When we think of corrosion, we often associate this with a material (usually a metal) which is damaged or degraded when exposed to the weather (or to water). A more precise definition of corrosion could be:

Corrosion is an electrochemical reaction between a Metal and its Surrounding Environment under the formation of corrosion products.

In order to understand why this takes place, we must look a bit closer into the chemistry of metals. Different metals may behave differently, but they all follow the same principles or “Laws of Nature”, as we may call these. As the most commonly used metal, we will use iron as an example in the following illustrations, but the same principles apply to all metals.

Iron can exist in many different forms, such as pure metallic iron, iron oxide (rust), iron sulphide, iron carbonate, iron chloride, etc. If iron shall change from one form to another, this will in some cases require use of energy and in other cases release of energy. Iron will always strive to be in the form with the lowest energy level, which is iron oxide (this is a law of nature). This is why almost all naturally occurring iron found in nature is in the form of iron ore, which is basically iron oxide. It is near impossible to find a piece of metallic iron in the landscape. When we convert iron ore to metallic iron we must use a lot of heat and therefore energy (furnace in iron works). Metallic iron will always strive to get rid of this extra energy by converting back to iron oxide (rust), and this can be seen when steel is corroding. So when we are fighting corrosion we are actually fighting one of the laws of nature. We can never eliminate or defeat a law of nature. We can try to neutralise its effect or block its consequences, but the moment our defence is weakened or damaged, corrosion will immediately start up again. This is why even a tiny pinhole in an otherwise perfect protective coating is enough for corrosion to take place.

We take advantage of the law about release of energy when a metal change form (corrodes) when we make batteries used in torches and pocket lamps and all other portable electrical device. Batteries typically contain two metals with different electro-potential and an electrolyte (water that can conduct electricity). When one of the metals starts to corrode inside the battery, it will release energy in while forming of ions which will move through the electrolyte to the other metal. The result is that a surplus of electrons will accumulate on the non-corroding metal. If these two metals are connected via an external connection, electrons will flow through that connection and we have electricity which can be used to light a torch.

Some metals corrode more willingly and faster than others. Metals which corrode very little are often called noble metals, such as gold and silver, while metals which corrode very easily are less noble. We can rank different metals in accordance with how noble they are, as shown in a galvanic series table (or electro-potential series). The further away from each other the two metals in a battery is in the galvanic table, the “stronger” the battery will be (greater voltage). If two different metals are used in the same structure, we can tell which one will corrode by looking at the galvanic table (the less noble will corrode).

 

The noble metal in a battery is called the cathode and the less noble metal is called the anode. The anode will slowly dissolve into ions (corrode), while the cathode will be protected from dissolving. When old used batteries start leaking, this is caused by the anode being perforated through corrosion and the electrolyte is escaping.

 

 

The driving force behind corrosion is the same law of nature that creates electricity in a battery. Whenever we have a corrosion cell we will also have a cathode, an anode and an electrolyte. The anode and cathode may be two different metals in direct contact with each other (bi-metallic corrosion) or two different areas on the same piece of metal, but with slightly different electrochemical potential due to for example impurities or crystalline structure. The electrolyte may be seawater or rain water or simply moisture in the air. This driving force is influenced by a number of factors, such as:

 

Difference in electro-potential between anode and cathode

The greater electro-potential difference, the greater corrosion (greater driving force).

 

Difference in size between anode and cathode

A smaller anode area will corrode more quickly when connected to a larger cathode area, while a larger anode area will corrode more slowly when connected to a smaller cathode.

 

Conductivity of electrolyte

Conductivity of the electrolyte relies on the concentration of ions. Sea water has a higher concentration of ions than tap water, which is why corrosion is more rapid in seawater than fresh water.

 

Temperature

Corrosion is an electro-chemical reaction. As for all chemical reactions, they will go faster at higher temperatures and slower at lower temperatures.

 

Conductivity of the external connection

The electrical resistance of some materials makes it difficult for electron flow in the material. The opposite term is electrical conductivity. A material with high electrical resistance can be used to separate dissimilar metals and thereby minimise corrosion. Highly electrical resistant materials such as rubber or PVC are often used as isolation materials.

