CORROSION CONTROL DEPARTMENT

Structural Asset Integrity Management (SAIM)  

The aging structure of onshore and offshore installations present the oil and gas industry with a constant and growing challenge where corrosion control is concern.Aging is, in general characterized by deterioration, which in severe environment can be significant with serious consequences for people, product and installations integrity if not managed properly.

Corrosion Control
• Corrosion Control is one of the key issues in structural asset integrity management(SAIM). This mitigation consist of:
• Design (material)
• Coating
• Cathodic Protection
• Inhibitors

 

Pipeline being coated with tape.

• Corrosion
• Most metals occur in nature in the chemically combined state. Energy, generally in the form of heat, must be supplied to produce metal from an ore.
• When exposed to natural environments such as a saline atmosphere, moist soil, or water, metal has a strong tendency to lose this added energy by returning to a combined state. This tendency takes the form of corrosion.

 

The Corrosion Cell

• The chemical changes brought about by corrosion are associated with the flow of electrical currents.

• Current discharges from the metal surface at anodic areas (corrosion) and returns to the surface at cathodic areas (no corrosion).

• Where the moisture volume in contact with the metal is small (e.g. condensation on the wall of a storage tank), the separation between anodes and cathodes will be small (i.e. 1mm) with the electric currents correspondingly small.

• Where the moisture volume is large (eg a pipe buried in moist soil), the separation between anodes and cathodes can be very large (even many km) with the electric currents correspondingly relatively large.

• Since the metal loss is directly proportional to the amount of electric current discharging at anodes, it is evident that the corrosion taking place on structures buried in the ground or immersed in the sea can be made much more serious than corrosion in the atmosphere.

• Corrosion is always associated with differences – either in the metal itself or its environment. Where these differences are significant (e.g. a mixture of two types of metal) any resulting corrosion is likely to be equally significant.

Consequences of Corrosion

If corrosion is judged to be inevitable, so also is:

• Continuous damage to process plant, structural assemblies and other equipment
• Consequent shutdowns for repair or replacement work
• Contamination of the environment
• Loss of product
• Loss of operating efficiency
• Over design
• Unfavorable publicity associated with hazards to the public and/or environment
• Customer alienation.

 

                                                                                Crude oil spill due to pipeline failure                                 Ruptured Pipelined                                                                          Pipeline repairs

 

• Corrosion is an insidious process, often difficult to recognize until deterioration is well advanced. Its effects are serious but, if considered at the correct time, they can be economically controlled.

• Solutions to engineering problems are often based upon the consequences of short-term failure without the consideration of the less apparent long-term consequences of failure due to corrosion.

• The use of the term 'Corrosion Control' –as opposed to 'Corrosion Prevent'- is significant. It is very difficult to ensure complete freedom from corrosion under most practical conditions due to the difficulty in monitoring the performance of anti-corrosion techniques and methods. This will particularly be the case where the metal surface cannot be seen.

• The following corrosion control methods should be considered:
1. Changing the Environment
2. Design
3. Barrier Coating
4. Cathodic Protection

• Changing The Environment
• The most obvious application is in the placement of pipework above ground instead of underground. This gives an optimum solution since the condition of the pipes can be viewed and subjected to routine inspection.
• The routine drainage of water separating out in oil storage tanks may also be considered under this heading.
• The importation of special back-fill material for buried pipes (e.g. sand or limestone dust) can be beneficial in two ways. The pipe environment is made more uniform and damage to the pipe coating by rocks and stones is avoided.

• Design
The placement of pipework above ground may also be considered under this heading.

• Accessibility for painting and visual inspection
• Avoidance of mixed metals –e.g. use of stainless electrical earthing rods instead of copper.
• Avoidance of water traps
• Incorporation of wearing surfaces

• Barrier Coating
• The term is self-explanatory: an impervious barrier is formed such that the metal surface is not in direct contact with an electrolyte.

• There are other specialized coatings which are active in that they contain chemical corrosion inhibitors to provide a form of cathodic protection (eg galvanizing).

• Barrier coatings will include normal paints and pipe-wrapping tapes or factory applied pipe coatings of coal tar, bitumen or epoxy.

• Assuming that a barrier coating is and remain, perfect throughout its required life – ie no pinholes, scratches or areas of exposed metal – corrosion will be completely prevented

                                                                                        Surface preparation                                                                          Coating Application                                                                 Tape application

• Except in special cases, such perfection is not practical. Particularly in the case of buried or immersed structures, this means that whilst the total metal loss due to corrosion will be reduced, any corrosion which does take place will be concentrated at the coating imperfections. \

• Thus, pitting attack is promoted and the rate of penetration will be significantly increased – most undesirable for tanks or pipelines. To eliminate this phenomenon cathodic protection is applied.

• Cathodic Protection

Cathodic Protection can only be used where the metal surfaces to be protected are in contact with a large volume of electrolyte such as when buried in the soil or immersed in the sea.

• Principles
In the freely corroding state a number of anodes and cathodes are formed. Cathodic protection involves the introduction of an artificial anode which is allowed to corrode and produce its own corrosion current. This deliberately-created corrosion current can effectively 'swamp' all the small naturally occurring currents. The result is one (or a series of) artificial anodes and one cathode (i.e. the structure to be preserved). When fully effective the cathode is maintained free from corrosion. The amount of protective current required is directly related to the surface area of metal exposed to the environment.

 

                                                                                           Impressed Current Cathodic Protection System (ICCP)

 

                                                                                                                          Sacrificial Anode System

 

Sacrificial Anodes
• Cathodic Protection in its simplest form. The dissimilar metals – copper and steel – create a simple electric cell. The steel forms the anode and the copper the cathode.

Impressed Current
• Here the artificial anode(s) are of inert material and current is forced to flow by using a direct current power source such as a battery charger (more correctly called a transformer rectifier). This system is used where larger amounts of protective current are required.