Pipeline corrosion environments

Rubber sheets are the most important materials for the construction and fabrication of much chemical plant equipment such as storage tanks, reactor vessels, pipelines, seals, hoses and rubber lined mild steel equipment of different sizes and shapes, inflatables, etc., and almost all equipment subjected to different corrosive environments. The rubber sheets can be either plain or fabrics coated with rubber. The fabrics used for coating are nylon, rayon, cotton or various other synthetics. Rubberized fabric cords are also used as reinforcing members in various products. 

Stress Corrosion Cracking One of the serious forms of pipeline corrosion is see. This form of corrosion consists of brittle fracture of a normally ductile metal by the conjoint action of a specific corrosive environment and a tensile stress. In the case of underground pipelines, see affects the external surface of the pipe, which is exposed to soil/ground water at locations where the coating is disbonded. 

External corrosion of pipelines is due to a continuously varying environment of different kinds of soils, sand, marshy land, brackish water, riverbeds, and sea organisms and microorganisms, especially SRB. In addition to the corrosive environment of soils due to moisture, minerals, oxygen, chloride, and so forth, which are the prime factors for initiation of corrosion, the following factors further enhance corrosion. 

Erosion corrosion is a severe problem in transportation underground pipelines. This is because crude transportation occurs at high speed, which can cause cavitation corrosion this, when coupled with a corrosive environment due to retained brine, can cause erosion corrosion, leading to severe hole formation. 

Cathodic protection A means of applying an external electric current to reduce corrosion virtually to zero. A metal surface can be maintained in a corrosive environment without deterioration as long as the cathodic protection system provides the external current. Cathodic protection can be galvanic, such as that on galvanized pipe, or it can be impressed current, such as that usually found on large potable water tanks and pipelines. 

Extensive investigations of potential criteria of gas pipelines were sponsored by the American Gas Association in 13 field stations in the U.S.A., Canada, and Australia. Results published by Barlo (1994, 1996, 1997) distinctly point to the complexity of the problem. A large effect was shown of the properties of the corrosion environment, i.e., the moisture in the soil, its resistivity, and its corrosivity, and of the temperature on parameters of cathodic polarization. The last edition of NACE Standards RP 0169-... 

Offshore platforms are, in essence, similar to buried pipelines because in both, external and internal surfaces are exposed to corroding environments in buried pipelines the external surface of the pipe is exposed to the soil (which is a corrosive environment), and its internal surface is under the corrosive impact of the fluid that is going through, either water, oil or the like. In case of offshore platforms, the whole immersed stmcture is exposed to seawater (a corrosive medium), and the internal surfaces of the systems such as seawater injection systems or oil storage facilities can be considered locations at which corrosion is occurring internally. 

As shown by Figure 1, also corrosive environment and sand under deposit can lead to degradation (in this case corrosion) of the pipeline wall. The gravel pack is again indicated as safety barrier because it can prevent sand production and, thus, sand deposit where the flow is slowed down by line bends. 

This is one of the oldest techniques of corrosion protection of steel in a corrosive environment, such as water or soils. Cathodic protection has been applied very successfully to the protection of pipelines, storage tanks and underground structures in petroleum and oil industries. Cathodic protection has been widely applied for the protection of concrete structures. In principle, a small negative voltage is applied to the steel in concrete which makes all the steel surface the cathode and eliminates the anode areas on the steel surface. This technique has been discussed in detail in Chapter 5. The cathodic reaction at the surface leads to increased alkalinity which passivates steel. 

At high temperatures, active sulfur can directly react with metals at any locations in contact with processing media. This sulfidation attack is a kind of uniform corrosion process. H2S is the most corrosive compound in these sulfides. The typical corrosive environments are located at the bottom of the towers of the atmospheric and vacuum distillation systems, pipelines, atmospheric heavy oil heat exchangers and vacuum residuum heat exchangers, the bottom of the main fractionating columns of catalytic cracking equipment and the coke retarding equipment. 

Types of damage can be classified as uniform or localized metal removal, corrosion cracking or detrimental effects to the environment from the corrosion products. Local attack can take the form of shallow pits, pitting, selective dissolution of small microstructure regions of the material or cracking. Detrimental effects are certainly not the case with buried pipelines, but have to be considered for environments in vessels and containers. It is usual, where different results of reactions lead... 

The greatest possible safety against corrosion damage is achieved by passive protection with coatings in combination with cathodic protection. Therefore coating and cathodic protection of pipelines that have strong safety requirements are compulsory in order to protect both people and the environment [2-5]. 

Only a small amount of the metal used in underground service is present in the ground water zone. Such structures as well casings and under-river pipelines are surrounded by ground water. The corrosion conditions in such a situation are essentially those of an aqueous environment. 

Oilfields in the North Sea provide some of the harshest environments for polymers, coupled with a requirement for reliability. Many environmental tests have therefore been performed to demonstrate the fitness-for-purpose of the materials and the products before they are put into service. Of recent examples [33-35], a complete test rig has been set up to test 250-300 mm diameter pipes, made of steel with a polypropylene jacket for thermal insulation and corrosion protection, with a design temperature of 140 °C, internal pressures of up to 50 MPa (500 bar) and a water depth of 350 m (external pressure 3.5 MPa or 35 bar). In the test rig the oil filled pipes are maintained at 140 °C in constantly renewed sea water at a pressure of 30 bar. Tests last for 3 years and after 2 years there have been no significant changes in melt flow index or mechanical properties. A separate programme was established for the selection of materials for the internal sheath of pipelines, whose purpose is to contain the oil and protect the main steel armour windings. Environmental ageing was performed first (immersion in oil, sea water and acid) and followed by mechanical tests as well as specialised tests (rapid gas decompression, methane permeability) related to the application. Creep was measured separately. 

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