In order to avoid corrosion of the pipeline,you must first know its types

In the last article we learned what is pipeline corrosion and its process,you must want to avoid it.As the saying goes, “know the enemy and know yourself, and you can fight a hundred battles with no danger of defeat”,the first thing you must do is to know its types.So in order to avoid corrosion of the pipeline,you must first know its types.The following are some common types of pipeline corrosion.

Uniform corrosion of  pipeline

Uniform corrosion of pipeline also known as general corrosion,it causes uniform loss of the material along the surface of the pipe, resulting in a continuous thinning, or wall loss, of its solid structure.

Uniform corrosion is characterized and expressed by mass loss per unit area and per unit time, e.g., mm/year.Stainless steel is generally considered to be corrosion resistant when the corrosion rate is less than 0.1 mm/year.

Problems with uniform corrosion on stainless steel are most likely to occur with exposure to strong acid and hot alkaline environments. In general, inorganic acids are more aggressive than organic acids, with hydrochloric acid being one of the worst.Cast irons and steels corrode uniformly when exposed to open atmospheres, soils and natural waters, leading to the rusty appearance.

This deterioration can be prevented by the selection of suitable piping materials,combined with corrosion methods such as cathodic protection and surface coating.We can supply many types anti-corrosion coatings for the pipeline products,such as 3PE,2PE,FBE,3PP,2PP,galvanized coating.

Localized Corrosion of pipeline:

Unlike general corrosion, local corrosion is specific to an area of the metal structure.Local corrosion can be divided into three types:

Pitting corrosion of pipeline

Pitting corrosion is a severe localized corrosion that can lead to cavities or pits in the surface of a pipe.In some cases, these pits can puncture the pipes.Pitting corrosion is caused by:

• Pipe material defects or surface defects
• Mechanical damage to the protective passive film
• Penetration by an aggressive chemical species, such as chlorides

Internal pitting corrosion is an important factor in the degradation of pipelines and assets in the oil and gas industry.Pitting corrosion may occur in both carbon steel and corrosion resistant alloys (CRAs).Pits usually differ in shape and depth.Improper selection of piping materials may be one reason.

This corrosion can be prevented by:

• Selecting a pipe material for the specific service environment, such as the temperature and chemical concentration of a reactant (resistant to pitting)
• Devising cathodic or anodic protection

Crevice corrosion of pipeline

Crevice corrosion is regarded as localized corrosion.The problem of crevice corrosion mainly occurs in chlorine-containing solutions, and the crevice is wide enough to permeate the solution, but narrow enough to cause stagnation.It often appear in those gaps such as gaskets, washers, insulation material, fastener heads, surface deposits, disbonded scoating, threads, lap joints and clamps.Crevice corrosion is most likely to occur in seawater applications.

There are factors that influence crevice corrosion. These include:

• The type of crevice: either metal to metal or metal to nonmetal
• The geometry of the crevice: including the size of the gap, its depth and the surface roughness
• The composition of the metal: the structure of the alloy composition and can be Cr, Mo, or others.
• The environment: the pH, halide ions, temperature and oxygen1

How can crevice corrosion be prevented?

• Replace riveted joints with welded butt joints.
• Eliminate crevices in lap joints through continuous welding and soldering.
• Drain existing solutions on surfaces and avoid creating stagnant conditions.
• Use solid and non-absorbent gaskets.
• Use higher alloys.

Filiform corrosion of pipeline

Filiform corrosion is also known as “underfilm Corrosion” or “filamentary corrosion”.Occurring under painted or plated surfaces when water breaches the coating, filiform corrosion begins at small defects in the coating and spreads to cause structural weakness.

Filamentous corrosion occurs on a metal surface covered by a thin organic film, usually 0.1 mm thick.Filiform corrosion can be visually recognized without using a microscopy. Filiform corrosion has been observed on surfaces of coated steel, magnesium, and aluminum with thin coatings of tin, silver, gold, phosphate, enamel, and lacquer. Filiform corrosion has also been observed on paper-backed aluminum foils.

Selective leaching of pipeline

Selective leaching also known as dealloying, paring, selective corrosion, or demetalification. It is a common type of corrosion of some solid solution alloys. When selective leaching occurs, one component of the alloy is removed preferentially from the material.Selective leaching or graphite corrosion occurs when noble metals and more active elements form alloys.This can lead to loss of the reactive elements from the surface of the pipe, resulting in loss of strength and premature failure.

