The Danger Of Internal Corrosion In Fluid Handling Equipment

Industrial & Manufacturing Articles

The internal corrosion of fluid handling lines is a critical phenomenon that determines the structural integrity of the component. If this process is not understood and mitigated against, it can have devastating consequences. Not only does this disintegration cost suppliers a significant amount of money to correct, but if it is not protected against, then it can pose significant dangers to human life. If you currently work with fluid handling equipment, like that from PFC Equipment, Inc, it's extremely important you understand the mechanisms in action within these members. 

Types of Corrosion

Water by itself is not corrosive; however, when mixed with other substances, it can form a highly corrosive liquid that attacks the integrity of equipment. The two main types of corrosion are: 

• 'Sweet' corrosion - water mixes with carbon dioxide.
• 'Sour' corrosion - water mixes with carbon dioxide and hydrogen sulphide. 

These processes can attack the equipment globally (along the entire surface) or locally (within a concentrated region). The latter is generally considered to be the worse corrosion mechanism as it causes differential corrosion between unprotected and protected areas. 

'Sweet' Corrosion

Sweet corrosion refers to the deterioration of metal in contact with carbon dioxide and water. Typically, the mechanical process can be broken down into four stages: 

• Carbon dioxide dissolves in water and creates a highly corrosive liquid.
• Carbonic acid is released as a byproduct of this reaction.
• Carbonic acid reacts with steel, which causes pitting or material loss.
• The carbonic acid may act to form a film over the surface.

Sweet corrosion may also be referred to as "CO₂ corrosion" and occurs in the absence of hydrogen sulphide. CO₂ corrosion can cause significant internal damage to equipment, the extent of which is determined by a number of factors: 

• Temperature - Sweet corrosion rates increase with temperature until a protective film can form on the equipment and slow down further damage.
• pH level - In general, corrosion rate decreases with increasing pH levels.
• Flow rate - High flow rates accelerate the process of sweet corrosion by eroding the line. This gives the water and CO₂ mix a chance to attack bare steel, which will deteriorate quickly. 

 'Sour' Corrosion

Sour corrosion is a similar phenomenon to sweet corrosion; however, it occurs in the presence of both hydrogen sulphide and carbon dioxide. This form of corrosion attacks the member by 'hydrogen cracking', a process that is well understood in the fluid handling industry but remains difficult to protect against. The main forms of hydrogen cracking induced by sour service are: 

• Hydrogen Induced Cracking (HIC) - This is associated with non-metallic inclusions in the member. The hydrogen in the solution recombines to form hydrogen gas, which collects under any deformation in the pipe and causes visible blistering on the surface.
• Stress Oriented Hydrogen Induced Cracking  (SOHIC) - SOHIC is a byproduct of HIC and is characterized by small 'ladder-type' cracks. These cracks align perfectly perpendicular to the stress direction, which gives rise to its name. The main difference between SOHIC and HIC is that SOHIC tends to form locally around welds as opposed to normal steel. 
• Soft Zone Cracking - This is a similar phenomenon to SOHIC; however, it occurs in weld areas where the material has been softened by exposure to intense heat.   

Localized Corrosion

Localized corrosion is not a phenomenon by itself; rather, it is the result of sweet or sour corrosion concentrated over a small area. This concentrated attack gives rise to three main forms of damage:

• Pitting - Small groups or 'pits' that have been attacked due to structural discontinuity with the rest of the member. This can lead to small groups of 'craters' experiencing high corrosion whilst the rest of the surface remains relatively unaffected. Pitting can introduce a cyclical process whereby the pits that were previously attacked become hot-spots for further attack, leading to high damage over a small area.
• Flow Induced Localized Corrosion - Occurs where turbulent conditions cause the metal's protective layer to disintegrate, leaving the bare material open for localized attack.
• Mesa Corrosion - If the entire surface is exposed to moist carbon dioxide and high temperatures, a protective layer can form as mentioned above. However, if there are any areas where this layer doesn't form, corrosion can occur. This means that corrosion occurs only on the unprotected areas, whilst the rest of the metal remains unaffected. 

Evidently, there is a strong need to mitigate against these destructive processes, such that members handling fluid retain their structural integrity over their design life. There are a number of methods commonly used to do this; however, it is noted that a combination of different methods is the most suitable way to reduce the risks associated with internal corrosion. 

21 January 2015