Hidden Corrosion Lies Beneath the Painted Panel
Updated: Oct 9, 2020
Premature corrosion can cost plenty. What a bus operator doesn’t know about this menace can lead to accidents and unsafe situations and certainly deplete the bottom line.
Minnesota Technology published an article 10 years ago stating the costs associated with corrosion-induced replacement and repair exceeds $350 billion annually. It is safe to say that number is far greater today and largely safety related.
Dissimilar metals react and cause them to corrode. Aluminum with stainless hardware will always corrode — sometimes sooner than later. While composite skins may cover up the issue, the remaining corrosion underneath can become a safety issue. Anyone at any bus repair center will attest to this based on what they see during the repair process. Even on newer buses corroded bolts may need to be cut off or drilled out. Electrical connections may show that the road ice chemical magnesium chloride is seizing, shorting or destroying the electrical wiring.
Articles and studies on dissimilar metal corrosion in other vehicle manufacturing industries such as truck, trailer, fire truck and ambulance are profuse, but not much has come from the bus and motorcoach industries. It raises the question if the problem lies hidden beneath the composite/plastic skins, or if this vehicle category is somehow immune from this serious issue; what is metal and what is not.
A good example of this is the account of new $750,000 fire truck that showed significant dissimilar metal corrosion even though the vehicle was only 45 days fresh off the line. The road ice chemicals accelerated the corrosion process during the delivery from the manufacturer to the municipality. The difference here is the corrosion was visible, where bus manufacturers can cover up the problem with plastic. A semi-trailer manufacture did an ASTM B-117 salt spray test that showed a frame rail bursting from road ice chemicals being trapped between a bolted assembly. This manufacturer was using Mylar tape/film to separate dissimilar metals. However, this idea of using Mylar tape dates back to the 1970s and can create a safety issue.
Asked what they do about dissimilar metal corrosion, several manufacturers at a recent national bus trade show said they do not have dissimilar metals on their bus builds. Some indicated their products are composite or plastic. Their response to further questioning about the steel or aluminum frames, compartment hinges and seat bolt downs was that paint takes care of any dissimilar metal issue. That error in thinking is clearly illustrated in the photographs showing the difference between paint and corrosion product.
What to do:
Take a proactive stance on corrosion with the bus manufacturer. When placing an order or writing up specs for a bus purchase, make corrosion prevention one of the requirements. Instruct the manufacturer what preventative measures are necessary.
Corrosion prevention is like a three-leg stool. One leg is to create a barrier between all dissimilar metals that are to be assembled. This barrier coating needs to stay moist to keep it from cracking and falling out during stress related movement. The second leg is to keep moisture from getting into corrosion prone areas and acting as an electrolyte that allows the dissimilar metals to make contact. The third leg is to ensure this corrosion barrier is rich enough in zinc to act sacrificial and keep the other metals from the corrosive reaction. Simple as it may seem, choosing a coating that provides all three of these legs of the stool will make the bus last longer and more than likely prevent a costly safety issue.