There are many kinds of corrosion which can attack metals and can cause leaks, explosions, fire, in equipment like pipelines and pressure vessels, but also can cause cracks and embrittlement of metals used for construction. Corrosion is often identified with atmospheric rusting of iron base alloys, but that is only a subclass of one of the many possible mechanisms of corrosive attack.

Some Types of Corrosive Attack on Metals

• General corrosion
• Galvanic cells
• Under-deposit corrosion
• CO₂ corrosion
• Top-of-line corrosion
• Weld attack
• Ringworm corrosion
• H2S corrosion
• Hydrogen Induced Cracking(HIC)
• Erosion corrosion
• Corrosion Fatigue
• Pitting corrosion
• Crevice Attack
• De-alloying
• Microbiological corrosion
• Intercrystalline corrosion  
• Stress Corrosion Cracking
• Hydrogen Embrittlement

General corrosion

It is reserved for straightforward dissolution of a metal in corrosive water. Theoretically, corrosion is evenly spread over the surface of the metal. Also referred to as "weightloss corrosion" or wall thinning.

Example: dissolution of steel in HCl. Simple systems like this serve to demonstrate the electrochemical basis of corrosion reactions,

• anodic reaction: Fe ----> Fe⁺⁺ + 2 e^-
• cathodic reaction: 2 H⁺ + 2 e^- ----> H₂

Part of the exposed surface, normally one half of the reaction, supports the anodic reaction; the remainder supports the cathodic reaction. The rates of these reactions adjust themselves until electrical equilibrium is obtained. Each reaction has an electrochemical potential associated with it; at equilibrium these are equal. Evenly distributed corrosion is uncommon in practice.

Galvanic cells

When different metals are exposed to a corrosive medium while in metallic contact with each other, an electric cell is formed. The current increases the corrosion rate of the least noble (base) metal (e.g. zinc), while the more noble one (e.g. copper) corrodes less, and supports most of the cathodic reaction. This is the basis of corrosion protection with sacrificial anodes. The effect depends on how far apart the metals are in the electrochemical series.

Under-deposit corrosion

This corrosion occurs when two pieces of metal, which are in metallic contact, are in different environments, as for example different concentrations of corrosive agent. Also sets up an electrical cell current which increases corrosion of electrode immersed in smallest concentration; a differential concentration cell. Happens often under deposits of sand, tar, debris etc., in combination with the presence of dissolved oxygen.

Carbon dioxide corrosion

This corrosion is very important in natural gas industry. CO₂ can causes severe corrosion of carbon steel, often in the form of sharp-edged holes. This is sometimes called mesa corrosion (mesa = table, also name of sharp-edged mountains of this shape).

Ringworm corrosion

It is a special form of CO₂ corrosion, caused by partial annealing of carbon steel pipe after upsetting the ends to enable thread cutting.

Weld attack

It is reserved for stainless steels, where welding and subsequent heat-treatment can lead to depletion of Chromium (Cr), which is scavenged in the form of carbides and in this form is no longer effective in reducing the corrosion rate.

H₂S  corrosion

H₂S  can cause pitting corrosion on several alloys. In most cases a protective sulphide is formed; weak spots in this layer can lead to enhanced corrosion. With ferritic steels, dissolved  H₂S causes hydrogen atoms to diffuse into the metal. The reason for this is that sulphides interfere with the reaction of H-atoms, originating from corrosion reactions, to  H₂ molecules. This causes the metal to become brittle. There are two forms of damage

• sulphide stress corrosion cracking (SSCC),
• hydrogen induced cracking (HIC).
  

HIC is caused by high pressure  H₂Sgas bubbles which grow at intermetallic impurities like manganese sulphide stringers. Also referred to as stepwise cracking. SSCC is controlled by keeping the hardness of the steel below 22 Rockwell. For SSCC reference is often made to a guidance (as opposed to solid advice!) document from NACE (MR0175) which also specifies the amount of  H₂S which can be harmfull. HIC is controlled by specifying low sulphur steel.

Erosion corrosion

Caused by erosion of a protective layer (oxide, carbonate, sulphide) by erosive liquid flow. The metal then tries to grow a new layer via corrosion, which is removed again etc. has little to do with erosion of the metal itself.

Corrosion Fatigue

The mechanical process of fatigue crack propagation can be enhanced when the metal is immersed in a corrosive medium e.g. water (H₂O). This effect can be reduced by application of cathodic protection, although by overdoing it, it becomes worse because of Hydrogen Embrittlement.

Pitting corrosion

A mode of corrosion of stainless steels (or alloys in passive condition). Caused by attack of chlorides on a protective (passive) layer, which is enhanced by presence of oxidising chemicals. Anodic reaction occurs at weak spots, remaining surface supports cathodic reaction. Remedial measures: more molybdenum in alloy, more Cr and Ni, cathodic protection.

Crevice Attack

Corrosion of stainless steels at spots where two surfaces are pressed together, e.g. steel to plastic, or to the same steel (bolts and nuts). Caused by loss of passivity in the resulting water film at these crevices by a differential concentration cell.

De-alloying

In some alloys one of the components can corrode selectively. Example is brass, where the zinc sometimes dissolves preferentially, leaving behind the red copper matrix which is weak and brittle. For badly understood reasons, the remedy is alloying with 0.1% arsenic.

Microbiological corrosion orMicrobiological influence corrosion (MIC)

Microbes (bacteria) can cause corrosion, even on stainless steels. The bacteria do not directly eat the metal, but their waste products are corrosive. They also can cause the development of differential concentration cells, leading to pitting. The most common type are the sulphate reducing bacteria or SRB's, which convert sulphates to sulphides and H₂S. Counter measures are: not giving them anything to eat (sulphates, nitrates, sources for carbon which they also need), and killing them by injection of bactericides or chlorination. This is difficult because they hide behind slime deposits which they create.

Intercrystalline corrosion

An alloy (e.g. stainless steel) can suffer local (normally the grain boundaries) depletion of an essential element for corrosion protection (e.g. chromium), as the result of heating. This is called sensitisation. When such an alloy is exposed to oxidising media, the corrosion proceeds along the grain boundaries and the alloy disintegrates into grains.

Stress Corrosion Cracking (SCC)

Specific combinations of alloy and environment can lead to SSC, when the metal is mechanically stressed while being exposed to this environment (the stresses can also result from the fabrication process!). The metal then fails at a load far below its nominal mechanical strength. Almost every alloy and metal has its specific enemies in this respect.

For example, when stainless steel in hot chloride containing water, and there is a sufficient supply of dissolved oxygen, a network of branching transgranular cracks results. More oxidising conditions i.e. more anodic potentials enhance this type of stress corrosion, which is therefore called "anodic cracking". Intergranular cracking where cracks follow grain bounaries is also possible. Cathodic SCC also exists, and is caused by hydrogen embrittlement.

Hydrogen Embrittlement

A generic name for all kinds of problems caused by the dissolution of hydrogen in metals, which causes loss of ductility. One of the most important examples is the SSCC, caused by the presence of H₂S.