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Types of corrosion

Types of corrosion

Corrosion can occur in many different forms and can be classified by the cause of the chemical deterioration of a metal,10 different types of corrosion can be specified:

General corrosion
 

The basic resistance of stainless steel occurs because of its ability to form a protective coating on the metal surface. This coating is a "passive" film which resists further "oxidation" or rusting. The formation of this film is instantaneous in an oxidizing atmosphere such as air, water, or other fluids that contain oxygen. Once the layer has formed, we say that the metal has become "passivated" and the oxidation or "rusting" rate will slow down to less than 0,05 mm. per year (0.002" per year).

Halogen salts, especially chlorides easily penetrate this passive film and will allow corrosive attack to occur. The halogens are easy to recognize because they end in the letters "ine". Listed in order of their activity they are:
fluorine
 

General corrosion
 

Also known as overall corrosion or general attack, this type of corrosion occurs when there is an overall breakdown of the passive film formed on the stainless steels. This general attack is the most common form of corrosion and is cause by a chemical or electrochemical reaction that results in the deterioration of the entire exposed surface of a tube surface. It's the easiest to recognize as the entire surface of the metal shows a uniform "sponge like" appearance. The rate of attack is affected by the fluid concentration, temperature, fluid velocity and stress in the metal parts subject to attack. This form of corrosion is not of too great concern from the technical standpoint, because the life of equipment can be accurately estimated on basis of comparatively simple tests.
 

Pitting corrosion
 

Pitting is a form of extremely localized attack that results in holes in the tube walls. It occurs when the corrosive environment penetrates the passivated film in only a few areas as opposed to the overall surface. As stated earlier, Pitting corrosion is therefore simple galvanic corrosion, occuring as the small active area is being attacked by the large passivated area. It is one of the most destructive forms of corrosion and also one of the most difficult to predict in laboratory tests.. It is generally promoted by low-velocity or stagnant conditions and by the presence of chloride ions. Once a pit is formed, the solution inside it is isolated from the bulk environment and becomes increasingly corrosive with time. The high corrosion rate in the pit produces an excess of positively charged metal cations, which attract chloride anions. In addition, hydrolysis produces H+ ions. The increase in acidity and concentration within the pit promotes even higher corrosion rates, and the process becomes self-sustaining.

Similar to pitting is crevice corrosion ,this corrosion occurs any time liquid flow is kept away from the attacked surface. It is common between single or twin ferrule fittings and tube clams surfaces, we find in many split seal applications. Salt water applications are the most severe problem because of the salt water low PH and its high chloride content. Due to the tight connections no oxygen is available to passivate the stainless steel, chloride pit the passivated stainless steel surface, the low PH salt water attacks the active layer that is exposed. The corrosion unhampered under the tight fitting clamp.
 

Galvanic corrosion
 

Galvanic corrosion occurs when two dissimilar metals are in contact in a solution. The contact must be good enough to conduct electricity, and both metals must be exposed to the solution. The driving force for galvanic corrosion is the electric potential difference that develops between two metals. This difference increases as the distance between the metals in the galvanic series increases. When two metals from the series are in contact in solution, the corrosion rate of the more active (anodic) metal increases and the corrosion rate of the more noble (cathodic) metal decreases. Three conditions must exist for galvanic corrosion to occur; electrochemically dissimilar metals must be present, the metals must be in electrical contact and must be exposed to an electrolyte.
 

Intergranular corrosion
 

All austenitic stainless steels (the 300 series, the types that "work harden") contain a small amount of carbon in solution in the austenite. Carbon is precipitated out at the grain boundaries, of the steel, in the temperature range of 565°C (1050° F). to 870°C  (1600° F). This is a typical temperature range during the welding of stainless steel.
 

This carbon combines with the chrome in the stainless steel to form chromium carbide, starving the adjacent areas of the chrome they need for corrosion protection. In the presence of some strong corrosives an electrochemical action is initiated between the chrome rich and chrome poor areas with the areas low in chrome becoming attacked. The grain boundaries are then dissolved and become non existent.
 

There are three ways to reduce this type of corrosion:
 

Chloride stress corrosion
 

Stress corrosion cracking ( SCC ) is the brittle failure of a metal by cracking under tensile stress in a corrosive environment. Chloride is the main contributor to SCC of stainless steels. High chloride concentrations, resulting from high chloride levels in the makeup water and/or high cycles of concentration, will increase susceptibility. If the tube piece is under tensile stress, either because of operation or residual stress left during manufacture, the pits will deepen even more. Chloride stress cracking is a serious problem in industry and not often recognized by the people involved. This is the main reason that Hastelloy C is recommended for several severe industry applications. Stress cracking can be minimized by annealing the metal, after manufacture, to remove residual manufactured stresses also chromate and phosphate have each been used successfully to prevent the SCC of stainless steel in chloride solutions, 2 issue to be noted:
 

Erosion corrosion
 

Also known as flow- assisted or flow- accelerated corrosion, This is an accelerated or increase in rate of deterioration or attack on a metal because of relative movement between a corrosive fluid and the metal surface, resulting from the combination of mechanical and chemical wear. The liquid velocities in some tubes prevents the protective oxide passive layer from forming on the metal surface. The suspended solids also remove some of the passivated layer increasing the galvanic action. You see this type of corrosion very frequently appears near the eye of a pump impeller. Erosion corrosion is characterized in appearance by grooves, waves, round holes and valleys which usually exhibits a directional pattern.
 

Fretting corrosion
 

Fretting corrosion occurs as a result of repeated wearing, weight and /or vibration on an uneven, rough surface. The corrosion results in pits and grooves with occurs on the surface of the tube. As mentioned earlier, 300 series stainless steel passivates its self by forming a protective chrome oxide layer whenever it is exposed to free oxygen. This oxide layer is very hard and when it imbeds into a soft elastomer it will cut and damage the shaft or sleeve rubbing against it.
 

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.
 

De- Alloying
 

De-Alloying or selective lauching, The process fluid selectively removes elements from the piping or any other part that might be exposed to the liquid flow. The mechanism is:
 

Micro Organismen
 

These organisms are commonly used in sewage treatment, oil spills and other cleaning processes. Although there are many different uses for these "bugs", one common one is for them to eat the carbon you find in waste and other hydrocarbons, and convert it to carbon dioxide. The "bugs" fall into three categories:
 

If the protective oxide layer is removed from stainless steel because of rubbing or damage, the "bugs" can penetrate through the damaged area and attack the carbon in the metal. Once in, the attack can continue on in a manner similar to that which happens when rust starts to spread under the paint on an automobile.

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