The struggle for pretreatment between raw aluminium and paint


It is well known now that some well described Qualicoat pretreatment procedures are very critical in obtaining good adhesion of powder coating and sufficient corrosion protection.

Proper etching/pickling to remove the most sensitive Aluminium top layer and sufficient rinsing are two of these important pretreatment steps.

Even though job coaters are following Qualicoat procedures most of the time when adhesion or corrosion issues occur the failure can be found in the pretreatment steps and not in the conversion bath or in the powder coating cycle. It seems that the newly developed Cr (VI) free conversion coatings are less forgiving than former chromate conversion coatings.

AD Chemicals and her customers are working closely together to determine the critical pretreatment steps. Pickling, racking and rinsing procedures can be optimized to guarantee the best performance possible with Cr(VI) free conversion coatings.

In this presentation we will discuss some factors that might influence the quality of your powder coated aluminium.



Each powder coating line has it´s own challenges. You have to be aware of the best processes prior to the painting of Aluminium objects. Companies often think that it is an easy step to switch their traditional Chromating process to a Chrome (VI) free process. Unfortunately this thought is a big misunderstanding. The starting point must be that a pretreatment line with chromium (VI) is not the same as a chromium (VI) free pretreatment line.

Our opinion is that previous pretreatment lines based on Chromium (VI) had possible processing problems, but the reactivity of the Chromating process had the ability to eliminate the possible defects that occur. The whole pretreatment industry is convinced that Chromium hexavalent has unique properties. Properties like self-healing, corrosion characteristics (For example filiform corrosion) will belong to the past after the sunset date for Chromium (VI) applications in 2017. This radical change requires a different view to the pretreatement lines and it’s chemicals products. “The struggle for pretreatments between raw aluminium and paint”.

The ultimate goal is to work with Chromium (VI) free pretreatments while maintaining the quality properties between Aluminium and paints. Also to meet the required specifications of Qualicoat.

First let’s focus on the variables of the pretreatment line and the quality of the Aluminium.

Concerning the quality of the Aluminium, an assessment of the quality is essential. However, jobcoaters have no influence on this question. They are depending of the Aluminium quality which they received from their sources. The only thing the jobcoater can do is to ask for a quality certificate of the Aluminium or a certificate of analysis of the formation of the Aluminium. Only the structure of the pretreatment line and the chemicals is in control of the jobcoater.

For example: Structure of a Pretreatment line(spraying), chromium (VI) free application

  1. Degreasing /Acid pickling >1.0 g/m2
  2. Rinse
  3. Rinse
  4. D.I. Rinse
  5. Chromium (VI) free pretreatment


Stage 1: Pretreatment tunnel, Pickling phase

Due to the increasing demands from Qualicoat (for example, sea-side > 2 g/m2), the pickling rate (etching-rate) has to be increased in the pickling (etching) phase. This means that you have to increase the aggresivity of the chemicals in the pickling phase. Important note is that when there is a stop in the line speed aluminium profile will be exposed by wet pickling solution and aggressive acid vapor. This “pollution” can dehydrate on the Aluminium surface. It is highly recommended to use a water curtain in case of an outage, as this will prevent acid drying effects. Experience has shown that dried-acid residues on the aluminum surface will not be removed totally in the rinsing phases. Residues can be a possible influence for the chromium (VI) free pretreatment. The formulation of the acid pickling is also a factor to prevent acid drying problems. AD Chemicals monitors it’s formulation at costumers to prevent undesirable effects. Other important items to prevent undesirable effects are good maintenance of the spraying nozzles, temperature, spraying pressure, contact time and the build up of the Aluminium content in the pickling solution.

Stage 2 and 3: Pretreatment tunnel, 2x normal rinse (tapwater)

The both rinsing phases are very important. The difficulty in these phases is that the rinsing water reaches all shapes and surfaces. Especially the seams of insulated profiles have to be rinsed well. In the seams acid residues stick and the rinse water flows over the pollution, this effect results in a minimum mixture with rinse water. The temperature of the rinse water is recommended to 18- 20oC, lower temperatures of the rinse water result in more viscous water and the rinsing effect will be less. The quality of the rinse water will be controlled by pH and conductivity.

Stage 4: Pretreatment tunnel, D.I. rinse water

The best rinsing quality will be if the conductivity is < 30µS and with a pH between 4,5 and 6,0.

At these parameters the risk of contaminations on top of the aluminium surface is minimized. This phase is highly important because any pollution can influence the last phase, the chromium (VI) free pretreatment phase.


Stage 5: Pretreatment tunnel, Chromium (VI) free pretreatment

The chromium (VI) free preatreatments of AD Chemicals are based on a precipitation reaction, where the conversion layer will be made. This layer is a cross linking layer for organic coatings. The thickness of this layer is very important. A layer that is too thick, results to bad adhesion and will be humidity sensitive.

A layer that is too thin, influences the adhesion at the bending tests. By guaranteeing the parameters like conductivity, pH and concentration, the chromium (VI) free layer can be assured. There are several types of chromium (VI) free pretreatments available for Aluminium. The most common types are based on the reactive groups titanium/zirconium (marked named by chromium free pretreatments) and chromium (III). All those Chromium (VI) replacements confirm the 1000hours Salt Acid Spray, corrosion resistance test requirements of Qualicoat.


One very important question bothers AD Chemicals:

“Is the requirement of 1000 hours corrosion resistance which Qualicoat prescribes enough?”

AD Chemicals doubts this statement of Qualicoat. When you only focus on the 1000 hours corrosion resistance, that has been described in the past, Qualicoat is right. However, the corrosion resistance of chromium (VI) based pretreatments fulfills the 2500 hours. This was a fair value for Chromium (VI) based pretreatments on Aluminium. AD Chemicals has investigated their chromium-free systems on the corrosion resistance of 2500 hours and compared it with the traditional chromium(VI) systems.

Internal research shows that the chrome free pretreatment of AD Chemicals precoat CR-FREE A-021, that has been approved since 2003, achieved a corrosion resistance of 2500-3000 hours.

Recently, the latest products of AD Chemicals (Precoat TVL & Precoat COLOR) also achieved the 2500-3000 hours corrosion resistance.

So AD Chemicals goes for long corrosion resistance!!

By proper process optimizations and pursuing the parameters and variables with fine chemicals, Chromium (VI) free pretreatments are perfect replacements of Chromium(VI) based pretreatments.