aluminium, the concentration of the chemistry, the temperature, the duration of the pickling as well as the relevant application.Regarding the safety and environmental instructions has to mention, that acid and alkaline pickle are caustic. So always personal protective equipment and goggles are necessary to wear. Acid pickle, which contains hydrofluoric acid in a concentration of more than 1%, a classification according to Regulation (EG) No 1272/2008 (GHS) is regarded as toxic for the entire formulation.
Sources of errors at the pickling process are for example an insufficient degreasing, existing dirt particles, and remaining metal oxides according to defective pickling or a not removed deformation layer.
This results on the one hand in the appearance of filiform corrosion, which is a conspicuous and bizarre form of metal corrosion. Especially areas at the sea and places with industrially polluted air are affected.
Furthermore, zinc deposition on aluminium can occur in acid etch degreasing, since pure aluminium pre-treatments are the exception. In addition, galvanized material is often pre-treated.
Incorrect storage can also lead to different etching rates on the treated product, called pickling shadow, by either moisture or oily intermediate film or cardboard.
Too strong alkaline pickling or lack of acidic follow-up treatment can also be a source of error.
However, a perfect pickling offers an optimal basis for the best corrosion protection and best possible adhesion!