Types of corrosion
Corrosion can occur in many different forms and can be classified by the cause of the chemical deterioration of a metal. Ten different types of corrosion can be specified:
- General corrosion
- Pitting & Crevice corrosion
- Galvanic corrosion
- Intergranular corrosion
- Chloride stress corrosion cracking
- Erosion corrosion
- Fretting corrosion
- High Temperature corrosion
- Micro organisms
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, 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:
- astatine (very unstable.)
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 caused by a chemical or electrochemical reaction that results in the deterioration of the entire exposed surface of a tube. 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 a technical standpoint, because the life time of equipment can be accurately estimated on the basis of comparatively simple tests.
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. Therefore, Pitting corrosion is simply 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 the presence of chloride ions. Once a pit is formed, the solution inside it is isolated from the bulk environment and becomes increasingly corrosive over 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 type of 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 which we find in many split seal applications. Salt water applications are the most severe problem because of the salt water's low PH value and its high chloride content. Due to the tight connections no oxygen is available to passivate the stainless steel, the low PH salt water attacks the active layer that is exposed. The corrosion soon reaches under the tight fitting clamp unhampered.
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.
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:
- Anneal the stainless after it has been heated in this sensitive range.
- When possible use low carbon content stainless if you intend to do any welding on it. A carbon content of less than 0.3% will not precipitate into a continuous film of chrome carbide at the grain boundaries.
- Alloy the metal with a strong carbide former. The best is columbium, but sometimes titanium is used. The carbon will now form columbium carbide rather than going after the chrome to form chrome carbide. The material is now said to be "stabilized"
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 manufacturing, the pits will deepen even more. Chloride stress cracking is a serious problem in industrial applications and rarely recognized by the people involved. This is the main reason why Hastelloy C is recommended for several severe environments. Stress cracking can be minimized by annealing the metal after manufacturing to remove residual manufactured stresses. Also, chromate and phosphate have each been used successfully to prevent the SCC of stainless steel in chloride solutions, two issues to be noted:
- Chlorides are the main problem when using the 300 series grades of stainless steel. The 300 series is the one most commonly used in the process industry because of its usually good corrosion resistant properties.
- Take care when insulating or painting stainless steel tube. Most insulations contain chlorides and tubing is frequently under tensile stress. The worst condition would be insulated, steam traced, stainless steel tubing. If it's necessary to insulate stainless steel tube, a special chloride free insulation can be purchased.
Also known as flow-assisted or flow-accelerated corrosion, this is an acceleration or increase in the 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 prevent 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. 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 exhibit a directional pattern.
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 itself 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.
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.
Dealloying, also called selective leaching, is possible when the process fluid selectively removes elements from the piping or any other part that might be exposed to the liquid flow. This mechanism occurs in three steps and is accelerated by heat:
- Metals are removed from the liquid during a deionization or demineralizing process.
- The liquid tries to replace the missing elements as it flows through the system.
- The undissolved metals often coat themselves on the mechanical seal faces or the sliding components and cause a premature seal failure.
These organisms are commonly used in sewage treatment, oil spills and other cleaning processes. Although there are many different uses for these "bugs", a common one is for them to eat the carbon that is found in waste as well as other hydrocarbons and convert it to carbon dioxide. The "bugs" fall into three categories:
- Aerobic, the kind that need oxygen.
- Anaerobic, the kind that do not need oxygen.
- Facultative, the type that goes both ways.
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 in a manner similar to rust spreading under the paint of an automobile.