CorrView International, LLC merilis serangkaian review yang merupakan hasil dari Ultrasonic Piping Investigations selama 18 tahun. Tinjauan terhadap berbagai jenis atau tipe korosi sering membantu dalam mendeterminasi penyebab korosi.
Here’s a look at 21 different corrosion types and failure conditions. In many cases, however, a combination of conditions will exist within the same piping system. Pada bagian 1 telah disajikan type 1 – 10, selebihnya disajikan pada bagian ke 2 ini.
Enough water can be hidden inside pipe insulation to suggest an actual piping failure when opened. This is due to the fact that fiberglass insulation offers very little true moisture barrier and allows humidity to condense at the cold pipe surface. Wet insulation is therefore a sure sign of some form of a problem with fiberglass insulation; water will eventually penetrate to the outside to produce wetness, discoloration, and crystallization, thereby providing telltale evidence of a problem. If acted upon, corrosion under insulation (CUI) problem can be avoided and the often mild deteriotarion present at early stages of such a problem can be corrected. If allowed to continue, substantial piping damage is likely.
An aluminum or vinyl jacket covering over otherwise improperly or insufficient insulation can actually hide the build-up of water at the pipe and allow years of additional damage to take place. In multiple cases where grooved or clamped pipe was in use, the accumulation of water inside its vinyl elbows, tees, or other fittings severely deteriorated the absolute weakest link in the entire piping system—its connection bolts—to cause catastrophic failure.
Cold water pipe, insulation failure, and area humidity produces condensation and wet pipe conditions, which are a prerequisite to mold growth. Mold typically develops at HVAC system piping after an extended period where wet pipe conditions have existed, and where years of opportunity likely existed to correct the problem long before it began. While not related to its corrosion condition or contributing to a failure of the pipe itself, the presence of mold typically raises all forms of health issues, which are in many cases followed by very aggressive and costly legal action. Water leaks due to pipe failure behind walls, overflowing condensate drains, and other sources of water will produce the same result.
The replacement of all pipe insulation and sheetrock walls is often the only solution for a mold condition. Although not technically a piping corrosion problem unless the water-saturated insulation has destroyed the piping as well, widespread mold contamination can result in losses well exceeding the cost of pipe replacement alone.
Microbiologically influenced corrosion (MIC) is, by far, the most severe and threatening form of corrosion to HVAC piping and fire protection systems. It is caused by the presence of various microbiological agents under specific environmental conditions and can, in some cases, result in an advanced and widespread failure of entire piping systems within only a few years. An MIC presence usually signals a very severe threat to the entire system, requiring extensive cleanings and repeated 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 the life of the system.
Microbiologically influenced corrosion produces large and deep pits due to its utilization of the steel pipe itself as an energy source (often as an alternative to oxygen), as well as through the production of strongly corrosive metabolic by-products, such as sulfuric acid, which further assist 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 commonly found in closed chill water piping, especially those winterizing with glycol, and has been documented to destroy copper, brass, and stainless steel pipe.
35 years ago, the standard piping specification for all HVAC and fire protection systems called for ASTM A53 grade B seamless stock. This is due to the well-recognized higher failure rate and vulnerability of seamed or welded pipe. Today, it is difficult to find seamless pipe installed unless it was specified in its design. Seamed pipe has not become better manufactured; it’s just cheaper. Seamed pipe has a greater vulnerability to corrosion at the seam due to many causes. In many examples, poor manufacturing practices produce an internal or external seam that is incomplete. On the inside, this incomplete weld seam then becomes a focal point for rust and micro-organisms to establish and promote higher corrosion activity, often leading to pinhole failure. A common dissimilarity in electrical potential between the pipe itself and weld filler presents another threat. The zinc protective coating at the weld is sometimes lacking for seamed pipe, initiating a very premature yet defined line of galvanic attack.
Defective seamed pipe is widespread throughout the foreign market. It is less common in better U.S. piping manufacturers but still present. Under low-corrosion conditions, defective seamed pipe can still produce system-wide failure problems, but under the higher corrosion conditions often encountered today, it can amplify the threat and consequences of a severe corrosion problem.
Grooved piping is a well-respected and proven pipe assembly process and method with decades of success. Most failures occur due to either incorrect installation or a severe corrosion condition. Where the outer groove is swaged or rolled into place, the pipe wall is displaced internally and no actual wall loss occurs. Where the groove is cut into the outer surface, substantial pipe wall is removed—similar to the wall loss at threaded pipe. As a result, any high-corrosion condition will reach the base of that outer cut groove first to produce a failure ranging from a pinhole leak to a total pipe separation. And because of the depth of the cut groove, no prior indication of a corrosion loss may have occurred to other areas of pipe. In addition, the end gap between piping sections often allows another corrosion front to act against the pipe from its end dimension.
