VQ wrote:Aluminium does corrode.
Having done a little research re my bus's condition I can now offer the following insight:
Experience shows us that bus structures often rust or corrode due to water leaking into the structure at window sill level
, especially in steel bodied buses.The corrosion rate is severely accelerated if insulation material and/or road dust is between the inner and outer panels, the reasons for this will become obvious as I progress. It is not just the exterior skin that is vulnerable either; but as the frame itself expands and contracts in response to sunlight, heat and cold, water can penetrate into rivet holes and thus deeper into the bodywork. The environment in which a bus operates (eg. northern tropics) significantly affects the rate of corrosion and hence it does vary from region to region. To lengthen a buses life and to reduce the downtime associated with body inspections, companies such as Volgren have been utilising Aluminium in bus construction since the early 1980's, my '83 Volgren B10M is one of their early one's.
Aluminum is actually a very active metal, meaning that its nature is to oxidize very quickly. While a weakness for most metals, this quality is actually the key to its ability to resist corrosion. When oxygen is present (in the air, soil, or water), aluminum instantly reacts to form aluminum oxide (oxidation). This aluminum oxide layer is chemically bound to the surface and it seals the core aluminum from any further reaction such as corrosion, protection can be further enhanced with surface treatments such as anodising. Aluminum’s oxide film is hard, and instantly self-renewing. This is quite different from oxidation (corrosion) in steel, where rust puffs up and flakes off, constantly exposing new metal. So how does Aluminium corrode?
As soon as Aluminium is exposed to a substance or condition which destroys its protective oxide coating it will corrode. In some conditions, the build up of simple dirt left in a buses window rubbers is enough to start the process. The oxide film is generally stable in the pH range of 4.5 to 8.5, but as soon as highly organic soils outside of this pH range, such as non-draining clays and organic muck, such as leaf litter, come into contact with Aluminium, the oxide film will break down and corrosion starts.
Other metals will also break down the protective oxide film. Aluminium should not come into direct contact with other metals such as steel framing and steel fasteners such as screws & rivets. As a preventative measure, whenever possible, aluminum should be isolated from other metals with a non-absorbent, non-conductive, insulator like bitumastic paints or polymer sleeves and washers. It is recommended that stainless steel fasteners be used wherever possible.What about "Electrolysis"?
Corrosion by way of electrolysis occurs in Aluminium when an external electric current is produced by improper or insufficient isolation of the electrical system, often referred to as stray current.
This stray current from operating systems in the bus and from the static charge of the vehicle itself, travels around looking for a ground. When the ground is located, the electrical current is discharged, no harm done.
But what happens when there is no ground to be found, or an ungrounded electrical device in the bus creates excess electrical current? If this excess current cannot be discharged by means of a ground, it continuously travels through the system, looking for a way out. During its travels, the charge causes electrochemical reactions with the various surfaces it contacts. When it locates an extremely favorable material, it goes to work devouring the material completely, aluminum affected by electrolysis will corrode rapidly in the form of flakes. Think of it as a very hungry termite, trapped in a box made of several different kinds of wood, it travels around inside the box, looking for a way out. As it goes along, it samples the various woods. When it finds the tastiest piece, it sets about devouring that piece until it can escape. Radiators are particularly vunerable to electrolysis because the water in the tank acts as one big conductor and therefore it's a bit like a big battery storing the stray current, in a vehicle generating a load of stray current, it's not uncommon for a new radiator to rust out in a few months. So what's the solution to corroding aluminium?
Once corrosion has been detected, treatment is a must. In some cases, the damage is superficial enough to be removed (scraped), and then for the repaired area to be protected with anti-corrosion products. In more severe cases the affected aluminium may need to be totally removed and replaced. It should never be ignored.
You'd be surprised at how many 'bus nuts' I've spoken with that said an Aluminium bus couldn't rust.
While it's true that they are more resistant and less likely to rust, if those window channels are neglected and leaf matter etc is left to accumulate, or if the wrong products are used in body repairs, Aluminium can definately rust out.
As far as our dear 'old' buses go, and regardless of whether it's an aluminium or a steel body, the best preventative maintenance is a simple regular inspection of corrosion prone areas (WINDOW RUBBER CHANNELS!) to ensure that leaf litter and dirt is being removed/washed from these areas on a regular basis. Parking any bus, aluminum or steel body, under a tree canopy is a definate NO! NO!
Ebay is full of examples of otherwise good buses that have been ruined as a result of being left under trees.
It wont be long before I strip the paint off those ugly blisters on my B10 (top picture). Hopefully it's only a case of a scrape and a paint with an anti-corrosion treatment, but at least I now know what's probably caused it, and I wont be ignoring it. I'll post some pics as I go. I definately wont be saying "Aluminium buses don't rust!!"