Tinjauan terhadap berbagai jenis atau tipe korosi sering membantu dalam mendeterminasi penyebab korosi. CorrView International, LLC merilis serangkaian review yang merupakan hasil dari Ultrasonic Piping Investigations selama 18 tahun.
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 ini disajikan type 1 – 10, selebihnya akan disajikan pada bagian 2.
A piping failure is often the first sign of a corrosion problem. Yet in many examples, signs of an impending pipe failure have been evident for months or years, and gone ignored. Failures can be minor (in the form of a pinhole leak) or catastrophic, with significant losses due to water damage as well as the cost of pipe replacement.
Pipe repairs take on various forms, ranging from temporary clamps to the replacement of entire piping systems. In many examples, the denial of a corrosion problem results in multiple or staged repairs over years, wasting valuable time that could have otherwise been used to correct the problem and minimize far greater corrosion damage. All too often, one or multiple individual failures are fixed without further investigation into the hidden cause. The absolute reliance on favorable but typically inaccurate corrosion rate data presented by corrosion coupons, often in contrast to blatantly obvious physical indicators such as thread leaks and high rust deposits, allows a high corrosion condition to continue unabated, thereby producing a much greater repair problem once the true corrosion problem is finally realized.
Every pipe thread is an inherent point of weakness, with approximately 50% of the pipe wall cut away. Often, a thread leak is the very first sign of a corrosion problem and will prompt further investigation. Left to continue unaddressed, however, total pipe separation often occurs to produce devastating water-related damages. While the condition of large piping mains is normally the greatest concern to a building or plant manager, it is the threaded pipe that typically causes the greatest damage. A high pitting condition will cause failures at random areas of the threads, allowing water to pass through, even though sufficient pipe wall remains to prevent a larger failure. For small leaks where the rate of evaporation exceeds water loss, dissolved iron oxide and other deposits accumulate at the threads to suggest a corrosion problem originating at its outside surface. In reality, however, it is an internal corrosion problem.
For more uniform but high corrosion activity, a more dangerous condition exists at the threads since the pipe wall is reduced more evenly and does not provide the telltale leak indication of a problem. For all forms of thread leaks, the potential always exists to suffer a total thread failure.
Galvanic corrosion can occur when different metals are joined together, and is greatly dependent on already existing corrosion conditions and the piping system involved. It is more common in open condenser water and process water systems than in chill water or fire protection systems, for example. It commonly occurs between a carbon steel pipe joined to a brass valve, with the most serious examples found at galvanized steel to brass valve connections. A bluish-green deposit at the valve and lack of leakage at its opposite steel-to-steel connection provides confirmation that a galvanic condition exists. It is most prevalent where corrosion activity is already high, and can produce widespread failures and total pipe separations.
In many cases, a leak at a thread between black steel pipe and a brass valve or copper pipe is automatically assumed to be due to galvanic activity when a much larger and more threatening high corrosion problem is actually the cause. In such cases, pipe replacement with dielectric insulating fittings is a costly misdiagnosis that can lead to years of continued higher corrosion activity and eventual system failure.
Internal rust deposits, commonly termed tuberculation, are an inevitable death sentence for most piping systems. They are the lighter and less dense end product of steel pipe corrosion. Once established by high and uncontrolled corrosion conditions, internal deposits initiate much greater deep pitting. The highest corrosion loss is more likely at horizontal lines and in low flow or dead end areas where rust and other deposits will settle, but can affect vertical lines and main risers as well. Random areas of high tuberculation are actually the result of a high pitting or cell corrosion condition directly beneath it, with the volume or height of the rust deposit directly proportionate to the depth and volume of pipe wall loss.
Contrary to manufacturers’ claims, standard fiberglass insulation provides an ineffective moisture barrier for cold water pipe. Condensed area humidity then produces a secondary, and typically hidden, corrosion condition at the pipe exterior. Most common at chill water and dual temperature systems, external corrosion losses will well exceed internal corrosion rates by up to 10 times or more. Over decades and hidden from view, insulation failure can destroy entire piping systems. In most examples where the insulated pipe is not hidden from view, an insulation failure problem is very easy to recognize. Missing, damaged, walked on, and broken insulation defines a likely problem worth investigating. Water damage, dripping pipe, discoloration, or crystallization of the outer insulation surface are further signs suggesting a potential problem. Aluminum and vinyl outer jacketing provides little resistance to moisture migration, but does shield the often telltale discoloration of the below insulation from view. In addition, vinyl jacketing often holds the water condensed at the cold pipe surface, producing a much greater threat of exterior pipe corrosion.
The threat of insulation failure is greatest in smaller diameter piping due to its inherently lesser wall thickness combined with generally thinner insulation applied. Lower temperatures at supply side piping can significantly increase this threat.
Of all the forms of corrosion caused to piping systems, weathering damage due to rain, snow, atmospheric conditions, or cooling tower overspray is the easiest to prevent. The piping is exposed and accessible, with corrosion activity always visually obvious. Most weathering damage requires decades to produce a failure, and is simply due to a lack of maintenance. Smaller diameter piping is always most vulnerable due to its inherently lesser wall thickness.
Under deposit corrosion represents one of the most damaging forms of corrosion to a piping system. Often called “cell corrosion”, it is typically very aggressive and localized, causing deep penetration of the metal surface with lesser general corrosion in the surrounding areas. Due to surface deposits, electrical imbalance, or some other initiating mechanism, all corrosion factors attack a select number of individual sites. In some cases, pitting is extended throughout the entire metal surface, giving it an irregular or very rough surface profile. In other instances, pits are concentrated in specific areas, leaving the majority of the metal surface in a like-new condition. Galvanized pipe is highly susceptible. Microbiological attack is often associated with under deposit corrosion. Regardless of the underlying reasons for an internal piping deposit condition, the presence of rust deposits signals a multitude of potential problems to varying degree. As we have stated repeatedly, under deposit corrosion is an inevitable death sentence to any piping system. Therefore, effectively removing such deposits so that chemical inhibitors can again reduce corrosion losses must always be considered a highest priority.
Corrosion Under Insulation (CUI) is a widespread and typically hidden threat. The underlying source of the problem is a misconception that fiberglass and/or soft foam insulation provides a moisture barrier, and is the primary choice of insulation based on reducing heat transfer, rather than reducing humidity migration. Most cold water piping has insufficient insulation thickness, is of the inappropriate type, is improperly installed, or all three. Damage, breaks, and areas of missing insulation allow the easy migration of moisture. This is a widespread and very massive problem in apartment and condominium properties where dual temperature heating and cooling risers have been installed at exterior building columns. Now 45 or more years after construction, the riser piping is almost totally destroyed from the outside—with no options available other than total pipe replacement at an enormous cost and inconvenience.
Smaller diameter pipe is more vulnerable to CUI due to the typically lesser insulation thickness applied and inherently thinner pipe wall. Supply side pipe is always impacted worse due to colder surface temperatures. The addition of threaded ends adds another layer of vulnerability for smaller pipes commonly found at chill water and dual temperature systems.
Piping insulation is an important and necessary requirement of any mechanical plant operation. While insulation at steam, condensate, other hot water lines is needed to prevent heat loss, it is even more important to chill water and cold piping services since moisture condensation can severely damage the pipe. It seems illogical, but proper insulation is often overlooked, resulting in potentially significant piping and equipment damage. Drain lines, vents, and other small diameter fixtures that are unlikely to produce any measureable heat transfer loss from the system often remain uninsulated, ignoring the threat of constant sweating and exterior pipe wall loss.