MadSci Network: Engineering

Re: Can electro-magnetics cause aluminum siding to lose its coating?

Date: Thu Jun 29 23:32:21 2000
Posted By: James Griepenburg, , Chemical consultant, Chemmet Services
Area of science: Engineering
ID: 958665313.Eg

I do not have any specific references but I have looked over several GM 
and Valeo painting procedures and talked with several painting engineers.  
I recommend that you call the company that manufactured your siding and 
ask for the laboratory.  The chemists and engineers there have probably 
seen the phenomenom and if they haven't they will be excited about it.  A 
search on IWON.COM for 'painting aluminum siding' and 'painting aluminum'  
gives many informative hits that are mostly anecdotal. 

Aluminum forms a hydrated oxide coating when exposed to air and moisture.  
Paint is usually non adherent to this coating and poorly adherent to 
alkaline treatments of this coating.  The most effective surface 
treatments of aluminum are acid washes, phosphating or chromating 
treatments, acidic anodising [oxidation], and the method used by at least 
one siding manufacturer who uses several washing steps followed by a 
specific [probably acidic] primer and then the top coat.  If the cleaning 
steps are compromised delamination of the paint can happen and poor 
cleaning is usually the cause [of these rare events according to one 

Can there be an electro chemical cause?  Yes.   If the aluminum is anodic 
to a more noble metal such as iron or copper and there are electrolytic 
and electrical connections It is possible for the Al to corrode under the 
paint starting where the paint is breached and the metal exposed to water 
and air.  When this happens it is usually obvious around the contact 
points but corrosion can happen at a distance if there is a electrolyte 
connection[liguid water].  If the Al is cathodic to a more active metal, 
only Magnesium comes to mind as a possibility,  a galvanic reaction could 
reduce and destroy the adherent oxide layer and cause paint to peel.  This 
would be noticed.  An electric potential could substitute for the metals 
but the electrical and electrolytic connections would still have to 
exist.   If the siding were subjected to an extremely high potential, say 
a lightning strike, dielectric breakdown could conceivably deatroy the 
adherent layer and cause the paint to falloff.  Such a phenomenom would be 
similar to dielectric breakdown in a capacitor.

Paint adherence problems are tough to diagnose at a distance and usually 
require considerable process and environmental study and surface chemical 
analysis to even arrive at a testable hypothesis let alone a complete 
solution.  Talk to the siding manufacturer and their laboratory people.  

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