Corrosion is derived from the Latin "corrosus" meaning gnawed away.  Corrosion may be further defined as a gradual destruction of a material, a substance, or an entity, usually by solution or other means attributed to a chemical process. Many different corrosion mechanisms exist. The most common types are generally well understood. For each, the process is complex, incorporates many factors, and varies according to metal and specific operating conditions.

Atmospheric corrosion

• Air pollution (both man made and natural such as volcanic gases)
• Airborne salt spray or droplets
• Temperature
• Moisture.

Corrosion can be categorized in some common types

• uniform corrosion
• pitting corrosion
• galvanic corrosion
• crevice corrosion
• concentration cell corrosion
• graphitic corrosion

Uniform or General Corrosion
The metal loss is uniform from the surface. Often combined with high-velocity fluid erosion, with or without abrasives.

When corrosion occurs uniformly over a wide area of the metal surface producing a general thinning of the metal leading to eventual failure. The rate of general corrosion is usually relatively predictable.

Pitting Corrosion
The metal loss is randomly located on the metal surface. Often combined with stagnant fluid or in areas with low fluid velocity.

Pitting corrosion is quite often evident on metal surfaces where no uniform corrosion is present and is usually highly localized. Pitting usually proceeds fairly rapidly and is accelerated by the presence of chlorides and is particularly common at the base of breaks in coatings. Pitting corrosion takes advantage of the different metallurgical phases present on the surface of most common modern alloys. Generally it is considered to be the product of localized anodic dissolution where the anodic portion of the corrosion cell is dwarfed by the larger cathodic portion.

It often termed "under deposit corrosion". It is deep penetration of the metal surface with little general corrosion in the surrounding area. Due to surface deposits, electrical imbalance or some other initiating mechanism, all existing corrosion potential attacks a select number of individual sites.

In most cases, pitting is extended throughout the entire metal surface, creating an irregular or very rough surface profile. In other instances, pits are concentrated in specific areas, leaving the majority of the metal surface in like new condition.

Pitting is the most common form of corrosion found where there are incomplete chemical protective films, and insulating or barrier deposits of dirt, iron oxide, organic, and other foreign substances at the pipe surface. It is prevalent at galvanized steel pipe, where any failure of the galvanizing invokes a pitting condition.

Pitting corrosion may include: crevice corrosion, water-line attack, under deposit attack, impingement or erosion corrosion attack, and concentration-cell corrosion.

Galvanic Corrosion
This is an aggressive and localized form of corrosion due to the electrochemical reaction often found between two or more dissimilar metals in an electrically conductive environment.

It occurs because the more electronegative material (the anode) is attacked by the more electropositive material (the cathode). The anodic metal develops deep pits and groves in the surface. The most common example of such corrosion activity, widely found throughout HVAC and process plant operations, is the direct connection of brass valves to carbon steel pipe, or between copper tubing and steel pipe, where the steel serves as the anode, and the brass or copper the cathode. Carbon steel pipe, without the protection of a galvanic insulator, will show the highest rate of corrosion under such conditions, usually developing over many years.

The severity of pipe loss due to galvanic activity is often found relative to the general corrosion activity of the piping system itself, with little or no galvanic activity found where extremely low general corrosion rates exist. Under conditions of high corrosion rate activity, galvanic losses often become aggressive, making an existing pipe corrosion problem significantly worse at the threads, its already most weakened area.

While galvanic corrosion is generally assumed to involve only dissimilar metals, millivolt potentials can actually be measured between similar metals under certain conditions. New steel pipe installed during a repair or renovation is often more electronegative than older existing pipe, and therefore may suffer from some degree of galvanic attack.

Crevice Corrosion
Occurs at places with gaskets, bolts and lap joints where crevice exists. Crevice corrosion creates pits similar to pitting corrosion.
Crevice corrosion occurs in sheltered, localized areas such as  crevices, joints, bolted and threaded parts and under existing corrosion deposits. It is the result of concentration of salts, acids and moisture which results in the formation of an occluded corrosion cell in such sheltered areas. A small anode is created in the crevice with the remainder of the body acting as a large cathode so corrosion at the crevice is highly accelerated as well as concentrated.

Concentration Cell Corrosion
Occurs where the surface is exposed to an electrolytic environment where the concentration of the corrosive fluid or the dissolved oxygen varies. Often combined with stagnant fluid or in areas with low fluid velocity.

Graphitic Corrosion
Cast iron loosing iron in salt water or acids. Leaves the graphite in place, resulting in a soft weak metal.

The following types of corrosion are on the basis of galvanic process

Electrolytic Corrosion
When moisture is present creating an electrolyte to permit electron flow, the moisture may come from condensation after takeoff as the trapped air cools at altitude or after landing as the cool aircraft causes condensation of the local humidity. This is very insidious and is part of the reason for commercial airliners to have the lower skin replaced periodically.

It usually happens at the faying (that’s an old ship builder’s term) surfaces. Sometimes it cannot be found until it has been taken rivets out and separated the parts (rib from skin, etc).

