Magnesium Corrosion

[Scottaber]

Having a problem with corrosion showing up on ZE41A-T5 magnesium cast housings. Parts are coated with Dow 17 (with the exception of Dow 7 or Dow 19 on some drilled areas). A top layer of Rockhard PT961 is applied over most of the Dow 17 areas. Parts are coated with a cosmoline material, and securely packaged with dessicant. After as little as one month of storage, Widespread corrosion shows up on areas that were machined. Corrosion Most notable on areas that only received Dow 17 but it is also seen coming up from areas that received Rockhard, causing the Rockhard to peel. Any ideas as to what could be causing this?

 

[CoryPad]

Incomplete cleaning during initial coating? The anodizing treatments can be highly acidic, so maybe that initiated corrosion.

Are you sure Cosmoline is safe for Mg? I am not familiar with it, so maybe it is not suitable for Mg.

The anodizing treatment on Mg can be quite porous, which can allow corrodents to contact the Mg surface. That is why it is important to use a sealant, like RockHard, to prevent corrosion.

Can you re-trace all of your process steps after machining? Even simple ones like handling/conveying. Then, see if your Mg housings are in contact with corrosive materials.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[swall]

Casting porosity, uncovered by machining may be trapping cutting fluids and the DOW pretreatment chemicals and if these don't get completely removed they can later seep out and cause corrosion under the final coating.