NEWS | Oct. 17, 2019

Aviation: Battling Airframe Corrosion with Primers and Topcoats

Protecting airframes is job #1 for aviation units. That’s why coating systems are needed to prevent corrosion to aviation assets.

The typical coating consists of an aluminum pretreatment, typically a conversion coating such as MIL-DTL-5541, an epoxy primer, and a chemical agent resistant coating (CARC) topcoat.  

The conversion coating and primer are controlled by Navy specifications while the CARC specification falls under Army control. Both services have made significant performance improvements over the last several years. What may not be apparent is the significant advancements in eliminating the use of hexavalent chromium in pretreatment and primers.

Hexavalent chromium has long been used to reduce corrosion risk. Unfortunately, hexavalent chromium is also a health risk to operators if they don’t use the proper safety gear when removing coatings.

When it comes to airframe corrosion, hexavalent chromium-free options are now available for the exterior airframe mold lines. Check out TM 55-1500-345-23, Technical Manual Painting and Marking of Army Aircraft (Jul 15). This TM provides hexavalent chromium-free options under MIL-DTL-53072, Chemical Agent Resistant Coating (CARC) System Application Procedures and Quality Inspection.

Options for pretreatments and primers include:
 
1.  Aluminum conversion coating options include MIL-DTL-5541, Chemical Conversion Coatings on Aluminum and Aluminum Alloys, Type II. These conversion coatings do not contain hexavalent chromium and are authorized on exterior mold lines.
 
2.  Class N primers also have no hexavalent chromium and are authorized for many applications in place of Class C primers, particularly for exterior aluminum and composite airframes. They may also be used on magnesium components when the complete conversion coat and enamel coating system is in place.
        
Non-hexavalent chromium conversion coatings and primers provide a safer option for Soldiers who apply and remove the coating.

In addition, Navy and Army Research Labs are working on the next generation of conversion coatings and primers. The new conversion coating provides superior corrosion protection using zirconium oxide and should be available in 2020. The new primers will contain some magnesium or aluminum metal oxides and should enhance overall corrosion protection when used with the new conversion coatings. The new primers should be available in FY21.

It’s important to note the application requirements for these conversion coatings and primers are different from the legacy chromated products. Always use approved products, follow procedures specified in the TM and ensure the use of appropriate personal protection equipment during application and removal of coatings.

If you have any doubts about the safety of using topcoats and primers, contact your local industrial hygienist (IH) or safety representative.  If you have technical concerns, contact your local Aviation Engineering Directorate (AED) or Liaison Engineer (LE).

For more guidance about currently approved coatings and new products on the horizon, contact the AMCOM Environmental hotline at (256) 313-1711 or email: