Galvanic Corrosion - Column

Automotive Finishing, Fall, 2001 by Tom Doppke

Fasteners are the building blocks of almost all industrial products. The coatings that protect them against weather, chemicals, sunlight, fading, aging and all the environmental bricks thrown at them are understood only marginally by the fastener experts. The finishing industry understands part of the story but not always all the requirements of the fastener. Between this is the middle ground that this column is dedicated to.

Various laws and regulations have impacted the fastener industry as much as others have the finishing business. Lightweight metals, aluminum and magnesium, thin gauge sheet steel and plastics all have forced attachments into different paths than those followed in former years. Unfortunately, yesterday thinking still abounds in the fastener field.

There is a great lack of information in the fastener field about galvanic corrosion. Complaints about rust are ''corrected'' by changing to stainless steel, adding finishes that are unsuitable for the job or just plain shot gunning ideas.

Galvanic corrosion is a well-known material phenomenon resulting from the coupling of two dissimilar materials under conditions favorable for the transfer of electrons from one material to another. Contributory to this is the role that finishes play in fastened joints. Primary to the understanding of the effects of the various finishes when coupled to various materials is some explanation of what galvanic corrosion is and how it occurs.

When dissimilar materials (usually metals) are in contact with each other in the presence of an electrolyte, an electric current flows from one to the other. This is called a galvanic current. Galvanic corrosion is that part of the corrosion on the anodic member directly related to the galvanic current. The fundamental relationship involved in galvanic corrosion is covered in Kirchov's Second Law, written as:

EMF = IR

From which the familiar formula: [E.sub.c] - [E.sub.a] = [IR.sub.e] [Ir.sub.m] is derived.

Where [R.sub.e] = resistance of electrolytic portion of the galvanic circuit

[R.sub.m] = resistance of metallic portion

[E.sub.c] = effective potential of cathode

[E.sub.a] = effective potential of anode

Defining these conditions in terms of a fastened joint, the anode is either the fastened material or the finished fastener, and the cathode is the opposite, depending upon the placement of the two in the electromotive series. The electrical contact is the fastening (threaded joint, rivets, etc.), and the electrolytic bridge between the anode and cathode is the moisture plus salt or other conductive substance.

The galvanic table is a ranking of metals from the most reactive to the least. The closer together on the table the joined metals are, the less galvanic current is generated. Obviously, the less the current flow, the less corrosion there will be. So step one is to try to keep the two metals as close together in the galvanic table as possible (See Table I). However, this is not always possible, as Figure 1 shows. The aluminum frame section was attached with rivets, a common fastening technique. However, the rivets were standard steel parts, no special coating beyond a flash zinc plate (0.00015 inch) that lasted only a few months. The moisture gathered at the surface interfaces, and the galvanic action corroded away large areas so badly that the fasteners themselves were lost in some instances.

Solutions? Since the easiest measure is to reduce the electromotive potential between the two coupled surfaces ([E.sub.c] - [E.sub.a]) and to increase the electrical resistance in the circuit of the joint ([IR.sub.c] [IR.sub.m]), a look at what can be done economically is needed. Making the cathodic area as small as possible and the anodic area as large as practical can effectively reduce the potential. That is, use the most "noble" or cathodic finish or material as the fastener or fastener coating and make the attached metal surface the anodic side. While this could have worked for our aluminum attachment, the use of numerous steel rivets in the figure effectively cancelled out the solution (the total cathodic area, sum of all the steel rivets, was still too large for effective reduction of potential).

Ships are often protected from seawater corrosion by the attachment of zinc anodes to the hull. These corrode in a sacrificial manner while protecting the steel surfaces. The largest use of magnesium in industry today is for anodes in water heaters. They corrode in place of the steel tank, greatly increasing product life. Replacing the anode can lengthen the heater's working life several years.

Another way to reduce galvanic action is to design attachments so that moisture drains away from the area. If no conductive substance (road salt, corrosion products, etc.) is present and/or no moisture is there to conduct the current, the effects of bi-metallic coupling are minimal (there is always a percentage of atmospheric humidity).

The use of the proper finish greatly increases the chance that current flow between the two metal suffaces will not occur or will be slowed. Chromates were excellent inhibitive coatings but are now being phased out of all coating usage in fasteners because of global environmental concerns. The proper finish should be as close to the metal being joined in the galvanic table as possible. It should also have some resistance to wear and other corrosion. Examples of inexperienced fastener people choosing a coating based on its galvanic protective properties and having it fail due to the finish softness or rubbing off during installation abound. Many of the zinc-based paint coatings have found favor within the automotive arena lately due to their galvanic protection, increased corrosion resistance and ease of application without the usual fastener problems of thread and recess fill.