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Replacement of Chrome (VI) Pre-Treatments for Aluminium. Which Product Should I Use?

Introduction:
With the increased environmental pressure on chrome (VI) salts in industry there are now several alternative non-chrome (VI) based pre-treatments available from the leading suppliers of these types of product.

This article will outline what materials are available from Henkel, in what applications to use them and the various benefits that each process has. We will cover chrome (III) phosphate, chrome (III) based, zirconium based, titanium based and zirconium hybrid products.

Chrome(III) phosphate:
In this category there is Alocrom 407/47 (Alodine 407/47); despite the bath containing chrome(VI) this process gives a chrome (VI) free conversion coating with comparable performance to Alocrom 1200 coatings. This technology has been in use for many years already, is well accepted and approved. In the past this type of product was used in the food industry such is the safety of the coating.

The bath can be made up using mains water (provided it is not too hard) or deionised water; the bath is normally run at elevated temperature although it can be used at ambient also. Pre-cleaning of parts to be processed is necessary to remove oil and grease but it is not essential to deoxidise the aluminium surface before treatment unless it is heavily oxidised. This process is not normally recommended for aluminium castings due to the potential for smut formation.

Corrosion performance without paint will be 168 hours of NSS for an electrically conductive surface and up to 336 hours for a thicker non-conductive surface.

With a paint coating over the Alocrom 407/47 1000-2000 hours of acetic acid salt spray can be achieved.

Approvals: BS 6496, 6497. Qualicoat specification.

Chrome (III) / Zirconium Oxide:
In this category there is Alodine 5992 which produces a chrome (III) chromium oxide / zirconium conversion coating, the process liquid contains no chrome (VI). This is a spray or dip process that gives a good treatment on copper containing aluminium alloys. The coating produced is colourless to faint green in appearance.

The bath must be made up in deionised water and is run at 30-50°C typically. Deionised water rinsing after treatment is recommended also.

Parts to be processed must be degreased and deoxidised prior to the treatment stage.

Approvals: Alodine 5992 meets the requirements of MIL-C-5541.

Non Chrome (Coloured):
In this category there is Alodine 160/161 which produces a manganate reinforced zirconium conversion coating. This is a dip only process, it should not be sprayed although this process can be used by brush or swab by making up the solution at ten times the concentration that it would be used for dipping and then applying in the same way that Brush Alocrom 1200 is used.

Parts to be processed must be degreased and deoxidised prior to the treatment stage.

Alodine 160/161 baths must be made up in deionised water and it is run at 15-30°C. Process times in the bath are 15-180 seconds.

The coating produced has equivalent corrosion performance to Alocrom 1200 when coated with paint and as an un-painted surface. The Alodine 160/161 coating also has the same electrical conductivity as an equivalent Alocrom 1200 coating.

As stated above the coating is coloured, the process is capable of producing coatings that are practically colourless through various shades of gold to light brown. This is achieved either by adjustment of the concentrations in the bath or by varying the dipping time.

In all cases the processed parts should be rinsed with deionised water.

Alodine 160/161 has also been proven to be a very good pre-treatment for magnesium components. The magnesium must also be thoroughly degreased and deoxidised before treatment in order to achieve the best results.

The Alodine 160/161 is not Qualicoat approved and also it is not recommended for aluminium alloys with high copper content.

Non Chrome (Colourless):
In this category there are 2 products to discuss these are Alodine 4850 and Alodine 5200.

Alodine 4850 is a zirconium based process that produces a colourless conversion coating. It can be sprayed, dipped or manually applied. The working solution must be made up using deionised water.

Parts to be processed must be degreased and deoxidised prior to the treatment stage.

The treatment bath is run at 20-40°C and parts should be exposed to the chemistry for 30-90 seconds. A light deionised water rinse is usually employed although a “no rinse” approach can be used in some cases.

The Alodine 4850 process is Qualicoat approved and GSB approved and also meets the performance requirements of BS 6496 / BS EN 12206-1.

Alodine 4850 can also be used as a chrome free post passivation treatment for phosphate processes.

Alodine 5200 is a chrome free titanium based treatment that also produces a colourless, bluish/yellowish conversion coating on light metals. It can be used in spray or dip applications at 20-50°C. Contact time can be 5-180 seconds. A light rinse with deionised water is normal followed by oven drying prior to painting.

Parts to be processed must be degreased and deoxidised prior to the treatment stage.

Alodine 5200 is also a good pre-treatment for magnesium with several customers already using it for this.

Alodine 5200 fulfils the requirements of MIL-C-81706 and MIL-C-5541 for US army land vehicles. It is also approved by Boeing for the treatment of titanium (BAC 5861).

Non Chrome Multimetal:
In this category we have Bonderite 1455. This product is very useful if the user needs a process capable of treating multimetal components in the same plant.

Bonderite 1455 produces a modified titanium phosphate conversion coating. It can be used on aluminium, steel and zinc surfaces to give a very good key for paints and adhesives and hence a high level of anticorrosion performance.

Bonderite 1455 can be used in spray, dip or manual applications typically at 1-3% concentration at ambient temperature. It can also be used as a wipe on treatment where it is used at higher concentration. Rinsing is not necessary but a light rinse on parts to remove any drips or pools prior to drying is advantageous.

Aluminium parts to be processed must be degreased and deoxidised prior to the treatment stage. This can be achieved by abrasion in application processes.
Painted aluminium treated with Bonderite 1455 will pass 1000 hours acetic acid salt spray but the process does not have Qualicoat approval.

Conclusion:
To sum up, these are some of the non-chrome (VI) conversion coatings available from Henkel; they demonstrate the types of technology available and the industry approvals that can be worked to when using these products. It then comes down to what product or process meets a specific customer requirement.


More information on Henkel products, supplied in the UK by Stowlin Croftshaw, here

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