Or could the acidity simply be common in the small bits of water we get with the fuel? That it concentrates in tank bottoms, as condensate extracts what is in the fuel and drags it down? Can't say, other than the tanks with no Raycor water were also tested by extracting the fuel with 5% distilled water and found to have neutral pH.
When I looked at the corrosion testing coupons after just 6 days hours of exposure to our synthetic corrosive mixture, only the most robust inhibitors were doing well, and uninhibited samples were doing very poorly. Premature tank failure from internal corrosion seems likely in most cases.
(No pictures--Sorry. I don't like disturbing the coupons until the exposure is finished)
Interestingly, some formulas that did fairly well in e-10 gasoline testing are not impressive. Not too surprising, now that I think about it, as the ethanol in gasoline has a major effect on corrosion chemistry and inhibitor action. With e-10 the salt solution is always in contact with everything as the ethanol dissolves the water and salt. Not true in diesel. For example, Seafoam did well in e-10 but is failing badly in diesel. One treatment, Valve-Tech, which claims to help suspend water, is actually making the situation significantly worse, with the fuel becoming discolored in just 36 hours. We saw the same trend in testing gasoline additives; any additive that suspended water tended to make corrosion worse, presumably because it made the water more available...
... Which should remind the reader that my gasoline testing has all centered on e-10 for the selfish reason that it is what I have. It is what most of us have. While the lack of ethanol is not likely to make the winners do badly, ethanol may be the reason some failed; they were never formulated for it.
Biocides are failing. Well, they never made anti-corrosion claims.
What is working? Star Tron is doing very well, at least on the upper sections. I believe this is in part because Star Tron is an effective emulsion breaker, preventing water suspension. In the past Star Tron has been unimpressive, generally not doing much, but then again, the effects were always positive. This time it is headed towards being a clear winner. Stabil is doing well, again; while it has never been "best" in in any of my testing, it is always in the upper quarter, suggesting versatile formulation.
The bottom line?
- Check the pH of water collected in your fuel/water separator. If the pH is less than 5.5 you have a serious problem. Empty and clean the tank, begin using a good inhibitor, begin practices such as vent filters and on-board polishing to keep the fuel dry, and begin using a biocide to prevent infection.
- Check the vapor space of your fuel tank for signs of corrosion (such as the sending unit area). Corrosion from acid products seems to appear first in the vapor space.
- If I had a metal--steel or aluminum--tank I would consider installing corrosion coupons somewhere in the fuel train; a corroded tank deep in the bilge sounds ugly. Perhaps a washer of the appropriate metal in the filter housing would work for many with minimum hassle and maximum probability that changes would be noticed.