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Pitting ABCs

Tank corrosion problem getting worse, becoming more expensive to fix

OVER the past few years, in breakout sessions with carrier groups during the National Tank Truck Carriers’ winter and annual meetings, Qualawash Holdings president Terry O’Brien has noticed that participants in the chemical segment sessions inevitably address the issue of trailer pitting.

It’s getting worse, not better, and it’s getting far more expensive to remedy, according to O’Brien, who spoke during the National Tank Truck Carriers’ Tank Cleaning & Environmental Council Meeting held June 3 and 4 in Denver, Colorado.

“The major reason for pitting is the stainless-steel tank trailer,” O’Brien said. “There is a chromium oxide film that protects the tank surface. If that film is broken in any way exposing the low-carbon steel layer, the trailer becomes susceptible to pitting.

“Behind the corrosion dot you see is a crevice that has grown beneath it. Product can get up in there. We’ve had a number of tanks that are two years old, and all of sudden they’re pitted. They get a shell thickness test and don’t pass, so they can no longer haul hazardous materials. In some cases, you can’t even put them into foodgrade service because of all the chemicals that have been carried in them. It takes an asset that was quite valuable and makes it worthless in some cases.

“Is there something happening in the cleaning process that’s changing or causing this pitting? We haven’t changed a whole lot as far as cleaning practices. It’s somewhat embarrassing in that we haven’t evolved too much, but it’s safe to say caustic is still caustic and detergents, while they have changed, don’t have pitting property in them, generally. We’ve tried to zero in on a number of things that might cause pitting, and we haven’t been able to come up with anything.”

He listed the causes of stainless steel pitting corrosion:

•  Halides.

“It’s anything that has a chloride in it, so that’s bleach and salt. They end with “ide”: fluoride, chloride, bromide, iodide. Review your MSDS for synonyms, such as hydrogen chloride solution, that reveal halide. Don’t haul that in stainless steel. And if you do, get the tank cleaned as soon as possible.”

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•  Waste products.

“People haul waste, and you don’t know what’s in it. So it’s very likely that it has some halide in it. Anything that is a waste product itself will potentially pit a trailer. We suggest customers stay away from waste altogether and find somebody else to haul it.”

•  Product set-up.

“We are seeing more and more customers top-loading trailers, which can lead to pitting. If you had a cut on your finger and kept a Band-aid on it, it tends to get white-chalky and doesn’t heal properly. If you leave it open to air, it forms a scab and heals. It’s similar to tanks. A tank needs air to breathe. If you keep loading product on it over and over and it doesn’t have a chance to breathe, it will pit.”

•  Non-corrosive products.

“These may seem harmless—mayonnaise, detergents, soaps, water-treatment products, chlorinated solvents, waste products—but some pretty benign products can do big damage.”

•  Change in formulation.

“Check the MSDS for ingredients. There are different formulations. We’ve had a carrier who hauled the same product in the same tank for 10 years, and all of a sudden, they say the tanks are pitted. When you dive deeper and go back to the shipper, ‘Oh, yeah, we tweaked the formulation, and now we have this product.’ And it may be a chloride of some kind.”

•  Temperature change.

“A lot of that pitting happens along the steam pan. It happens a lot more on international containers that get more heating than standard tank trailers, but it’s a real issue. The solution is to maintain minimum operating temperatures, avoid heating the system, and avoid stagnation.”

•  Improper cleaning.

“By scraping the tank interior, you’re basically breaking that chromium oxide layer. Hydro blasting is best. Suitable cleaning materials are soft cloths and sponges, plastic brushes—soft cleaning pads with no particles and stainless cleaners with no particles. You’re trying to stay away from anything that is metal on metal that might scratch the tank or gouge it. Unsuitable materials are iron brushes and steel wool, scouring pads, and abrasives and sandpaper.”

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He said prevention is the best route: clean and dry as soon as possible after delivery (clean means bare-metal clean); frequent inspections; and early detection and repair.

Linings and coatings for tank trailers can provide protection from corrosion (ensures wall thickness and safe transport) and maintain product purity (prevents leaching).

Lining/coating types include: high-bake phenolics; epoxies; modified epoxies; vinyl esters and polyesters; natural and chlorobutyl rubber; Teflon; and kynar.

To select a lining/coating, he said the following need to be considered: lading; corrosion rate; pressures; erosion; cleaning procedures; temperatures; and condition of lining.

Not all products are compatible with all trailers, O’Brien said. The customer must know the construction and accessories, the prior product, the best source (shipper), and provide critical answer: Will the product attack or pit shell/coating/lining or components? Does density exceed rating? Does temperature exceed rating? Does viscosity require special venting or valving? Do regulations require special equipment, venting, or valving? Is insulation and/or heating required? Does tank have proper pressure and safety relief devices?

“You are responsible for proper packaging of products you haul—not the salesperson and not the customer,” he said. ♦

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