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Aircraft Aluminum Polishing

Posted by Jay Schneider. Filed under Aviation.
December 16, 2010

Hmmm….aluminum polishing -  a task that is close to may heart! As an FAA Licensed Air Frame and Power, (Engineer for those in other than in the US), I have thousands of hours experience on this daunting task that some say is one of the most demanding and frustrating surfaces to brighten. Fact is some say it IS the hardest substrate to polish correctly. This blog series will cover the entire spectrum of aircraft aluminum polishing.

 

How to approach the surface. There are many theories covering the topic of polishing aircraft aluminum – or commonly called Alclad or clad aluminum. Alclad is a trademark of Alcoa used as a generic term to describe corrosion resistant aluminum sheet formed from high-purity aluminum surface layers metallurgically bonded to a high strength aluminum alloy core material. Alclad is heat-treated aluminum, copper, manganese, magnesium alloy that has the corrosion resistance of pure metal at the surface and the strength of the strong alloy underneath. This process adds decades to the expected life-time expectancy to the aircraft structural part provided the aluminum was properly heat treated. If improperly cared for, aluminum will oxidize after sometime and corrode as does most non-ferrous metals. Prior to polishing, let’s take a look at some of the surface detriments aluminum can behold – this way we will have a good over all snap shot of what we are dealing with:

 

Electrolytic Corrosion

I prefer to use this term when moisture is present creating an electrolyte to permit electron flow. The moisture may come from condensation after takeoff as the trapped air cools at altitude or after landing as the cool aircraft causes condensation of the local humidity. This is very insidious and is part of the reason for commercial airliners to have the lower skin replaced periodically. It usually happens at the faying (that’s an old ship builder’s term) surfaces. Sometimes you cannot find it until you have taken rivets out and separated the parts (rib from skin, etc).

Salt Water Corrosion

This I consider an extreme form of Electrolytic corrosion but it is one that just about everyone understands. This is what Navy aircraft have to fight the most. In most specifications for a new airplane the Navy will give a minimum thickness of aluminum alloy such as .026 inch. Anything thinner loses a high percentage of its strength by the time corrosion is noticed.

Galvanic Corrosion

This type of corrosion is galvanic corrosion that is accelerated by being in the path of an electrical power circuit.

 Intergranular Corrosion

This can be caused by poor processing during manufacture of the alloy (hence the need to be sure it meets a given specification). Usually it is caused by selecting the wrong chemical in attempting a conversion coating on a given alloy. Poor control of heat treatment processing is also a cause.

Over-etching aluminum can cause this type of corrosion.

Exfoliation Corrosion

This type of corrosion is similar to intergranular corrosion except the corrosion follows grain boundaries and "large" chunks fall out. Extrusions can be susceptible where grain boundaries are stretched and/or rough surfaces occur during the extruding process.

Fretting Corrosion

Fretting corrosion is caused by two surfaces rubbing together at a very small amplitude.

Stress Corrosion

The high-strength heat-treatable wrought aluminum alloys in certain tempers are susceptible to stress-corrosion cracking, depending upon the product, section size, direction and magnitude of stress.

Stress corrosion cracking is where the internal stresses (residual stresses) vary across a section so that when they are loaded with additional outside forces, the grain boundaries at the surface start to break. When a part (sheet, extrusion, etc) is quenched and the outside layer cools too quickly, tension stresses are set up on the outside and compression stresses in the middle. This is sometimes taken care of by stretching or shot peening.

Local stresses (assembly stresses) can also be caused by selection of too small diameter of high shear fasteners (such as bolts), shrink or press fits, taper pins, and clevis joints in which tightening of the bolt imposes a sustained bending load on the clevis lugs.

Embrittlement

Improper heat treatment can cause an embrittlement of an otherwise ductile material. Titanium makes a very good light weight spring but Titanium embrittlement can be caused by Cadmium. The cadmium, under pressure and/or heat, will flow (infuse) between the grains of titanium. This weakens the grain boundaries and when the titanium is stressed, a crack will initiate.

Cad plate applied on steel bolts and not subsequently baked can cause the bolts to break from "hydrogen embrittlement.

Galling

Galling is a condition where two parts (made from the same alloy) slide over one another and start marring or gouging the surface between them.

References

1. "Dissimilar Metals," MIL-STD-889
2. "Metallic Materials and Elements for Aerospace Vehicle Structures," Military Handbook, MIL-HNBK-5
3. "Aircraft Corrosion Control," available from EAA

Now that we have an understanding of some of aluminum surface detriments, we can determine what steps to take to correct the problems and brighten the finish. Which is how we will start my next blog - 

Keep the Blue Skies Up and Happy Motoring!

Scott

Scott S. Mc Lain

Vice President

Lake Country Mfg.

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