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Understanding corrosion


May 8, 2008
By Rik Robinson*

The proper treatment of corrosion will increase the professional image of your autoglass shop.

Recent reports from autoglass technicians claim that existing and reoccurring corrosion are the biggest problems they face today. The existing corrosion is often on older vehicles where the paint has been damaged by stone chips or dents. The reoccurring corrosion mainly comes from an improperly done repair.

20a
Light corrosion is indicated by light metal discolouration; typically orange. It can be removed with 80 grit
sandpaper or a wire wheel.
Photos Courtesy Of Jim McConkey, Apple Auto Glass.

Why do we continue to see corrosion after the vehicle has been repaired, considering all of the new primers that are available today? The first reason is the limited understanding of where the corrosion is coming from. The second reason is the attention to detail that is paid during the treatment of these corroded areas. The best primers in the world will not work if they are not properly applied. Most urethane manufacturers and many automotive aftermarket paint suppliers have products that will work well for the inhibition of corrosion if they are applied correctly. The key is to follow all of the manufacturer’s instructions. These products and the procedures required for their application often involve many steps. Each step must be followed. This is where the attention to detail comes in. Understanding the key points of each procedure and ensuring they are addressed is what is required to be successful.

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The autoglass technician first needs to know where the corrosion is coming from. Technicians often inadvertently expose bare metal on the pinchweld during the removal of the glass. The tools used to cut through the urethane holding the glass in place can damage painted surfaces in areas that are difficult to see and even harder to repair. The damage to the painted surface now exposes the metal to oxygen and water. This is the same for stone chips and dents that break through the paint exposing the metal underneath. The simultaneous exposure of oxygen and water to the metal is necessary to obtain corrosion, but this can be avoided.

20b
Moderate corrosion typically has some red spots. It can be removed with a wire wheel, media blast or chemical rust remover.

Oxygen can get in by diffusion almost anywhere; the metal structural part is needed, so the obvious candidate to exclude from this mix is the water. The most important function of the conversion layers on metals, consisting of phosphate and chromate compounds, and the paints or other coatings, is to prevent the water from getting to the metal surface. The primers used to repair these areas must also be able to seal out the water and prevent it from reacting with the metal. The application of these primers is critical. There can be no exposed metal or areas where water can get in if the primer is to do its job. Paying attention to this detail is often overlooked, as this is the cause of most of the reoccurring corrosion that is seen. The technicians want to blame the primer but the application is normally the culprit. Following the primer manufacturer’s recommendations and ensuring all of the corrosion has been removed prior to priming is what is needed to be successful.

Corrosion of the pinchweld is a very serious issue and must be corrected properly. To properly treat a pinchweld that is corroded, it is important to first identify the type and the amount of
corrosion that is present. The following will define these characteristics.

20c
Severe corrosion can be identified by deep ‘pitting’, dark red spots and raised edges. It can be removed with a wire wheel, media blast or chemical rust remover. Some cases require treatment by an auto body shop.

There are four levels of corrosion that have now been recognized by automotive OEMs:

  1. Light. Light metal discolouration, typically orange.
  2. Moderate. Moderate corrosion typically has some red spots.
  3. Severe. This can be identified by deep ‘pitting’, dark red spots and raised edges.
  4. Perforation. This level can vary from microscopic holes to loss of metal.

It is very important to note that level four or ‘perforating corrosion’ must be treated by an automotive body shop. The first step in corrosion treatment will always be to remove the corrosion to obtain a bright, corrosion free metal surface. Ideally, this should be done prior to trimming the remaining intact and well bonded adhesive bead.

The method used to remove the corrosion will depend on the type of corrosion that is present. These guidelines are consistent with automotive OEM recommendations and should be followed for removing corrosion.

  • Light: Remove corrosion with 80 grit sandpaper or a wire wheel.
  • Moderate: Remove corrosion with wire wheel, media blast or chemical rust remover.
  • Severe: Remove corrosion with media blast or chemical rust remover.
  • Perforation: The panel must be replaced and treated by a body shop.

20d
Perforation corrosion can vary from microscopic holes to loss of metal. The panel must be replaced and treated by an auto body shop.

Prior to treating the areas where the corrosion has been removed, make sure that these areas are smooth, uniform and completely rust free. Complete the treatment of these areas by following every step provided by the primer manufacturer, no corner cutting is allowed in this application. Ensure proper pre-treatment of the surface is done if required. Apply the primer deep into the scratch and ensure complete contact with the metal surface. Cover the area around the scratch to ensure no small sections are missed and that the entire surface is sealed. Allow for proper dry/cure times before proceeding. The vehicle is now ready to complete the glass installation following the steps provided by your urethane supplier.

The proper treatment of corrosion will not only reduce the number of call-backs, it will also increase the professional image of your technicians and your glass shop. This is the level of service that is required to improve your customer service and help you to compete in this highly competitive market. -end-

*Rik Robinson is the technical development manager for Sika Canada.


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