Gasoline Additive Corrosion Testing

4-2016, rev. 7-2024

I've been playing mad scientist again, conducting laboratory corrosion testing experiments for Practical Sailor Magazine.

In my experience at least, corrosion products often foul carburetors, and I've learned how to field strip and clean carbs far too well. Can storage additives actually reduce corrosion? I ran some modified ASTM tests to see. Since I've done this for years with engine coolant samples in my day job, this was something I feel very comfortable with. The set-up and procedure is a bit different, but it's still familiar ground.

Standard metal coupons are exposed to e10 in varying conditions for 1 month. To induce galvanic effects they are grouped copper/steel/brass to simulate a tank system and steel/aluminum/brass to simulate a carb bowl.

The test bottles look like this.

 

After exposure, the sealed control looks like this.

Without an additive, with the addition of 0.03% seawater and allowing limited venting, we get this.The black spots are corrosion pits, not dirt.

With a good additive we get this--effectively no corrosion even with salt

And with a poor additive it can be even worse than with nothing at all

Not a realistic test? Actually just an acceleration. The pitting looks very much like this 12 -year old Yamaha 9.9 carburetor; I have 4 of them and they all look about the same, just like the aluminum coupons.

And the successful additives weren't always the ones you would guess or be led to believe by advertising and marine catalogs, not at all. Unfortunately, If I give more than a teaser, PS will strangle me.

I will share this:

• Without air and water there is NO corrosion by e10.
• Without saltwater the corrosion is very, very slow. If salt spray, even the TINIEST AMOUNT can sneak into you tank, you're going to have trouble within weeks. The effect was dramatic.
• One of the very best additives did NOT come from the boat store. The worst ones did. Go figure.

It turns out that the NMMA (National Marine Manufacturers Association)has formed a working group to develop standards for gasoline additives. IT seems they have found that some are damaging their engines.