Some years back, I realized I was chasing the rust on my auto bodies and each time I carefully sanded, primed, and painted it away, it came back even worse. So I did a little research. It seems that rust is an electrochemical reaction, and once started, creates a surface that resembles the topography of the Himalayans. Most every rust preventive method fails because the surface prep is imperfect, and once started, it is virtually impossible to get rid of it all. The gold standard is to grind/sand blast back to clean metal, and dip it in molten zinc - heavy hot dipped galvanizing. This can last a long, long time, but it too will fail because the zinc is sacrificial. Plated zinc is much less effective but aided by several coats of paint the rust can be held at bay for reasonable lengths of time. The problem even here will be sheared edges - building up a thick enough protective film on sharp corners and edges is almost impossible - and not cost effective from the mfr's point of view when your part/swing arm will last 8 years and then they get to sell you a replacement. Welding brings in dissimilar metals - welding rods and wire are alloys - so we are introducing even more galvanic action.
The good news is you can delay the inevitable by frequent flooding of the hollow interior with oil. Waxes (some of them) dry out, shrink and crack and the protective film now has voids. Tar-like compounds are similar. Even worse, is when these thick coatings pull away from the metal and now we have a pocket that can trap moisture. Remember the topography? Once water can inflitrate up a valley under the coating the corrosion returns. Don't forget that as we fight rust, the departments of transportation are doing their best to make ice melting solutions that act faster and stick longer (they add sugar to the brine now) or are 'hotter' which also make bigger, deeper valleys in our steel. The folks who spray the interior of auto body panels with oil have a great idea. And it works if you don't do it only once. So if you want to regularly flood the interior of your swing arm with used motor oil, you can significantly prolong its life. How often do you spray? Please treat a significant sample of swing arms and report back to us. Oh, and note the brand of oil that works the best. (grin)
Seriously, navies have experimented with electronic devices that send an electric current through the ship in opposition to the polarity of the chemical reaction. This works until you find dissimilar metals in a salt water enviroment. Downsides? The hardware costs a fortune or two, and if you get it wrong you can electroplate the ship into the ocean so fast that you can call it a cloaking device as the ship disappears before your very eyes. Paint is cheaper.
