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Modern Aluminum Alloys

Use of aluminum in various parts of motors and drive housings has greatly increased over the past several decades, and it seems likely that it will continue to increase. Today’s aluminum has many advantages over other materials that make it the ideal material for motor and drive housings manufacturers. 

Strength:

There is a wide variety of aluminum alloys with various tensile strengths. Most easily cast alloys tend to be soft and have relatively low yields, in the area of 20,000 psi tensile, and 15,000 lb. yield.  In the past, aluminum largely appeared in the form of sand castings and since the material cost was high, alloys of a cheaper grade were often used. It is some of these cheap cast alloys that have given this material an undeserved reputation. Where aluminum is die cast, alloys may be used that have remarkable characteristics. Small NEMA Motor endshields are die cast, 380 aluminum with a tensile strength of 48,000 psi. Compare this with soft gray cast iron that has 20,000 to 25,000 psi tensile.

Aluminum die castings have extreme dimensional stability; that is, they do not warp or crack. Incorrect alloys used in die casting, or incorrect procedures, can cause cracks to form in the initial part; however, these are readily observable and occur during the casting process.

We have used die cast aluminum in UNIMOUNT, rolled steel and dripproof  motors for over 20 years and is now using it in Paint Free Wash Down motors. Literally millions of these die cast aluminum housings and components are in service and they enjoy an excellent record. The in plant failure of components due to cracking is considerably lower than with cast iron. In general, an aluminum component can be dropped on a concrete floor without serious damage due to aluminum ductility. This is not the case with cast iron due to cast irons brittleness.

Corrosion Resistance:

Question: Does aluminum corrode?

Answer: Yes. It corrodes very rapidly, more rapidly than most materials - on exposure to oxygen an oxide forms almost immediately. Oxides form on cast iron and steel, but at a much lower rate. 

Question: Does oxidation continue and destroy the material?

Answer: No. The aluminum oxide formed on the exposure to air is a corrosion resistant oxide and under normal conditions, oxidation stops or it takes place very, very slowly. The oxidation will turn the aluminum a slight grayish color. In the absence of abrasion and in normal atmospheres, this oxide forms a very efficient protective coat. In fact, anodizing of aluminum is merely a method of rapidly producing an aluminum oxide of greater than normal thickness and hardness, and is used where extreme protection is necessary.

In the case of cast iron, an oxide forms which continues to change its chemical composition from Fe0 to Fe2O3, to Fe3O4, and the rust continues - eventually through the iron. This is not true of aluminum. Steel forms these oxides very rapidly and eventual failure of an unprotected steel component in a normal atmosphere would be much more rapid than cast iron or aluminum. In fact, iron and steel are never used in the unprotected state. Aluminum is usually used unprotected. For example - piping, structural members, building siding, food containers, windows, doors, fences, gutters, etc.

Question: This is all right for an average atmosphere, but how about unusual conditions?

Answer: One of the conditions most usually brought up is that of exposure to sea air and water. Aluminum has a much greater resistance to corrosion than does iron or steel. A steel ship must be protected from the salt water or the corrosion of the steel is destructively rapid. Many ships are built of aluminum and, while they are painted for protection primarily against the growth of marine life, they are, on the whole, more resistant than steel to the action of salt water. Currently many large ships and tankers are built of aluminum. The Coast Guard is replacing its steel buoys with aluminum buoys. However, aluminum, like iron or steel, should be protected if it is exposed to salt water and salt air. The normal paint which a U.S. MOTORS® brand motor or component receives is ample for all but the most unusual conditions. Outboard and inboard stern driver units are aluminum and enjoy long service life.

Question: Why do some people still believe aluminum corrodes more than iron or steel?

Answer: Prejudice and/or lack of accurate information. The following is from "Kent’s Handbook" on the "Corrosion of Iron Alloys", and under "The Mechanism of Corrosion".

"SURFACE FILMS - Clean metallic surfaces exposed to air quickly acquire a film of oxide, which, at ordinary temperature, is thin and invisible. At higher temperatures it is thick enough to give well known characteristic colors - rust. The nature and properties of the film depend upon the composition of the metal itself and its environment. Aluminum and stainless steel, for instance, owe their high durability to the formation of a continuous and permanent file that is stable under most conditions of exposure". In general terms, aluminum is more resistant to corrosion by most common materials and atmospheres than cast iron, and far more resistant to corrosion than is carbon steel.

Question: How does paint affect corrosion?

Answer: All iron and steel must be painted immediately in order to prevent serious atmospheric and other corrosion from taking place. While aluminum is far more resistant than iron, it generally is painted, and when used in combination, all three - the aluminum, the iron, and the steel, are always painted. Here again, aluminum shows one very great advantage. The aluminum oxide which forms on the surface of the aluminum is a relatively impervious surface preventing additional oxygen from contacting the aluminum. This surface is hard and tenacious, so when a coat of paint is placed over it, there is little tendency for the oxide to leave the surface. This is the reason that paint has such a far superior clinging action to aluminum than to iron or steel. If iron or steel has any traces of rust under the paint, the oxide in the rust continues to attack the metal and form additional oxides which flake off, taking the paint with them, and exposing the base metal to the atmosphere.

Where extremely corrosive conditions do exist, aluminum can be easily protected. Anodizing is suitable for most corrosive conditions and epoxy paint also provides excellent protection. For extreme conditions such treatments as polyester dip and bake produce an almost indestructible finish. Such treatments are costly but are available if the conditions warrant.

The following table of chemicals and industrial materials, to which motors and gears, and drives may be subjected, shows the effects on aluminum. These are taken from The Aluminum Company of America publication "Process Industries Applications".

Table of Corrosive Chemicals and Industrial Materials

In Summary: 

  1. Aluminum, in the vast majority of applications, will exceed the corrosion resistance of cast iron and far outdistance steel. 
  2. Die cast 380 aluminum has a higher tensile strength than gray cast iron yet, in equal volumes, weighs about half. 
  3. Aluminum is an excellent conductor of heat. This fact makes it an excellent gearbox and motor frame material. Aluminum reduces oil temperature by conduction heat to the outside air much faster than cast iron or steel.