Approximately 40%, or 17 million tonnes, of the total world aluminium shipment is in the form of rolled products. This is used in packaging, architectural applications and transportation. All these products require modification of the surface to maintain durable performance.
Surface modification of aluminium involves:
There are three general approaches to cleaning:
All three provide a surface that is suitable for pretreatment.
The aluminium pretreatment process is often integrated with a preceding strip cleaning process, and often a subsequent coating process.
The pretreatment can be either a chemical or an electrolytic process, to replace or modify the natural aluminium oxide film. The chemical process is more usual, with different versions for food packaging applications (to satisfy regulations) and for architectural or transport applications.
Historically pretreatments for coil coated aluminium were chromium (VI) based formulations which provided excellent performance. However, as chromium (VI) is toxic, considerable effort has been deployed to develop more health- and environment- friendly alternative pre-treatments.
Current alternatives include:
With earlier conversion coatings, excess solution had to be washed off the metal surface leaving contaminated rinse water as an unwanted by-product. Later, no-rinse systems were developed where a precise amount of pretreatment solution was applied uniformly across the strip and then dried in place.
The pretreatment, typically a no-rinse chrome chemical type, is applied to each side of the strip by roller coating. A typical pretreatment is 120 mg/m2 dry coating weight.
The pretreatment is dried, typically by passing the strip through a hot air oven to achieve a metal temperature of approximately 40 to 60°C.
An alternative electrolytic pretreatment process is effectively anodizing in a hot electrolyte using liquid contacts. A cell containing an electrolyte (e.g. 20% sulphuric acid) is divided into 2 or more compartments, with electrodes in each compartment.
By using electrodes with slots through which the strip passes, both sides of the strip are pretreated simultaneously. The pretreatment oxide film is produced whilst the strip is subjected to the anodic half-cycle, and the film is modified by chemical attack during the cathodic half-cycle. Full pretreatment takes about 3 seconds.
After the pretreatment and drying stages, the strip is coated with a lacquer or paint depending on the final product application.
Pretreatment functionality is a combination of one or more attributes.
Therefore, both the chemistry and structure of the pretreatment layer play a part in determining performance. Achieving the correct structure can mean the difference between success and failure of the final product.
Aluminium pretreatment represents a small but important cog in a multibillion dollar industry. However, in the twenty-first century, industry requirements for control and minimisation of failure, coupled with increased competition and higher production speeds, places ever increasing demands on pretreated surfaces.
This blog post was originally written by Dr Colin Butler who has now left the company. Please contact Dr Junjie Wang if you have any questions.