Diesel Fuel Additive Corrosion Testing

Before any testing project can begin, we try to understand as much as possible. While on some subjects I struggle, some are closer to my heart and background. As an API tank inspector and 30 year chemical engineer, tank corrosion is one of the latter.

It's nice when there is an industry proven and well documented test protocol, but many boat applications lack these; they are just not high volume like automobiles. Often regulators try to apply automotive rules and standards, and like the fuel tank vent filters, they often miss the mark completely. Corrosion testing is one of those, since marine systems are completely different.in how they are used and what they see.

I came across a study by Battelle, funded by the API intended to pin the blame for ULSD corrosion problems in part on ethanol. While I don't think much of our whole ethanol policy, I feel Battelle failed to make a conclusive case; there are simply too many factors, even though they may be right in part. They knew this, careful stating that the hypothesis was possible but not assigning causality. The most important failings were that most of the samples displayed some evidence of treatment with additives (specifics not determined), that other corrosive chemicals were present, and that there was no control case (one tank was asymptomatic, but it contained similar contaminants). Thus, the sample group was not well controlled, the fault of the client and not Battelle. In fact, because the data doesn't fit the hypothesis I believe much of it is honest science, with the conclusions overly extrapolated by the client. The Ethanol trade group responds by accusing the API of bad science and warped publicity, an amusing example of the skillet calling the kettle black.

For example, look at the composition of the bottom water. Note that NY-1 was non-symptomatic, while the other 5 displayed serious corrosion issues.

Chloride. Since this wasn't in the fuel to begin with, I would like to assume this is seawater. The NC-1, NY-1 and NY-2 samples have enough sodium to support this--in fact the the bottom water is 7-25% seawater, and I would think marine tanks could rationally be higher.

Glycolate and formate. Interestingly, these are primary decomposition products on ethylene glycol based antifreeze, and they were only found in the non-symptomatic tank. I wonder if that tank was somehow exposed to antifreeze, which would stop corrosion.

Acetate. Almost certainly from acetobacter, as they have supported with DNA testing. Was the food source ethanol? That is IMHO a stretch, but that doesn't mean organic acids are not common bacterial by-products. These levels are high, like 5-20% vinegar.

Fluoride. Common ingredient in hydrodesulfurization catalysts, though I'm very surprised to see it in these concentrations. It must accumulate in the water phase, suggesting that some of the water in these tanks came with the delivery. Finely dispersed water droplets must have collected the ions. Could be significant contributor to tank bottom corrosion, similar in action to salt water. I found it curious that they never mentioned this data.

pH. Supports organic acid numbers, though there has clearly been some neutralization. Could biodiesel contamination also affect some diesel? Sure, but all we can do is go by what we have measured; low pH. Like many other analytes that are affected by biological activity and pH--nitrate, ammonia, carbonate, calcium which can precipitate--it hints at the current condition but not so directly at a contaminant source.

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While there is too much going on to nail down cause and effect, I think it's clear enough we can look at adding seawater and vinegar at dosages similar to those above. In practice, to insure some acceleration, I will increase them somewhat, using 25% seawater and sufficient vinegar to reach a pH of 3.5 (this isn't far from the pH of vinegar, so I will limit acetate to 35,000 ppm, which is a 40% solution of white wine vinegar). I think I can support my basis as more valid than the industry standard tests, which is designed around pipeline transport and uses distilled water and mixing.

In the end, we can prove which corrosion inhibitors will work all of the time, only which work in a specific set of circumstances, and that those circumstances represent a realistic scenario. Practical engineering with a few roots in chemistry.