Letter 30013

Chromate conversion coating highlights material grain [So. Africa] 

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These are components machined out of Aluminium, grade 6082 T6

These were treated with the chromate conversion coating process - Alodine / kenvert / Iridite.

Supplier A treated them, but could not attain the quality required. The colour was uneven, and too light.

The components were then re-treated by supplier B, who got the coating perfect, but at this stage some of the components revealed what appears to be the grain of the material.

"Small grain"

What does not make sense is that the components went through the same process simultaneously, but only some revealed the grain. The grain size appears in three categories: not visible, small and big.

"Large grain"

Can the process the components go through affect the grain of the material?

Were the components machined from 3 different batches of material?

Is there a way to fix or avoid the above mentioned problem?

How many times can you safely treat a component?

I am looking forward to replies. Regards,

Johan B
- Cape Town, South Africa

You have the best finish you can get on die cast !!! And each time you re-process die cast you get further inward and away from the more dense outer crust therefore the quantity of silicon in the surface varies and the cosmetic appearance varies.

Further, thin walls have more silicon on the surface than thick walls.

Processes are available to remove most of the silicon from the surface but not all job shops know how and/or will not set up the fuming treatments that will remove the silicon.

Robert H Probert Robert H Probert Technical Services Garner, NC, USA Editor's note:        Mr. Probert is the    author of Aluminum How-To

Do I understand correctly, if I derive from the above reply, that Aluminium 6082 T6 is Die-Cast? These components were milled from solid pieces of aluminium (bar/slab), and the newly exposed surfaces would thus be far from the original outer surface.

Johan B
- Cape Town, Western Province South Africa

You are looking at variable grain structure based of asymmetric cooling after "hot" rolling. It probably came from different lots of the same alloy, but you can see differences like this from the first feet of material to the last feet of a run on the same lot. The best answer is to not etch/clean it any more than is absolutely necessary, which will hold down the highlighting of the grain. Unfortunately, that does not hide scratches and makes reruns impossible.

James Watts
- FL

Johan,

Mr. Probert made a mistake-- alloy 6082 is a wrought alloy, not a die casting alloy, although it does contain some Si (0.7-1.3%). The grain size variation in the aluminium block is due to differences in recrystallization, which is affected by the thermomechanical process history (temperature, strain, strain-rate, etc.). Areas of the billet or slab that are more heavily deformed and/or see a higher temperature will tend to recrystallize, resulting in the "large grain" parts. Less working and temperature results in incomplete or no recrystallization. It is possible that these components are made from 3 different batches of material, or from material that underwent different processing (top of the slab vs. middle of the slab).

Toby Padfield
Automotive module supplier - MI, USA

John, I believe what you are seeing is the grain structure of the aluminum, which is typically more prevalent in the middle of the stock. The part does not look like a casting to me at all. The way to avoid this "mottling", is to avoid caustic etching as a prep prior to chromate, but as you mentioned, these parts had to be reworked, most likely stripped in a caustic bath. They could be stripped again, and hand finished to remove the mottled look, and rechromated with very little, or no caustic etch.

Marc Green anodizer - Boise, ID, USA

We carry out a great deal of chromate conversion on AL 6082. The finish you are getting is typical of 'machined from solid' components that have undergone rework, ie stripping of a cured chromate finish and re chromating. The removal of a properly applied and cured chromate conversion is quite difficult to achieve short of glass bead blasting or similar, the stripping process could easily be over etching the components surface. Alternatively your sub contractor may be over etching the components in the first instance.

Richard Mosley
- UK

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