White substance under the roof.

[Msingh15]

Hi,

Could anyone please explain what is this material accumulated under the roof of this aluminium kiosk?

I know it is because of the condensation issue but what is that white substance? is it the corrosion of protective layer of aluminium? or something else?

Thanks

 

[TugboatEng]

Aluminum oxide.

It is part of the self protective oxides that form on aluminum. Some alloys protect better than others. Oily residues and coatings will make the corrosion worse as they can cause crevice corrosion. An abrasive scrub here and there will keep the oxides from building up into a chalky layer. Acid based cleaners will also help remove it but will not be effective if there is any oily residue.

 

[Msingh15]

Thanks TugboatEng

But if it was oxidation of aluminium, wouldn't this have happened on whole roof rather than the middle channel?

Thoughts on this?

 

[3DDave]

Aluminum develops an oxide coating nearly instantly; it is usually very thin and transparent.

It's when there is an action that depletes this oxide that this sort of corrosion starts. There was probably a small defect that has been expanding for a long time. Look for any source of alkali contamination that would accelerate this.

https://www.alumeco.com/knowledge-technique/general/corrosion-of-aluminium-surfaces

Salt is also not kind, but alkali cleaners are sometimes used. If this is a low spot or the insulation is uneven then condensation could accumulate and allow mildly corrosive gasses to dissolve.

 

[EdStainless]

Minor variations in air flow, temperature, and surface condition will significantly change how a surface corrodes.
Are they cooking in here?

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P.E. Metallurgy, consulting work welcomed

 

[Compositepro]

It is oxidation, or white rust. It is a reaction that occurs in the presence of water. Dry aluminum will maintain a transparent and impervious oxide layer, which is very protective against further oxidation. Water of high or low pH will dissolve the protective oxide and cause oxidation that results in a very porous white oxide. The oxide is still actually transparent but the porosity scatters light, which makes it appear white. Just as solid sugar, salt, or glass are transparent but the powders appear white.

The corrosion is in the center of the panel because the panel has a slight sag, and condensation drains to the lowest point before dripping.

 

[Msingh15]

Hi

Thanks to all of you for your informative replies. Makes much more sense than before.

Ed stainless, no cooking in that enclosure. it used as an enclosure for high voltage switchgear.

Regards,

Manny

 

[EdStainless]

How old is this? If this has happened quickly then you should look at how this enclosure is vented.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[Kim_W]

There is some ''filiform'' distribution of the whitish layer. This may happen when the aluminum roof was covered with an isolator (decal, carpet, etc). Was that section of the roof in contact with anything like that which may hold up moisture, accumulated from condensation?
Im not an expert but it looks like early stage of poultice corrosion, where the white substance is aluminum hydroxide.

 

[BrianE22]

May be there is some warm convective air currents hitting that area more than others.