The pretreatment of aluminium before powder coating

Discover the importance of the pretreatment of aluminium for powder coating. Find out about the different methods and benefits of this corrosion-resistant process.

The pretreatment of aluminium

It is necessary

It is necessary to pre-treat the aluminium and aluminium alloys before the application of the powder coating to:

• ensure good adhesion of the powder coating to the aluminium surface and
• protect against any subsequent corrosion.

The pretreatment process is a “conversion coating” process, whereby a corrosion-resistant layer is formed on the aluminium substrate. This pretreatment requires the initial cleaning and etching of the aluminium surface, followed by the application of the pretreatment system [1].

Conversion of aluminium surface

Powder coating including pre-treatment is covered by EN 12206-1. The current version of the standard is EN 12206-1:2021.

The conversion of aluminium surface may be carried out using [2]:

• chromate conversion coating or
• non-chromate conversion coating or
• anodising.

A conversion layer is produced by application of the appropriatr solution. After the conversion process, the aluminium surface is normally rinsed with deionized water and dried

QUALICOAT and GSB

While both QUALICOAT and GSB set their own performance requirements for powder coating, they are regularly updated although largely based on  EN 12206-1, with additional requirements to ensure correct processing by the applicator [3, 4].

Hexavalent chromium pre-treatment

Chromate conversion

Chromate conversion layer vary in colour from an iridescent yellow to a golden tan. The layers are characterized by the presence of chromium and absence of phosphate [2] (Figure 1).

The mass per unit surface area of the conversion layer (conversion coating weight) shall be between 0,4 g/m² and 1 g/m².

Phosphochromate conversion

Phosphochromate conversion layer vary in colour iridescent to various shades of green. The layers are characterized by the presence of both chromium and phosphate [2] (Figure 2).

The mass per unit surface area of the conversion layer shall be between 0,4 g/m² and 1,2 g/m².

Drying

The drying temperature after chromate pretreatment, as measured on the metal surface, shall not exceed:

• 100 ºC, if the coating powder is applied without delay afrer pretreatment
• 65 ºC for chromate layers and 85 ºC for phosphochromate layers, if the coating powder is applied with delay afrer pretretament (less than 16 h).

May be prohibited

Chromates or phosphochromates have been the traditional pre-treatment for architectural aluminium since the introduction of powder coating. They are relatively simple to operate and provide proven corrosion protection in service and good powder adhesion.

Hexavalent Chromium (Cr VI) poses a health risk to workers due to potential occupational exposure and an environmental risk on any accidental release from the processing plant, as chromates are both carcinogenic and mutagenic. For this reason, the use of chromates was scheduled to be prohibited under ECHA REACH from 21 September 2017. The revised sunset date is now 21 September 2024 [1]. It may be postponed.

• It should be noted that where phosphochromates are used there will be no residual Hexavalent Chromium (Cr VI) on the surface of the pre-treated aluminium [1.
• It is beleaved that hexavalent chromium (Cr VI) is normally reduced to trivalent chromium (Cr III) during the powder coating curing process at 185 degrees Celsius.

Non-hexavalent chromium pre-treatments

Options for the pretreatment of aluminium for powder coating that do not use hexavalent chromium (Cr VI) compounds currently include [1]:

• trivalent chromium (Cr III)
• chromium-free
• pre-anodising or flash anodising.

Chromium-free and pre-anodising are the two main alternatives to the use of hexavalent chromium. Trivalent chromium is not typically used in pre-treatment for external applications [1].

Chromium-free pre-treatments

When the process is well controlled and maintained in accordance with the relevant standards, powder coatings utilising chromium-free pre-treatment have good corrosion performance. Statistical analysis has shown that there is no significant difference in corrosion performance between the alternatives and chromium-containing processes [3].

Pre-anodising treatment

If pre-anodising is selected it should be noted that this process is most suited to extruded sections. It requires more process stages, additional controls for operator exposure (e.g. sulphuric acid mist) and comprehensive management of waste waters [1].

Sources:

1. Pre-treatment Systems for Architectural Powder Coating and BS EN 12206-1:2021
2. EN 12206-1:2021
3. Qualicoat Specifications
4. GSB