Corrosion
Corrosion is a major factor in the selection of material for any industrial plant or machine because the material selected must resist the various types of corrosion it will be subjected to. This article focuses on various types of corrosion and their effects upon metals.
Corrosion is the deterioration of a material due to interaction with its environment. It is the process in which metallic atoms leave the metal, or form compounds, in the presence of water and gases. All metals and alloys are subject to corrosion. Even the noble metals, such as gold, are subject to corrosive attack in some environments.
The corrosion of metals is a natural process. Most metals are not thermodynamically stable in their metallic form; they want to corrode and revert to the more stable forms that are normally found in ores, such as oxides. Even though this corrosion cannot be eliminated, it can be controlled.
General Corrosion
General corrosion involving water and steel generally results from chemical action where the steel surface oxidises, forming iron oxide (rust). Most components in industrial plants are made from some form of iron or steel (an iron based alloy), so general corrosion is of high importance.
Some standard methods associated with material selection that protect against general corrosion include:
- The use of corrosion-resistant materials such as stainless steel, nickel, chromium, and molybdenum alloys. Note: Keep in mind that the corrosion is electrochemical by nature, and the corrosion resistance of the stainless steels results from surface oxide films that interfere with the electrochemical process.
- The use of protective coatings such as paints and epoxies.
- The application of metallic and non-metallic coatings or linings to the surface, which protects against corrosion, but allows the material to retain its structural strength (for example, a carbon steel pressure vessel with stainless steel cladding as a liner).
Galvanic Corrosion
Galvanic corrosion occurs when two dissimilar metals with different electrical potentials are in electrical contact with each other in an electrolyte. A difference in electrical potential exists between the different metals and serves as the driving force for electrical current flow through the corrodent or electrolyte; this current results in corrosion of one of the metals. The larger the potential difference, the greater the probability of galvanic corrosion. Galvanic corrosion only causes deterioration of one of the metals. The less resistant, more active metal, becomes the anodic (negative) corrosion site. The stronger, more noble metal, is cathodic (positive) and protected. If there were no electrical contact, the two metals would be uniformly attacked by the corrosive medium and this would then be called general corrosion.
Electric potential difference tables have been created that arrange metals sequentially from most active, or least noble, to passive, or most noble.
Galvanic corrosion is of particular concern in design and material selection. Material selection is important because different metals may come into contact with each other and form galvanic cells. Design is important to minimise differing flow conditions and resultant areas of corrosion build up.
In some instances, galvanic corrosion can be helpful. For example, if pieces of zinc are attached to the bottom of a steel water tank, the zinc will become the anode, and it will corrode. The steel in the tank becomes the cathode, and it will not be affected by corrosion. This technique is known as cathodic protection. The metal to be protected is forced to become a cathode, and it will corrode at a much slower rate than the other metal, which is used as a sacrificial anode.
Localised Corrosion
Localised corrosion is defined as the selective removal of metal by corrosion at small areas or zones on a metal surface in contact with a corrosive environment, usually a liquid. It usually takes place when small local sites are attacked at a much higher rate than the rest of the original surface. Localised corrosion takes place when corrosion works with other destructive processes such as stress, fatigue, erosion, and other forms of chemical attack. Localised corrosion mechanisms can cause more damage than any one of those destructive processes individually. There are many different types of localised corrosion. Pitting, stress corrosion cracking, chloride stress corrosion, caustic stress corrosion, primary side stress corrosion, heat exchanger tube denting, wastage, and intergranular attack corrosion, to name just a few.
Stress-Corrosion Cracking
One of the most serious metallurgical problems and one that is a major concern in the power generation industry is stress-corrosion cracking (SCC). SCC is a type of intergranular attack corrosion that occurs at the grain boundaries under tensile stress. It tends to propagate as stress opens cracks that are subject to corrosion, which are then corroded further, weakening the metal by further cracking. The cracks can follow intergranular or trans granular paths, and there is often a tendency for crack branching.
The cracks form and propagate approximately at right angles to the direction of the tensile stresses at stress levels much lower than those required to fracture the material in the absence of the corrosive environment. As cracking penetrates further into the material, it eventually reduces the supporting cross section of the material to the point of structural failure from overload.
Stresses that cause cracking arise from residual cold work, welding, grinding, thermal treatment, or stresses applied during service; the stress applied must be tensile (as opposed to compressive).
SCC occurs in metals exposed to an environment where, if the stress was not present or was at much lower levels, there would be no damage. If the structure, subject to the same stresses, were in a different environment (noncorrosive for that material), there would be no failure. Examples of SCC in the power generation industry are cracks in stainless steel piping systems and stainless-steel valve stems.
The most effective means of preventing SCC are:
- Proper design.
- Reducing stress.
- Removing critical environmental species such as hydroxides, chlorides, and oxygen.
- Avoiding stagnant areas and crevices in heat exchangers where chloride and hydroxide might become concentrated.
Low alloy steels are less susceptible to SCC than high alloy steels, but they are subject to SCC in water containing chloride ions. Nickel-based alloys, however, are not affected by chloride or hydroxide ions.
An example of a nickel-based alloy that is resistant to stress-corrosion cracking is Inconel. Inconel is composed of 72% nickel, 14-17% chromium, 6-10% iron, and small amounts of manganese, carbon, and copper.
Chloride Stress Corrosion
Chloride stress corrosion is a type of intergranular corrosion and occurs in austenitic stainless steel under tensile stress in the presence of oxygen, chloride ions, and high temperature.
It is thought to start with chromium carbide deposits along grain boundaries that leave the metal open to corrosion. This form of corrosion is controlled by maintaining low chloride ion and oxygen content in the environment and use of low carbon steels.
Caustic Stress Corrosion
Despite the extensive qualification of Inconel for specific applications, a number of corrosion problems have arisen with Inconel tubing. Improved resistance to caustic stress corrosion cracking can be given to Inconel by heat treating it at 620°C to 705°C, depending upon prior solution treating temperature, but other problems that have been observed with Inconel include wastage, tube denting, pitting, and intergranular attack.
Summary
The important information in this section is summarised below:
Corrosion Summary
- Corrosion is the natural deterioration of a metal in which metallic atoms leave the metal or form compounds in the presence of water or gases. General corrosion may be minimised by the use of corrosion-resistant materials and the addition of protective coatings and liners.
- Galvanic corrosion occurs when dissimilar metals exist at different electrical potentials in the presence of an electrolyte. Galvanic corrosion may be reduced by the careful design and selection of materials, and the use of sacrificial anodes.
- Localised corrosion can be especially damaging in the presence of other destructive forces such as stress, fatigue, and other forms of chemical attack.
- Stress-corrosion cracking occurs at grain boundaries under tensile stress. It propagates as stress opens cracks that are subject to corrosion, ultimately weakening the metal until failure. Effective means of reducing SCC are 1) proper design, 2) reducing stress, 3) removing corrosive agents, and 4) avoiding areas of chloride and hydroxide ion concentration.
- Chloride stress corrosion occurs in austenitic stainless steels under tensile stress in the presence of oxygen, chloride ions, and high temperature. It is controlled by the removal of oxygen and chloride ions in the environment and the use of low carbon steels.
- Problems occurring with the use of Inconel include caustic stress corrosion cracking, wastage, tube denting, pitting and intergranular attack. Inconel's resistance to caustic stress corrosion cracking may be improved by heat treating.