 

pH in the environment

Different metals react differently to the pH in their close environment. Corrosion of steel increases in acidic solutions, while strong alkaline solutions prevent corrosion (passivity). Zinc and Aluminium will show slow corrosion in near neutral solutions, but heavy corrosion in acidic and alkaline solutions.

 

 

Humidity

Atmospheric corrosion is clearly influenced by the relative humidity, typically increasing above and slowing down below 60% RH. In practice, little or no corrosion takes place at RH less than 50%.

 

Pollution

Air and water pollution may provide aggressive ions which may accelerate corrosion (including acid rain, soot and dust particles).

 

Passivity

Some metals corrode slowly due to a passive film of oxide forming on their surface, reducing or preventing further corrosion to take place. Examples: Aluminium, Stainless steel and Titanium. However, these passive films may be destroyed by aggressive species in the electrolyte or by mechanical damages, so corrosion may still take place.

 

Stress

Stress caused by various factors, such as mechanical, chemical or temperature variations may cause stress corrosion cracking.

 

Corrosion mechanism:

 

General corrosion

General corrosion is uniform by nature and results in a relatively uniform loss of steel thickness. Typically occurs on a piece of steel which may not be in contact with any other metal. Small anodic and cathodic spots are formed on the steel due to slight differences in salt level, oxygen content and steel microstructure (crystalline structure). Can be found both in submerged and atmospheric conditions. Risky if untreated, but less risky than other types of corrosion.

 

Steel with mill scale

Mill scale is more noble than carbon steel and will act as a cathode, making the carbon steel an anode. As long as the mill scale is intact it will protect the steel from corrosion (like a metallic coating). However, the mill scale is brittle and will during exposure crack, allowing water and dissolved salts to penetrate to the carbon steel. Heavy corrosion will develop on the steel, since this is anodic to the mill scale. Rust and mill scale must always be removed prior to paint application. Mill scale is most effectively removed by blast-cleaning.

 

Pitting corrosion

This typically takes place on metals with a natural protective film, such as carbon steel with mill scale, and aluminium, stainless steels and titanium with passivating oxide films. If these protective films have weak points or are damaged by mechanical or chemical exposure (e.g. chlorides), concentrated corrosion will occur on such spots. This can lead to perforation of the metal in a short time.

In pitting corrosion the metal at the top of the pit has access to the oxygen in the air and becomes the cathode. At the bottom of the pit oxygen is depleted and the metal becomes the anode. The deeper the pit is the less the oxygen available at the bottom and the corrosion rate increases. Pitting corrosion can be a very serious form of corrosion since its early stages may easily be overlooked, but may quickly penetrate the metal and cause leakage in for example pipes and tanks.

 

 

Galvanic (bi-metallic) corrosion

This is likely to take place when two  different metals (or alloys) are connected (electrical contact) to each other in the presence of an electrolyte. This will form a classical battery where the anodic metal will corrode and the cathodic metal will be protected from corrosion. The speed and intensity of galvanic corrosion will depend on things like electro-potential  between the two metals, the surface areas of the two metals, the electrolyte composition (conductivity), presence of aggressive ions (e.g. pollutants), etc. Example of galvanic corrosion situations could be carbon steel bolts used on a stainless steel construction, copper wire tied to an aluminium pipe, brass bushings used with a mild steel valve, etc.

 

 

 

Crevice Corrosion

A crevice means a tiny gap or opening. Water and dissolved salts can easily penetrate such small spaces through for example capillary action, but may have difficulties leaving the crevice,  allowing the water to stagnate. The initial corrosion will reduce the oxygen dissolved in the water inside the crevice compared to water outside the crevice. This will lead to an anodic zone  inside the crevice and cathodic outside, thus creating a concentration cell which causes corrosion inside the crevice. Crevices like this may easily be created in connections where two flanges are bolted together (e.g. a valve in a pipe)and on structures which are riveted together. Crevice corrosion does not require the presence of two different metals.

 

 

Cavitation

Cavitation is the formation of vapour cavities in a liquid – i.e. small liquid-free zones ("bubbles" or "voids") – that are the consequence of forces acting upon the liquid. It usually occurs when a liquid is subjected to rapid changes of pressure that cause the formation of cavities where the pressure is relatively low. When subjected to higher pressure, the voids implode and can generate an intense shock wave.

