Since most steels are magnetic, testing with a
magnet is an obvious means
of identification.  Galvanized steel is magnetic and its zinc coating has a characteristic speckled appearance that is hard to miss.

Some of the most widely used stainless steels are not magnetic and identification is a problem.  A rust-free surface on a heavy and somewhat shiny material is a good clue to its possible identity as stainless steel.

Copper and its more common alloys, brass and bronze, are readily identified by their appearance.

Aluminum and its alloys are best identified by their light weight and corrosion-free appearance.

Plastics are probably the most difficult materials to identify.  Many of them are white, translucent or transparent and positive identification is best left to the producer of the material, provided this is known.

Examine the surface
A clean, dry surface that resists physical disintegration is a must for routine, high-quality bonding.

Metal oxide films are not necessarily bad.  If the oxide is tightly bound to the metal underneath, a durable joint may be formed.  Aluminum and its alloys invariably form an oxide that is very tightly held, but iron does not.  Iron oxide, or rust, flakes off relatively easily and thus is not a suitable surface for adhesive bonding.

Other corrosion and oxidation products on metals may be highly visible and obviously must be removed.  Similarly, also remove obvious grime, grease and oil.

While removing evident contaminants is common sense, when examined under high-power magnification, it becomes apparent that most surfaces are far from smooth at the microscopic level.  The "hills and valleys" of a surface are usually contaminated with water, metal oxides, absorbed gases, processing lubricants, and perhaps loosely held products of a reaction between the surface and its surroundings.  Remove or substantially reduce these materials to achieve strong, durable bonds.

Which method to use?
There are three primary methods to prepare surfaces for bonding:
abrasion, solution cleaning or conversion techniques.  Often, a combination of methods is used.

Use abrasive techniques, including wet and dry blasting, sanding and brushing, when objects are large or when heavy layers of rust or other coatings must be removed.

Solution cleaning methods, including hot alkaline, solvent wiping and vapor degreasing, are economical for high production and effective for loosely held contaminants.   Exercise care to prevent contamination of wipe rags or solution tanks.

Conversion processes include anodizing, phosphating and chemical cross-linking for certain plastics.  Technically speaking, pickling (mild acid treatment) and etching (severe acid or alkali treatment) are conversion methods, but they are generally classified as solution treatments.

Aluminum and its alloys, and copper and its alloys, tend to be "softer" than steel and therefore solvent wiping is preferred over heavy grit blasting or similar methods.

In most cases, carbon steel, especially when there is surface rust, can tolerate various forms of grit blasting.  Stainless steel and galvanized steels can also tolerate grit blasting but, in most cases, solvent cleaning should be sufficient.

Test the surface with water
It is difficult to test surface cleanliness with any degree of certainty.  But the water break-free test is a fast and easy way to test for contamination.  Here, pour a small amount of water over the surface to be bonded and watch if it breaks into droplets.   If individual droplets form, it's usually because there is oil or grease
contamination.

This test will not detect salt or metal oxide contamination, and some surfaces such as polyolefins and Teflon break up a water layer even when they are clean.

Dennis Fisher is the adhesives technical manager for AlliedSignal's specialty chemicals division.

This article appeared in the February/March 1999 issue of MRO Today magazine.  Copyright 1999.

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