Salt Water Corrosion
It is an extreme form of Electrolytic corrosion but it is one that just about everyone understands. This is what Navy aircraft have to fight the most. In most specifications for a new airplane the Navy will give a minimum thickness of aluminum alloy such as .026 inch. Anything thinner loses a high percentage of its strength by the time corrosion is noticed.

Electrical Power
This corrosion is accelerated by being in the path of an electrical power circuit. Most homebuilt aircraft do not have a lot of accessories that are electric powered. They can stay away from this type of corrosion by having a wire for return circuit. More in the future on electrical bonding.

Intergranular Corrosion
This corrosion can be caused by poor processing during manufacture of the alloy (hence the need to be sure it meets a given specification). Usually it is caused by selecting the wrong chemical in attempting a conversion coating on a given alloy. Poor control of heat treatment processing is also a cause. Over-etching aluminum can cause this type of corrosion.

Exfoliation Corrosion
This corrosion is similar to intergranular corrosion except the corrosion follows grain boundaries and "large" chunks fall out. Extrusions can be susceptible where grain boundaries are stretched and/or rough surfaces occur during the extruding process.

For this corrosion critical part,use bar or rod material that has been wrought instead of extruded. In industry extruded stock is used a lot but it usually goes through several inspections before going on an aircraft.

Fretting Corrosion
Fretting corrosion is caused by two surfaces rubbing together at a very small amplitude. I sometimes think that it should not be considered so much a corrosion as a wear. It can be eliminated by placing a very thin sheet of Nylon or Teflon between the surfaces.

Stress Corrosion
Stress corrosion is the product of tensile stress (including residual stress remaining after fabrication) and localized corrosion which combine to produce a brittle cracking of metal under certain conditions.

Examples of environments which enhance stress corrosion are high pH amine solutions for most common steels and chloride bearing solutions for most stainless steels as well as certain aluminum alloys. The high-strength heat-treatable wrought aluminum alloys in certain tempers are susceptible to stress-corrosion cracking, depending upon the product, section size, direction and magnitude of stress.

Stress corrosion cracking is where the internal stresses (residual stresses) vary across a section so that when they are loaded with additional outside forces, the grain boundaries at the surface start to break. When a part (sheet, extrusion, etc) is quenched and the outside layer cools too quickly, tension stresses are set up on the outside and compression stresses in the middle. This is sometimes taken care of by stretching or shot peening.

Local stresses (assembly stresses) can also be caused by selection of too small diameter of high shear fasteners (such as bolts), shrink or press fits, taper pins, and clevis joints in which tightening of the bolt imposes a sustained bending load on the clevis lugs.

Embrittlement
Improper heat treatment can cause an embrittlement of an otherwise ductile material. Titanium makes a very good light weight spring but Titanium embrittlement can be caused by Cadmium. The cadmium, under pressure and/or heat, will flow (infuse) between the grains of titanium. This weakens the grain boundaries and when the titanium is stressed, a crack will initiate. Cad plate applied on steel bolts and not subsequently baked can cause the bolts to break from "hydrogen embrittlement." More on embrittlement in the future.

Galling
Galling is not a corrosion but with all the talk about corrosion between dissimilar metals, galling problems should be discussed. Galling is a condition where two parts (made from the same alloy) slide over one another and start ripping the surface between them.

Nuts and bolts are not made of the same material. If they were, they would gall and disassembly would be impossible. Each is made from a different alloy. To me it is interesting that the nut is always made the sacrificial member. If there is any shearing of threads, the nut will shear first. Many times if a nut is unknowingly sheared on assembly, it will fall off and the bolt will remain in place, averting disaster. More on bolts and nuts sometime in the future.

OTHER TYPE OF CORROSION 

Microbiologically Influenced Corrosion
Microbiologically Influenced Corrosion (MIC) is, by far, the most severe and threatening form of corrosion to HVAC piping systems. Corrosion rates of 100 MPY have been documented.

MIC is caused by the presence of various microbiological agents under specific environmental conditions - in some cases resulting in advanced and widespread failure of entire piping systems within a few years.

An MIC presence usually signals a very severe threat to the entire system - requiring extensive cleaning and sterilization at great expense. For many affected systems, MIC cannot be eliminated, and an elevated corrosion and pitting condition will exist for the remainder of system life.

MIC produces large and deep pits due to the microorganism's utilization of iron as an energy source (often as an alternative to oxygen), and through the production of strongly corrosive metabolic by-products such as sulfuric acid - which further assists the microorganism in dissolving pipe metal. MIC exists to varying degrees of severity, and is not exclusive to carbon steel piping systems or open condenser water systems.

MIC is less commonly found in closed chill water piping, in hot water heating and domestic water systems, and has been documented to destroy copper, brass, and stainless steel pipe.

High Temperature Attack
Under very high temperature metals need only the presence of oxygen or other oxidizing gases to corrode. This type of corrosion is referred to as high temperature oxidation, scaling or tarnishing and requires neither moisture nor dissolved electrolytes (salts, acids) to proceed.