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News

Single and two-component pickling processes for aluminium pretreatment

Henkel : 15 December, 2012  (Technical Article)
There are various technologies available for the initial treatment of aluminium, prior to giving it its weather-protection coating. Michael Frank, Technology Manager at Henkel, desribes the aluminum pretreatment options for window frames, building façades and other exposed construction surfaces are often made from aluminium. He sees advantages in both the appropriate use of a single-component system and also in the application of an individually metered two-component system.
Single and two-component pickling processes for aluminium pretreatment

The quality requirements on coated aluminium for façade applications are very high. Compliance with these is facilitated by quality specifications issued by internationally recognised approval bodies such as GSB or Qualicoat. The interaction between the metal pretreatment and the powder-coating process that invariably follows is of particular importance in this regard.

The primary focus of technological innovations in the field of aluminium pretreatment in recent years has been on replacing chrome(VI)-containing conversion agents by chrome-free processes based on zirconium or titanium. The conversion bath is just one part of a complex process - closer scrutiny of the individual process stages shows just how much importance attaches to the cleaning and pickling operations. The resultant, defined attack on the aluminium substrate is particularly crucial to the corrosion resistance of the coated article.

Single-component or two-component processes?

There are two basic pickling process types with which oxide layer removal can be variously achieved as recommended or specified by the GSB or Qualicoat.

In the first case – Process 1 (see flow chart) – the aluminium material is pickled under strongly alkaline conditions. The amounts removed from the aluminium surface in the process typically lie between 1 and 1.5g/sq.m. Because the oxides of certain alloyants of aluminum cannot be removed under alkaline conditions, the pickling deposit left on the surface has to be removed with an acid pickle in a secondary operation. This subsequent pickling process removes another 0.5 to 1g/sq.m.

two-stage process whereby <a href=aluminium material is pickled under strongly alkaline conditions" src="http://www.newmaterials.com/ckfinder/userfiles/images/Fullscreen capture 15122012 084212.jpg" style="border-bottom: 1px solid; border-left: 1px solid; margin: 3px 8px; width: 420px; float: right; height: 197px; border-top: 1px solid; border-right: 1px solid" />

 

 

Process 1

Two-stage process whereby aluminium material is pickled under strongly alkaline conditions

 

 

 

In the second process variant – Process 2 (see flow chart) – there is no alkaline pickling stage. Instead, an acidic pickling cleaner is used which, in a single operation, has to ensure the removal of the specified oxide layer thickness equating to more than 1 g/sq.m. The advantages of this technology are:

  • the entire pretreatment process is performed without any jump changes between extremely alkaline and acidic pH values, which in turn usually means reduced volumes of chemicals consumed
  • plant configurations can be implemented without certain pretreatment zones, thus reducing investment outlay.

acidic pickling cleaner is used in a single operation

 

 

Process 2

Acidic pickling cleaner is used in a single operation

 

 

 

Occupational safety and environmental compatibility

However, in order to be able to harness the benefits of a single-stage pickling cleaner, users need to know how to handle corrosive, fluoride-containing media. Fluoride-free pickling solutions – desirable from the point of view of occupational safety and environmental compatibility – are only alternatives in theory. Although it is possible to dissolve both aluminium and its alloyants to a small degree under acidic conditions, the layer thicknesses removed from wrought alloys are insufficient when it comes to meeting the high quality standards required for architectural applications. Hence such solutions are unsuitable both for two-stage pickling and as pickling cleaners.

There are three major parameters that influence the degree to which the required oxide layer removal is achieved:

  • the quantity of disassociated freely available fluoride ions,
  • the bath temperature,
  • the quantity of foreign ions left in the bath solution after the pickling process, predominantly Al, Si, Mg and Cu.

Process reliability with hydrofluoric acid

Contrary to certain assertions, the source of fluoride (ie whether hydrofluoric acid (HF) or complex-bound fluorides - silicofluorides) has little influence on process efficacy. Practical experience indicates that a reduction in material wear, such as in pumps, piping and pretreatment tanks, achieved through the use of complex fluorides, is also subject to conjecture. When using pure hydrofluoric acid, an automatic metering system made from fluoride-resistant material is obligatory. The acid also needs to be added to the pickling bath at a point where thorough mixing is ensured. Once these two prerequisites have been fulfilled, this aggressive medium can be handled such that process reliability and material protection are not compromised.

Two basic concepts

There are thus two basic concepts available:

  • A single-component product (1c) in which the acid-fluoride ratio is adjusted to a defined value;
  • or a two-component product (2c) in which the acid and fluoride fractions are metered in line with specific application requirements.

The following table shows a comparison of the relative benefits of both options.

 
 

1C Product Benefits 2C Product Benefits

Improved occupational safety as no handling of T+ coded products

High flexibility where pickling rate needs to be adapted to varying Al alloyant contents.

Process simplification as fewer chemicals need to be added and metered.

Efficient, demand-led consumption of chemicals

Optimal, low conductivities in the pickling baths and thus minimal rinse water consumption.

 

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