The process of basic corrosion is part chemical and part electrical. It requires a combination of moisture and oxygen. Once those two elements – both of which are a constant presence throughout any building’s life cycle – come together, and in contact with steel, a multi-stage process begins, as
- First, the iron (Fe) atoms that comprise steel lose some electrons and become positively charged. Positively charged ions attract negatively charged ions.
- Second, water (H2O) and oxygen (O), mix together and become even more negatively charged, thus attracting themselves to the positively-charged iron atoms mentioned above. The result is a chemical called iron hydroxide (4Fe(OH)2).
- Iron hydroxide continues to react with oxygen, yielding 2Fe2O3.H2O – also known as hydrated iron oxide OR brown rust.
As long as there is no barrier between the iron and the water/oxygen molecules – and as long as the electrochemical reaction is allowed (via poor design, materials selection and/or neglect) to take place, the steel will continue to react until all that is left is a pile of brown rust and the rest of the building rubble all around it.
A second type of corrosion that affects steel members is called bimetallic corrosion. This type of erosion occurs when a chemical reaction is caused by two metals coming in contact – or close contact – with one another. This type of corrosion is more common in metal alloys and is quite complex as there are multiple variations, but it is partly dependent upon any two metals’ respective positions in the galvanic series.
Bimetallic corrosion occurs most frequently in steel structures that are submerged or buried, but working with a reputable metal building supplier will ensure you building is designed with respect to any potential bimetallic corrosion, using proper precautions where necessary.
Certain environmental pollutants, toxins and compounds can exacerbate either one of the above forms of corrosion, which is why your building’s location plays a key roles in the types of materials and protective coatings that are used. Buildings most susceptible to environmental corrosion are those in an industrial or manufacturing areas where off-gassing and toxic emissions are higher than normal, as well as buildings located in coastal environments, exposed to higher levels of salinity.
Thus, the typical causes of corrosion on structural steel members include:
- Uncoated steel.
- Steel that has not been coated with respect to the particular environment.
- Steel that has not been properly maintained.
- Lack of a vapor barrier and/or adequate insulation inside the building.
- Unaddressed maintenance issues such as leaky roofs, plumbing leaks, standing water, etc., that lead to chronic exposure to moisture.
- Incorrect design/construction of the building foundation.
The consequences of corrosion are many and varied and the effects of these on the safe, reliable and efficient operation of equipment or structures are often more serious than the simple loss of a mass of metal. Failures of various kinds and the need for expensive replacements may occur even though the amount of metal destroyed is quite small. Some of the major harmful effects of corrosion can be summarised as
- Reduction of metal thickness leading to loss of mechanical strength and structural failure or breakdown. When the metal is lost in localised zones so as to give a crack-like structure, very considerable weakening may result from quite a small amount of metal loss.
- Hazards or injuries to people arising from structural failure or breakdown (e.g. bridges, cars, aircraft).
- Loss of time in availability of profile-making industrial equipment.
- Reduced value of goods due to deterioration of appearance.
- Contamination of fluids in vessels and pipes (e.g. beer goes cloudy when small quantities of heavy metals are released by corrosion).
- Perforation of vessels and pipes allowing escape of their contents and possible harm to the surroundings. For example a leaky domestic radiator can cause expensive damage to carpets and decorations, while corrosive sea water may enter the boilers of a power station if the condenser tubes perforate.
- Loss of technically important surface properties of a metallic component. These could include frictional and bearing properties, ease of fluid flow over a pipe surface, electrical conductivity of contacts, surface reflectivity or heat transfer across a surface.
- Mechanical damage to valves, pumps, etc, or blockage of pipes by solid corrosion products.
- Added complexity and expense of equipment which needs to be designed to withstand a certain amount of corrosion, and to allow corroded components to be conveniently replaced.
By retarding either the anodic or cathodic reactions the rate of corrosion can be reduced. This can be achieved as
Conditioning the Metal – This can be sub-divided into two main groups
- Coating the metal , in order to interpose a corrosion resistant coating between metal and environment. The action of protective coatings is often more complex than simply providing a barrier between metal and environment. Zinc coating in iron or steel confers cathodic protection
- Alloying the metal to produce a more corrosion resistant alloy, e.g. stainless steel, in which ordinary steel is alloyed with chromium and nickel. Stainless steel is protected by an invisibly thin, naturally formed film of chromium sesquioxide
Conditioning the Corrosive Environment – Corrosion Inhibitors – A corrosion inhibitor is a chemical additive, which, when added to a corrosive aqueous environment, reduces the rate of metal wastage.
- anodic inhibitors – as the name implies an anodic inhibitor interferes with the anodic process, examples of anodic inhibitors include orthophosphate, nitrite, ferricyanide and silicates.
- cathodic inhibitors – the major cathodic reaction in cooling systems is the reduction of oxygen. Zinc ions are used as cathodic inhibitors.
- adsorption type corrosion inhibitors– many organic inhibitors work by an adsorption mechanism.
Electrochemical Control – Since corrosion is an electrochemical process its progress may be studied by measuring the changes which occur in metal potential with time or with applied electrical currents.