Due to the potential for far greater than just a pinhole failure, and the possibility of total pipe separation, any leaks at a grooved clamped fitting should be investigated thoroughly.
We have documented more pipe damage due to external corrosion than internal causes. Most is due to insulation failure and the fact that it is hidden from view until a leak, maintenance, or some other event prompts a visual investigation. For uninsulated pipe, such as roof level condenser water lines, a surprisingly high volume of pipe is left to deteriorate and could have been avoided through simple maintenance. The severity of an outer surface corrosion problem can be misleading, but with approximately 18 times as much less dense rust produced from its original volume of pipe steel. In most examples, surface rust is minor, and can be easily addressed by the maintenance staff by mechanical wire wheel and the application of an effective rust reverser and outer protective coating.
Left to continue, surface rust develops into stratified layers under which deep pitting accelerates, and against which only sand blasting will prove effective at its removal.
The first sign of a corrosion problem is usually revealed at the cooling tower. Rust deposits at the pans represent the pipe wall that was once part of the circulating system. White deposits at the tower fill represent a potential scaling condition. Discolored and turbid water are yet another indication that corrosion activity is high and that chemical water treatment is lacking. Algae and other organic growths not only interfere with operations, but also accelerate many other corrosion processes and promote microbiologically influenced corrosion. In many cases, cooling tower maintenance and the removal of rust deposits is performed without ever investigating the underlying cause of the problem—and without considering that the volume of rust deposits exposed at the tower is nothing compared to the volume of rust deposits still inside and firmly attached to its walls.
Most heavy rust deposits are produced after decades of high corrosion activity, only falling loose to be carried to the cooling tower after some form of shock to the system, such as a spring start-up or temperature change. They are rarely captured by most filtration systems or removed or dissolved by most chemical adjuncts.
Soft foam insulation allows moisture to infiltrate to cold pipe surfaces and produce destruction of the steel pipe common to fiberglass insulated systems. Over a relatively short time, soft form insulation deteriorates—hardening, cracking, and shrinking to produce large gaps for moisture to enter. In addition, the foam actually degrades chemically to become slightly acidic, bonding itself to the pipe or rust layers so securely that removing old soft foam insulation becomes extremely difficult. Where high humidity is present and condensation to a cold pipe surface is a serious concern, hard cell “foam glass” insulation is overwhelmingly recommended. As a second choice, a heavier thickness of fiberglass, painted with a high solids coating to act as a moisture barrier, is an option. Soft foam insulation should only be used for temporary or short-term applications.
High corrosion losses at copper pipe resulting in failure are rare. Contrary to common belief, however, copper is not immune to the effects of corrosion. Corrosion conditions resulting in high corrosion losses against steel pipe, and possibly producing a corrosion rate 10 times above normal, will also produce an equally elevated corrosion rate against copper components. Although a 0.3 MPY corrosion rate against copper is normal for an HVAC piping system, high corrosion rates of 3–4 MPY have been measured.
Failures at copper piping systems are mostly related to specific conditions or events, such as acidic and low pH water, galvanic activity and improper grounding or stray voltage, or high steel corrosion activity resulting in the migration of iron oxide into the copper lines.
Brass is found less and less at most commercial properties, having been replaced by easier to install and far cheaper Type L copper pipe. Corrosion activity at brass pipe is typically very low, allowing it to easily provide nearly 100 years of reliable service life
Under certain water quality conditions, however, and where the water supply is more aggressive, the zinc chemical component is leached out of the brass to produce small pinhole failures or fractures and splits. Greenish-white deposits are a common signature of dezincification, which can be confirmed by metallurgical lab analysis. Most dezinzification occurs where water quality is aggressive yet still requires 75 or more years to occur, a length of time generally exceeding the expected service life of most building structures.
Brass is commonly found at older domestic hot water piping systems instead of galvanized pipe due to the effect that the heat has against the zinc protective coating. With the galvanized steel pipe always failing first, and given access difficulties in any domestic pipe replacement projects, the brass hot water component is also replaced, making brass pipe an increasingly less common concern.
Microvolt differences in ground potential between building piping and the building’s structural steel has been cited in some examples of very localized pipe failures. This normally occurs at steel piping supports and hangers, and with direct metal-to-metal contact. Requiring highly sensitive electrical instruments to make a positive diagnosis, ultrasonic testing performed further removed from the area of failure will often show far less and even normal corrosion activity.
Although this form of electrolysis rarely occurs, we consider it prudent to insulate metal to metal contact, and especially where steel pipe is exposed to water, cooling tower overspray, and other weathering conditions.