 

Cavitation is a significant cause of wear in some engineering contexts. Collapsing voids that implode near to a metal surface cause cyclic stress through repeated implosion. This results in surface fatigue of the metal causing a type of wear also called "cavitation". The most common examples of this kind of wear are to pump impellers, control valves, pumps, propellers and impellers, wear on ships’ propellers and rudder, mostly caused by repeated impact on the same spot of air bubbles or vacuum bubbles.

 

 

 

Erosion

Erosion corrosion arises from a combination of corrosion and the physical abrasion caused by a rapid moving  fluid, for example inside a pipe. Virtually all alloys or metals are susceptible to some type of erosion corrosion depending on the moving fluid. The best way to limit erosion-corrosion is to design systems that will maintain a low fluid velocity and to minimize sudden changes in line size or direction (e.g. elbows).

 

Materials that rely on a passive layer are especially sensitive to erosion corrosion. Once the passive layer has been removed (eroded), the bare metal surface is exposed to the corrosive environment. If the passive layer cannot be regenerated quickly enough, significant damage can occur. Fluids that contain suspended solids are often responsible for erosion corrosion.

 

 

Selective Corrosion (preferential corrosion/de-alloying)

An alloy is a mixture of two or more different metals, for example copper and zinc forming brass. If one of the active alloying elements is more active than the other(s), it will corrode away from the rest of the alloy. In the case of brass, zinc may corrode away so the yellow brass- colour of the object changes towards the red-brown copper colour. After leaching of the zinc has occurred, the mechanical properties of the remaining metal are impaired and the metal may crack.

 

In cast iron, affected areas change from grey to black by the iron corroding away, leaving behind the more noble ingredient carbon, making the dark colour.

 

Marine aluminium is commonly alloyed with magnesium. Magnesium is more active  than aluminium and corrodes during exposure. Chlorides are highly aggressive to magnesium and to the passive layer of aluminium. Residues of this corrosion are  aluminium hydroxide.

 

Stress Corrosion Cracking

Stress corrosion is the result of combination of an applied tensile stress and a corrosive environment. Metal under tensile stresses can corrode at higher rates than normally expected. The stressed areas will develop microscopic surface cracks which can accelerate the rate of localised corrosion. Once the stress cracks begin, corrosion can accelerate at these sites and weaken the metal. The metal may ultimately be perforated. The tensile stress is often the result of expansions and contractions that are caused by big temperature changes or thermal cycles. The best defence against stress corrosion is to limit the frequency of the tensile stress.

 

Stress corrosion cracking of some metals is more prevalent in certain environments:

• Copper: In ammonia solutions
• Stainless steel: In seawater
• Carbon steel: In nitrate solutions

 

 

Microbiological (Bacterial) corrosion

Microbial corrosion (also called microbiologically -influenced corrosion or MIC) is corrosion that is accelerated by the presence  of microbes. These may produce aggressive substances / ions, such as sulphuric acid, which will accelerate corrosion and damage metals  and coatings. Typical areas with suffering from microbiological (bacteria) corrosion:

• Sewers
• Water ballast tanks
• Oil tanks

 

This type of corrosion can take many forms and can be controlled by biocides, coatings and CP

 

 

Common corrosion on Steel

Almost any corrosion can occur on steel since it is a widely used construction material in all environments. However, we may observe more often in real life:

• General corrosion
• Pitting corrosion
• Galvanic corrosion
• Stress corrosion cracking

 

Common corrosion of Stainless Steel

Stainless steel develops a passive layer in open air. Halogen salts (fluorine, chlorine, bromine, iodine) are aggressive to the passive oxide film. Stainless steel is often used to make pipes, process equipment, fittings, etc. These corrosion types are common for stainless steel:

• Pitting corrosion
• Crevice corrosion
• Stress corrosion cracking

 

Common corrosion on Copper and alloys

Copper and its alloys are widely used as heat exchangers, pipes, plumbing, etc. The common alloying elements (zinc, tin) used in copper alloys are more active . In situations where copper and its alloys are commonly used, the following corrosion types are often seen:

• Erosion corrosion
• Selective (de-alloying) corrosion
• Stress corrosion cracking

 

Common corrosion on Aluminium

Pure aluminium is rarely used for construction. Construction aluminium is commonly alloyed with magnesium, copper and zinc. Aluminium will react with oxygen to form a passive layer in atmospheric conditions. These corrosion types are commonly found on aluminium

