Protecting Metals from Corrosion And Rusting

How to stop rust and corrosion

One day someone had an accident and they discovered that if you stick two pieces of dissimilar metals into a bowl of acid things start to happen like blue flashes, sparks and in fact all manner of dramatic electrical things.

It's all to do with those tricky little electrons. You see they like some kinds of metal more than others. So given half a chance if metal is dipped in some nasty acid they get all excited and head off for a holiday in more comfortable surroundings. More electrons in one place than another is a potential difference, a voltage no less and we have - a battery.

Unfortunately this wonderful phenomenon has a down side. Two dissimilar metals and a bit of water or even moist air and those electrons start to go travelling and as they leave one piece of metal it starts to fall apart leaving oxides or salts which in the case of ferrous metals is iron oxide or rust. It is an electrochemical reaction. This is particularly bad in salty environments such as we find near the sea.

How do we stop rust?

Well the simplest and most common way is to insulate the steel from the environment by coating it with paint. This is usually fairly effective but any sort of flaw in the paint coating will allow rust to start. Once the coating is breached the electrical activity can be concentrated at that point and so rusting may progress even faster than if the steel wasn't painted at all. The addition of lead and zinc to paint may reduce rusting but has limited effectiveness because the paint tends to insulate the zinc and lead particles. Lead paint also has a tendency to keep the kids quiet.

Galvanising

A better option is galvanising. No I am not talking about the process of trying to get that lazy son out of bed. Galvanising is the application of a thin layer of zinc which, like lead, electrons prefer to leave. The zinc corrodes leaving an insulating skin of zinc carbonate on its surface which then prevents further corrosion.

Anodising

A similar concept is used on aluminium which is anodised – a chemical process used to create a dense protective layer of aluminium oxide (corrosion) on its surface.

Avoid dissimilar metals

Another way of reducing corrosion is to make sure that the metals used in contact with each other are the same. Copper, brass, aluminium, steel, etc. if they are exposed to the elements in contact with each other you will usually find that one will start to deteriorate.

Stainless steel

You can use stainless steel. By adding alloying materials, particularly chromium, to the steel the structure of the steel is changed to be very stable. The steel forms a very thin layer of chromium oxide on its surface to protect it from further corrosion. I won't bore you with the metallurgical explanation or of the difficulties of working with stainless steel but believe me a good marine grade stainless steel is very rust resistant.

Beware - there are many different types of stainless steel and some are still prone to rusting in the right, or do I mean wrong, circumstances. I have recently come across several stainless steel water tanks that have rusted through.

Sacrificial Anodes

Ships and steel structures immersed in sea water use sacrificial anodes made of magnesium or zinc. Electrons don't much like those. The whole point is to make the electrons flow out of the anode and not the steel and so stop the steel from rusting. The sacrificial anodes corrode in preference to the steel. Once the sacrificial anodes are exhausted they must be replaced or the steel will start to rust.

As I have said in previous articles your electric water heater and solar hot water heater have sacrificial anodes inside the water storage tank to stop it rusting the steel of the tank. When the sacrificial anode is exhausted your water heater is living on borrowed time I'm afraid.

Cathodic Protection

There is another clever method of stopping immersed steel from rusting known as cathodic protection. It is commonly used on steel oil rigs. We apply a very low voltage to cancel out the electrical activity that corrodes the steel. It can be amazingly effective. I once inspected some steel piles in the Red Sea where the electric current had been too high. The current had forced the paint coating off the steel leaving shiny bare metal sitting in salt water. I do hope I am not boring you with my metallurgical drivel.

Rust is a constant problem particularly on an island surrounded by sea with those wonderful salt laden breezes. Here in Bali we have all manner of steel that is exposed to these breezes, air conditioner units, steel boxes for electricity switch panels, ceiling fans, pumps, steel manhole covers and water heaters.

If you are close to the sea you need to consider corrosion as an ongoing maintenance problem. Here are some ideas:

• Install stainless steel ceiling fans. Be careful and make sure they are stainless steel, there are many fans around that look like stainless but are plated steel and will rust.
• Think about using stainless steel for electricity boxes and manhole covers.
• Consider installing covers on water heaters, pumps and air conditioner units to protect them but make sure that air conditioners have vents where they are needed for the fan.
• Situate external steel items on walls out of the way of prevailing winds, I saw a house recently with air conditioners totally rusted out on one side while they were still in excellent condition on the other.
• Some air conditioner manufacturers produce external units with a high quality paint finish to resist corrosion.
• After 5 years consider checking the sacrificial anodes in your water heaters.
• Don't use corrugated iron near the sea.
• Galvanise the steel or use stainless steel for external frames, balustrades, security grills and structures.
• Where possible use marine grade 316 stainless steel.