MAiNTENANCE CoRNER

By Duncan Bennett, Member #4171

This is a bit of a “left field” maintenance topic, but if your non-negotiable goal or at least prayer is to maintain the overall look of your bike and prevent male bolts and screws welding themselves into their respective female counterparts like a [awesome analogy too risque to print here] you need to think metallurgy. Galvanic corrosion to be precise. What is galvanic corrosion, and what does it mean when you pull up in Surfers Paradise alongside a group of fit looking young men/ladies (strike out whichever not applicable to your situation) in the middle of a small underwear shoot? It means that the polished stainless steel fuel tank you just installed to impress young men/ladies (go again with the striking out) is sucking electrons from that small busted-arse high carbon steel bolt you fixed it on with like a Dyson in a ‘roid rage. Losing electrons means rusting if you are iron, and corroding if you are any other metal, but either way you are disappearing. So one day your small busted-arse high carbon steel bolt will literally do that, your fuel tank will come adrift while on the M1, and unfortunately smack your fit young man/lady (you know what to do) pillion in the face. You won’t see either again. So how do we stop galvanic corrosion, or at least slow it down? It isn’t always easy with a new bike because the manufacturer will have a parts list, but not what the parts are made from, so over time we just have to accept that rust will start to appear.... Mirror stems are a traditional rust hotspot on BMWs from the late 1990’s and early 2000’s. These are made from low quality steel and chromed, so the electrons are sucked out until the rust gets started and the chrome starts to flake off, no coincidence that it starts at the bottom near the couple between the low quality and higher quality metals. This is a classic cause of galvanic corrosion - a small low quality metal part is coupled to a large high quality metal part - most of the bike including the engine block. So that is rule No.1 - small parts that you need to undo should always be of higher quality than the large parts, think 316 stainless steel for fasteners. The large mass of lower quality metal has more than enough electrons to keep the small high quality fasteners happy for a very long time.

Often we have no choice on small parts, think Bunnings who rarely seem to have the right fastener in the right quality and you end up with zinc

plated stuff. In this case you can use rule No.2; insulate. Think of those things you see up power poles and electrical wires in general - there is non-conducting insulation that keeps the wires separate from the metal bits of your bike that your sweaty regions are electrically attached to. You can simulate this insulation if you can use a piece of plastic tube on a through-bolt, use a fibre washer, or just coat it with anti-sieze, the electrical connection between dissimilar metals is not there or is very poor - think what happens on mild steel bolts and nuts over a long period of time if anti-sieze isn’t there - the rate of corrosion is slowed right down. Rule No.3 - offer up a sacrificial lamb, although literally attaching a small sheep to your bike doesn’t work terribly well and shouldn’t be attempted especially if you volunteer out at the RSPCA. It is actually a sacrificial anode, in other words the deliberate attachment of a piece of more easily oxidised metal to the bike. My first bike was a Honda XR250, last of the rear drum brake models. The drum brake housing was made of a magnesium alloy - very light. On one particular adventure near Norseman in WA, I decided to take a short cut across Lake Cowan, one of the huge salt lakes in that area. I almost made it, about 10m short of land the bike went through the crust. Impossible to pull out on my own, so I walked home. The bike sat in hypersaline ooze all night. The next day a friend and I managed to get it out, and I rode it home. I noticed the back brakes weren’t working, and assumed it was just the mud. After the longest and most thorough bike cleaning of my life, I was surprised to discover the bike was in tip-top condition, except for the rear drum brake housing. Which had almost ceased to exist. It had made the ultimate sacrifice and given up its electrons so that the rest of the bike could live, luckily we got the bike out when we did otherwise the next thing in the sacrifice queue might have been less easily replaced.