The most common example is dezincification, the selective removal of zinc in brasses. Many alloys are susceptible to selective leaching under certain conditions. This can lead to color changes or density changes in the affected material.In some cases, the addition of aluminum or tin can prevent leaching.A special case of selective leaching is the preferential erosion of inclusions.The metal in the affected area becomes porous, losing much of its strength, hardness and ductility.

Galvanic corrosion of pipeline

Galvanic corrosion may also be known as bimetallic corrosion and dissimilar metal corrosion.Galvanic corrosion refers to the corrosion damage caused by the electric contact between two different metals in the electrolyte,in which the more noble metal is protected and the more active metal tends to corrode.

For galvanic corrosion to occur, three conditions must all be present:

• Electrochemically dissimilar metals must be present.
• Metals must be in electrical contact.
• Metals must be exposed to an electrolyte.

How to solve this problem?This reaction can be prevented by using a combination of metals that are closer to the galvanic battery series,and by placing insulation between the two.The coating on the cathode surface will also help.

Intergranular deterioration of pipeline

Intergranular corrosion is a chemical or electrochemical attack on the grain boundaries of a metal. It often occurs due to impurities in the metal, which tend to be present in higher contents near grain boundaries. These boundaries can be more vulnerable to corrosion than the bulk of the metal.This problem can be prevented by selecting extra-low carbon stainless steel materials.

Cavitation Corrosion

Cavitation corrosion is a specific type of erosion which results from gas bubbles’ implosion on metal surfaces. It is usually connected with abrupt pressure variations associated with a fluid’s hydrodynamic parameters, such as in propellers, stirrer blades and turbine blades.

Parts of pipelines such as pump suctions, discharge pipes, elbows, tees or expansions or fitments at heat exchangers—even valve seats—may be extremely prone to this damage under certain operational conditions.

Areas of low and high pressure have a tendency to become induced under specific conditions such as high velocity. In areas with low pressure, vapor bubbles and gas can be produced. When such bubbles shift to high-pressure areas, they cave in and produce pressure waves, which can erode protective films, leading to heightened levels of corrosion.

Erosion corrosion

Erosion corrosion is similar to cavitation corrosion in that fluid movement removes corrosion from the metal surface, thus exposing fresh metal for further corrosion. The difference between cavitation corrosion and erosion corrosion is that fluid movement over the corroded metal surface enhances corrosion instead of high-speed fluid impinging on the metal surface. Erosion corrosion follows this basic pattern:

• Metal corrosion forms metal oxides and metal hydroxides on the metal surface.
• Fluid moving over the metal removes the oxides and hydroxides exposing fresh metal.
• The fresh metal corrodes.

This process continues to cycle from beginning to end until the metal component fails.

Stray Current Corrosion

Stray current corrosion is caused by the flow of stray currents through pipelines. This can cause localized pitting and pin holes on metal surfaces at the precise points where stray currents leave the surface.

Sources of stray electricity include:

• High voltage overhead or buried lines nearby
• Electric railways
• Electric welding machines
• Earthed DC source
• Cathodic protection

Damage can be reduced by controlling electricity leakages, bypassing stray current to an earthing station or the use of an additional protective system.

Environmental Cracking:

Environmental cracking is a corrosion process,which is the result of various environmental conditions that affect metals.Chemical, temperature, and stress-related conditions can lead to the following types of environmental corrosion:

• Stress Corrosion Cracking (SCC)
• Corrosion fatigue
• Hydrogen-induced cracking
• Liquid metal embrittlement

High-Temperature Corrosion:

Fuels used in gas turbines, diesel engines and other machinery, which contain vanadium or sulfates can, during combustion, form compounds with a low melting point. These compounds are very corrosive towards metal alloys normally resistant to high temperatures and corrosion, including stainless steel.

High-temperature corrosion can also be caused by high-temperature oxidization, sulfidation, and carbonization.

Those are the common types of pipeline corrosion.As a professional pipeline supplier,Haihao Group not only provides high quality pipeline products, but also gives appropriate anti-corrosion suggestions according to the use of customers to protect their interests.If you want to learn more about anti-corrosion coatings about pipeline products,feel free to contact us.Email:sales@haihaogroup.com