Corrosion is defined as the deterioration of a metal because of a reactionto the environment around it.
Corrosion monitoring is a method of determining and examining equipment, plant, tanks, vessels and pipes, for corrosion and is performed to ensure safe and continued long-life operation of equipment and facilities.
Corrosion monitoring enables:
Corrosion is a complex process, and hard to measure and can be general or very localized. The causes of corrosion include electrochemical, chemical, bacterial, erosion, or protective film loss. Areas affected by corrosion are not always easy to identify.
Corrosion is a safety and environmental hazard, with additional financial cost to industry in loss of production can be extensive, particularly to those dealing with hazardous or explosive products such as those of the oil and gas and petrochemical industries. This increases where plant and equipment have aged and become more prone to corrosion and the resulting equipment failures.
Proactive corrosion monitoring saves lives, the environment, and money, improves safety, reduces maintenance and replacement costs, and maximizes productivity.
Corrosion monitoring can be split into two areas – measuring and inspection.
Nondestructive testing (NDT) techniques find, identify and monitor the rate of corrosion damage. The most common NDT methods used to detect corrosion include Ultrasonic Testing (UT), Radiographic Testing (X-ray), and Magnetic Flux Leakage.
Corrosion measuring (monitoring) is a proven solution to risk reduction, minimizing unnecessary facility shutdowns, and providing asset life extension, delivering significant cost savings in plant replacement.
Monitoring offers direct corrosion measurements when equipment, plants and facilities are operating.
Corrosion monitoring systems use corrosion (weight loss) coupons, electrical resistance (ER) probes, linear polarization resistance (LPR) probes, and galvanic monitoring (ZRA).
More advanced monitoring methods include biological monitoring, environmental (ECM) monitoring, and ultrasonic thickness (UT) monitoring, and it is not uncommon to find multiple methods used together.
Corrosion coupon, (or weight loss coupon) testing is the oldest form of corrosion monitoring applied in all environments. A coupon is a sample piece of the metal or alloy matching the equipment or plant. Each is of an accurately known weight, and is placed into the process environment and removed after some time, and then analyzed in a laboratory for:
The disadvantage of coupons is when inserted in the environment, no corrosion data can be collected or reviewed. It is impossible to attribute corrosion rates to particularly changing process conditions.
Corrosion coupon, (or weight loss coupon) testing is the oldest form of corrosion monitoring applied in all environments. A coupon is a sample piece of the metal or alloy matching the equipment or plant. Each is of an accurately known weight, and is placed into the process environment and removed after some time, and then analyzed in a laboratory for:
The disadvantage of coupons is when inserted in the environment, no corrosion data can be collected or reviewed. It is impossible to attribute corrosion rates to particularly changing process conditions.
Corrosion Monitoring methods are a proven solution in reducing facility shutdown time, extending life, and saving companies costs. It is common to apply multiple techniques at the same time.
Probes measure the change in electrical resistance of a component and provide information about the metal loss. These probes can perform in all environments over long periods. They provide periodic or continuous data about the rate of corrosion when in service and may be used to monitor the effects on corrosion rates when process variables change
Probes utilize an electrochemical technique that can only work in conductive media and aqueous fluids. LPR probes work by applying current to an electrode and instantaneously measuring the corrosion rate. They can provide periodic or continuous data about corrosion when in service and may be used to monitor the effects on corrosion rates when process variables change
nother electrochemical technique that measures galvanic current passing between two dissimilar electrodes. The results provide information about the oxygen content in the media.
The commonly used NDT method that measures the component wall thickness. This method is sensitive and is capable of scanning large surface areas using ultrasonic waves generated from an externally mounted sensor.
Almost all metals are found naturally as ores. The manufacturing process of extracting the pure metal from the ores can involve the input of lots of energy, normally in the form of heat as does the alloying, forming, and manufacturing into finished products.
Corrosion is the process where the energy added to the metal during this manufacturing is released, and the metal is returned to its oxide state.
Almost all metals are found naturally as ores. The manufacturing process of extracting the pure metal from the ores can involve the input of lots of energy, normally in the form of heat as does the alloying, forming, and manufacturing into finished products.
Corrosion is the process where the energy added to the metal during this manufacturing is released, and the metal is returned to its oxide state.
Metal Ore ⇒ reduction (add electrons) ⇒ Pure Metal ⇒ oxidation (remove electrons) ⇒ Corrosion Products
Energy in ⇑
Energy out ⇓
Where there is an electrolyte present in the environment, such as seawater, water, or moisture in the soil, the corrosion (oxidation) process is electrochemical.
Corrosion takes place because of the formation of electrochemical cells. For the corrosion reaction to occur five factors are necessary. If any of these things are removed, electrochemical corrosion will not happen. The five factors needed are:
An ANODE, the metal or place on the metal surface where oxidation occurs (loss of electrons and where the signs of corrosion occur). The anode has a more negative potential than the CATHODE and is called less noble than the CATHODE.
a CATHODE - the metal or place on the metal where reduction occurs (gain of electrons). The CATHODE has a more positive potential than the anode and is called nobler than the ANODE.
an ELECTROLYTE - the electrically conductive environment in which the anode and cathode sit, typically seawater, water, or moisture in the soil.
An ELECTRICAL PATH - the ANODE and the CATHODE must be electrically connected, normally this is through the body of the metal but can be a cable or other electrical bond.
a POTENTIAL DIFFERENCE - a potential (voltage) difference must exist between the ANODE and the CATHODE; this is the difference in potential between the ANODE and CATHODE
Table 1. Schematic of the corrosion cell
The corrosion cell is powered by the potential difference between the ANODE and CATHODE, this causes a current to flow, the size of which is determined by the electrical resistance in the circuit according to Ohm’s Law. Remove any one of the elements and corrosion cannot occur.
The potential of metals can be measured against a known standard a common one is a copper/copper sulfate reference electrode (CSE). When electrically connected, the more negative potential metal will become the ANODE and the more positive potential metal will become the CATHODE. Table 1 gives a list of common metals and their potential
Table 1. Schematic of the corrosion cell
As can be seen, Mild steel has a range of potentials, so that it can easily create its corrosion cell and when copper is connected to clean mild steel in an electrolyte the steel is the more negative and becomes the ANODE to the CATHODE copper, the steel will then corrode as no corrosion normally occurs at the CATHODE.
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