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Monday, October 10, 2011

How to degrease aluminum?

Question:
What is the best cleaner to use when degreasing aluminum?


Answer:

First, the word "degreasing" is a technical term used by many metal manufacturers that means removing oil (with or w/o grime) on the surface of the metal. 

It comes from the historical use of a vapor degreaser using freon, where people dipped dirty metal parts in to the freon vapor, the vapor condensed on the part and dripped off while solvating off the oil, then more vapor formed to rinse. You did not have to rinse with this type of cleaner. Freon is ozone depleting so it is not commonly used any more, but because of this history the industries that used to use them do not like to rinse. 

Degreasers can also mean heavy duty alkaline cleaners, typically with corrosion inhibitors - sometimes these industries never even rinse these because they are just not that critical an application (removing grease from engine parts in a railroad engine that is going to be reassembled and have fresh grease put on it for example).

WHEN PASSIVE OXIDE LAYER BROKEN 
If the bare aluminum has been recently cut, abraided, polished, turned or drilled, then the passive oxide layer will have been broken and if it has not had time to reform in air (takes 15 min or so) or if it has an oily coating that interferes with air getting to the surface, then the aluminum is not passivated.  Unpassivated aluminum will be attacked by an alkaline cleaner, even ones with mild pH or silicate corrosion inhibitors.  For unpassivated aluminum we only recommend using Citranox or Citrajet for degreasing

If Citranox or Citrajet is not  rinsed off an aluminum salt will form on the surface.  For most applications this will not harm the aluminum, and will only cause slight brown discoloration mostly visible by magnified inspection and barely detectable to the naked eye as a golden brown discoloration. 

It is always recommended to rinse, and the first rinse water following the Citranox or Citrajet cleaning should be with cold tap water NOT DI.  DI water is more ion hungry, will expose more sites to undesirable oxidation from the dissolved  oxygen, and will cause discoloration of the unpassivated surface.  If tap water is not available as first rinse, then wait 15 minutes before rinsing in DI at the expense of any possible problems with dried on and/or redeposited residues from the cleaner. 

In heavy duty degreasing, people may be fine simply wipe rinisng or blowing off the parts and can probably live with any trace hazy discoloration.  For more critical cleaning where the term degreasing is less likley to be used, you absolutely have to rinse thoroughly. 

WHEN PASSIVE OXIDE LAYER PRESENT 
If the bare aluminum (or metal) has the protective oxide layer present and is in a passive state, then we recommend an alkaline cleaner: either mild pH or with silicate corrosion inhibitors. 

Note that anodized aluminum is an aluminum that has been electrically treated to form a stronger oxide or other coating that you typically would safely recommend an alkaline cleaner for, unless there has been cutting or anything that has gone thru the anodizing in which case you are back to all the concerns about bare aluminum.   Typically people do not cut or abraid anodized aluminum, they would do the anodizing after all the cutting or drilling.



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