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Hole in the Cart: Controlling Rust

Posted: August 1, 2014 by Stephen Petit

Rust is a red flag in every sense of the phrase. It can contribute to structural weakness, it’s a hazard and it just plain looks bad. To a customer or a safety inspector, a cavity in the metal is a tell-tale sign that you’re not taking care of your equipment.

The problem is in a state of flux, says Roy MacLean, sales supervisor for the Winkler, Man.-based trailer manufacturer Lode-King Industries. “It’s going to get worse before it gets better because of all those new chemicals that the highway guys are using to melt ice.”

But even if your metal survived winter, rainy spring and summer is another battle altogether. Corrosion is an electrochemical reaction that requires little more than two pieces of metal, air, and water. If you haul freight in Canada, rust belongs on your list of life’s inevitabilities.

You can’t see or feel it, but a piece of metal has an electrical charge to it, and not all metals have the same amount. When metal with a negative charge (like steel or aluminum) is placed near metal with a relatively more positive charge (stainless steel, copper, or brass), and you wet down the area, electrons from the negative side flow through the water to the positive as fast as they can, taking bits of oxidized metal along with them. The loss of electrons kicks off a process that creates a ferric hydroxide, or rust.

Chemicals found in acid rain and salty road spray only aggravate the problem. They help the electrons move from one metal to another even better than pure water. It’s not unheard of to find evidence of salt-heavy road grime conducting electricity between metals that are an inch or more apart.

It’s hard to stop corrosion, but folks try. At the foundry, a variety of metals can be alloyed with steel to make a more rust-resistant product (for example, adding chromium creates stainless steel, which is very resistant to rust). Equipment manufacturers will use paint, chrome plating, or galvanizing (a low-voltage zinc coating) to cover up the metal, or in some cases sacrificial anodes to draw current away from metal they want to protect.

As the guy who specs trailers or directs repairs, preventive maintenance is the key. Keep an eye on areas that are prone to nicks, scrapes, and chips and keep them clean and dry. Recoat bare metal ASAP, and apply a liberal coating of rust-control spray to crevices and connectors where moisture can creep in.

Good ideas all. But the first step should be to work with your trailer manufacturer or sales rep to spec components and features that are rust resistant.

Dissimilar metals and stress points: Consider the proximity of dissimilar metals — those with different charges — on your equipment. If a piece of negatively charged metal such as steel (at minus 0.58 volts), or aluminum, (at minus 0.7 volts), is next to one with a more positive voltage, such as stainless, (which can be minus 0.1 volts), the current will flow. The negative metal will be eaten away, starting as a pit in the surface and gradually growing from there.

You can reduce problems if the more positive stainless area is small and the negative aluminum area is large. A square foot of aluminum and a small steel rivet, for example, should be of no consequence. But if rust works its way under a rivet or beneath the paint, the low oxygen levels can actually increase the negative charge in the metal. One option is to replace the stainless with galvanized steel. Zinc’s charge is minus 1.0 volts, much more negative than aluminum or steel.

“The zinc is sacrificed, not the metal underneath it,” says Ray Camball, director of fleet sales for Trailmobile in Mississauga, Ont. “Compare trailers after seven years,” Camball says. “The ones with galvanized doorframes still look good. With the conventional painted steel frames, once the threshold plate has been scuffed up a little, oxidation begins and you get perforation through the vertical rear end of the threshold plate.”

If the zinc starts to wear out (look for brown discolouration), touch it up with zinc primer.

Electrical connections: Corrosion increases electrical resistance, which in turn boosts the load on the entire system. Spec sealed connectors where the harness attaches to the light fixtures — typically a boot-like rubber or plastic. And because corrosion can creep up inside insulated wires, especially if there’s an unprotected splice or connection, consider wiring with translucent insulation.

Brakes: Harsh de-icing chemicals have been linked to oxidation on brake shoes, a condition called rust jacking. Corrosion can force the lining off the shoe table and fracture it, sending tiny cracks scurrying away from the lining’s edge. Some brake suppliers are reformulating lining materials and working with new coatings designed to inhibit corrosion on new and relined shoes.

Structural corrosion: An upper coupler replacement on an eight-year-old trailer is a work order you don’t want to have to fill out. If you can afford a little more tare weight, spec a thicker-than-standard coupler plate.

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