1. The colourless conversion coating shows it is an oxidation process with possible
reaction to hydroxide.
2. Aluminium oxides and hydroxides are soluble in all saponifiers, so the conversion
coating will inevitably be attacked
3. Will the metal itself be attacked? The answer is yes, to some extent. Some
saponifiers are buffered to reduce, but not eliminate, attack of aluminium
4. If the resultant appearance is blackish, then your aluminium is surely an alloy. What
is happening is that the aluminium is being dissolved, leaving crystals rich in other
metals exposed. Pure aluminium leaves a whitish appearance.
Cure? None. Mitigation: adjust the saponification (product, time, temperature,
concentration) to minimise attack, consistent with adequate flux removal. This is called
walking a tightrope. Workaround: add all Al components after the cleaning process.
Brian
On 17 Dec 2003 at 16:12, Dhawan, Ashok wrote:
> Technetters
>
> Anyone who has experienced and resolved tarnishing of Aluminium
> surface upon cleaning with DI water + saponifier. The aluminium
> surface is protected with conversion coating (clear) which is turning
> black when exposed to saponifier solution.
>
>
>
> Ashok Dhawan P.Eng
> Manufacturing Engineer
> Solectron
> Snailmail:1455 Mountain Avenue
> Winnipeg MB R2X 2Y9 CANADA
> * E-mail: [log in to unmask]
> * Phone: (204) 631-7208
> * Fax: (204) 631-7294
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