• Pitting corrosion
• Galvanic corrosion

 

Common corrosion on Zinc (Zinc coatings)

Pure zinc is not a metal that is often used for construction. Zinc is mainly used as corrosion protection coating on steel, so galvanised steel is the most common construction material with zinc. Steel items of limited sizes are often galvanized, such as pipes, handrails, screws, bolts, thin plates. Corrosion of galvanised steel can include:

• General corrosion
• Galvanic corrosion
• Crevice corrosion

 

Check list

When evaluating possibilities of corrosion, check for:

• Sharp edges & corners
• Rough welding seams / Blow holes / Weld spatter
• Bi-metallic combinations
• Drainage / Stagnant water / Accumulation of water
• Access for paint application and maintenance work: Notch radius / Stitch welds / Crevices / Narrow gaps / Design of structure / etc.
• Flow: Turbulence / Cavitation / Crevices
• Exposure: In-/ Out-doors / Aggressive ions / Chemicals
• Environment: Temperature / Humidity / Stress (applied or residual or Cyclic)

Possible consequences of corrosion

Economic

• Replacement of corroded equipment
• Overdesign to allow for corrosion
• Preventive maintenance, for example, painting
• Shutdown of equipment due to corrosion failure

Safety

Structures may become weak and no longer safe as a consequence of corrosion, creating potential dangerous situations for human life, property and the environment.

How to prevent corrosion?

Corrosion is an electro chemical process with four basic elements:

• Anode
• Cathode
• Electrical connection
• Electrolyte

Corrosion will stop if one or more of these elements are eliminated

Structural design

An ideal structure can minimize corrosion risk by:

• Providing enough space for paint application and maintenance
• Minimising steel defects which can cause early breakdown of protective film
• Minimising gaps, crevices or other shapes that can entrap aggressive or corrosive substances
• Allowing free drainage from the surface
• Good design
• Avoiding dissimilar metals connecting or by using sufficient isolation materials to stop galvanic corrosion
• Providing proper planning and methods for handling, transportation, and avoiding construction damage during fabrication

Proper materials selection

We must find the most suitable material for a given environment. Modern materials technology has developed a number of new materials such as high strength plastic, corrosion resistant steel, chemical resistant concrete.

Insulation

High temperatures and increased electron transmission will accelerate the corrosion rate. Insulation can be used to:

• Control heat exchange
• Reduce corrosion current transmission

Environment control

The most practical way to minimize corrosion by environmental control is:

• Removal of moisture in the air by de-humidification using machinery designed for purpose
• Raising the steel temperature in order to avoid condensation

Can only be achieved in confined and enclosed areas.

Cathodic Protection

Cathodic protection is used as a corrosion preventing back up system in case of coating failure. Based on the principle of the galvanic cell, the more active metal can be used to protect a more noble metal. Another way is to supply electrons to the metal to be protected by impressed current. Cathodic protection is used widely for structures immersed in water or buried in soil.

Metallic coatings

Metallic coatings can protect steel by:

• Forming a dense barrier layer (noble metal) on steel surface, e.g.: Chrome plating
• Cathodic protection, e.g. Hot Dip Galvanising

Corrosion inhibitors

It is always difficult to apply a protective film in a closed system containing corrosive fluids or gases and to find any corrosion and treat it. Corrosion inhibitors are chemical compounds which can react with water and steel to form a passive layer which will protect the steel.

Paints and coatings

This is the most common method to protect substrates from corrosion. Thousands of different paints and coatings are available in the market to protect substrates by:

• Barrier effect
• Inhibitive (passivating) effect
• Cathodic effect

Coatings’ Barrier Effect

The coating creates a barrier which prevents seawater or other corrosive agents from coming into contact with the substrate. Example: Epoxy primers

Coatings’ Inhibitor Effect

In inhibitive coatings, moisture penetrates to reach the inhibitive primer where the reactive pigment is activated, which in turn passivates the metal substrate at the coating/metal interface. Inhibitive paints are not recommended for immersion service. Example: Zinc phosphate primers

Galvanic Effect

The paint contains metallic zinc powder and acts as a sacrificial anode. Example: Zinc-rich primers