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Black stains on aluminum from shipping

I work for a metal shop as QA Engineer. We fabricate aluminum enclosures for electronic devices which are plated by a subcontracted supplier. Plating is clear chromate according MIL-5541 type II class 3.

Weeks ago we detected some black stains over the parts which matched the pattern of creases in the polyurethane bag used for package parts. When removing the stain we found out that the aluminum under it had been "degraded"

We verified cleanliness of bags used, dry times on plating but we found nothing wrong.

Today we just received a complaint from our customer in regards of same issue. We are for sure that we did not ship stained parts so the marks or stains must appear after we ship the parts.

Our supplier is clueless as what could be causing such condition. Help is highly appreciated.
Regards

Isela Rodriguez
sheet metal employee - Chihuahua Mexico
April 21, 2009

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Class 3 at best is not much corrosion protection and is obviously adsorbing fumes from the poly packaging that degrades the thin (to nothing) coating. Recommendation: (1)package in certified sulfur free white paper, and/or (2) use Class 1A full yellow thick and heavy chromate.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

April 23, 2009

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May 19, 2009

Estimado Ingeniero

Haz trtado bolsas de silica gel? por otra parte lo que te mencionan anteriormente puede coincidir ya que yo tuve el mismo problema con mi cliente sobre cold roll pero el hecho de que empaques piezas:
Calientes,
Humedas,
Mal Tratamiento de Cromo
Bajo pH
Alto pH
Indica una mala aderencia del cromato con lo cual aparecen manchas blancas you obscuras.

Saludos

Sergio Samaniego
Mexicali, Baja California, Mexico

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Ed. note: Sincere apologies, Sergio, but this conversation can't continue in Spanish. This is a monitored site where we attempt to limit commerciality, flaming, etc., and I am unable to do that in languages other than English. Also, we solicit help from a long list of "regular readers" who also don't speak Spanish. Sorry, but postings must be in English. Regards, -- Ted.