MIC refers to corrosion caused by the presence and activities of microorganisms—microalgae, bacteria, and fungi. MIC deteriorates pipes by pitting or tunneling, thereby penetrating their cross-sections eventually degrading wall thickness. The formation of slime or tuberculation nodules can cause blockages or reduce flow. Low flow or stagnant conditions make systems more susceptible to microbial growth. In older systems, problems occur when there is a change of water source, water quality, new materials of construction, or new operating conditions. In order for MIC to grow, bacteria plus four other environmental factors need to be present - Metal, nutrients, water and oxygen.

Weathering damage is typically due to atmospheric conditions or piping being exposed to caustic materials in the air. The piping is typically exposed with corrosion activity always visually obvious. Most weathering damage requires decades to produce a failure, and is simply due to a lack of maintenance however if piping is installed in an especially corrosive environment may exacerbate the weather of piping. Smaller diameter piping is always most vulnerable due to its inherently lesser wall thickness.

Galvanic corrosion (dissimilar-metal corrosion) is an electrochemical process in which one metal corrodes preferentially, when in contact with a different type of metal. This type of corrosion is usually at piping joints and can been visually witnessed on the exterior of the piping however, internal corrosion downstream of the physical corrosion can be present. While most witness of galvanic action is at pipe joints, improper materials for pipe hangers can cause this type of corrosion as well.

Pitting is localized failure of piping. This corrosion type could be caused by electrochemical corrosion, localized MIC, coating failure, or surface deposits within the pipe creating localized galvanic action. This pitting corrosion overtime can extend throughout the piping causing thinning of the pipe wall eventually causing a wall failure and leak. Visual in nature on exposed uninsulated piping, pitting is typically the result of unchecked corrosion within a system.

Electrochemical oxidation(aka rust) is a chemical reaction where electrons from the surface of the pipe(internal or external) are transferred to another material(air, water). Water must be present in order to facilitate this electron transfer. This type of reaction actually degrades the metal at the atomic level expanding the space between molecules causing it to become porous and brittle over time.

Sediment/Crevice corrosion occurs when a foreign object within a pipe comes to rest in a particular spot causing a secondary type of corrosion to occur (electrochemical or galvanic action). Corrosion stars slowly around the object with the longer the item sitting the more corrosion occurs as more surface area of the pipe is exposed. Typically, but not always in dry type systems this corrosion tends to cause pinhole leaking.

• Invisible to the naked eye during visual inspections, internal corrosion can be caused by several factors as the inside of the pipe is its own environment.

• Internal corrosion can cause a degredation of the wall structure from the inside reducing pipe integrity. Unfortunately, internal corrosion issues are typically only realized when leaks occur or there is a system failure when called upon to activate.

• Internal corrosion can affect every system type(Wet, dry, double-interlocked preaction, deluge, etc) as typically these are closed systems each with their own unique environment. The types of corrosion that affects each type of system may be different but no system is completely safe from internal corrosion.

• Easy to see visually (and sometimes covered up by paint) external corrosion is caused by the oxidation of the piping systems typically due to environmental factors.

• External corrosion can cause pitting of piping and degradation of pipe wall integrity causing leaks within the system. When called upon to activate in a time of need these leaks can cause lack of water flow to the required area or worse, a complete failure of the piping structure at point of leak.

• Systems located near oceans, airports, major highways, within parking structures/garages or other locations with questionable air quality can be more susceptible to this type of corrosion.