1 Corrosion general
Corrosion is serious problem in the world, each year the over trillion dollars are lost in the world, every year industrial company spread another trillion dollars to prevent corrosion, from building, bridge to ship industrial process all the equipment with metal material is experience the corrosion.
Corrosion is a natural process that converts a refined metal into a more chemically stable form such as oxide, hydroxide, or sulfide. It is the gradual destruction of materials (usually a metal) by chemical and/or electrochemical reaction with their environment. Corrosion engineering is the field dedicated to controlling and preventing corrosion.
Inspection and find the corrosion rate and to set up anti-corrosion measure is very important to power plant successfully operation.
2 Type of corrosion:
There is different type of corrosion to effective for the power industrial which author are lists below:
- FAC corrosion, due to temperature and PH and material caused, most happen at HRSG and nuclear power plant
- Stress corrosion crack is the cracking induced from the combined influence of tensile stress and a corrosive environment
- Pit corrosion form of extremely localized attack that results in holes in the metal
- Line corrosion is happening at where pits are connected or nearly connected to each other in a narrow band or line.
- General corrosion, uniform corrosion characterized by a chemical or electrochemical reaction which proceeds uniformly over the entire exposed surface or over a large area
- Under deposit corrosion is a general term that refers to localized corrosion that develops beneath or around deposits present on a metal surface. There may not be one single corrosion mechanism suitable for explaining UDC that occurs in different processes or conditions
- Microbiologically influenced corrosion (MIC) refers to corrosion caused by the presence and activities of microorganisms. While microalgae, bacteria, and fungi do not produce unique types of corrosion, they can accelerate corrosion reactions or shift corrosion mechanisms
- Grooving corrosion, a specialized form of erosion corrosion which also occurs frequently within ships.
- Exfoliation and selective leaching is removal of one element from a solid alloy by corrosion processes
- Galvanic corrosion (also called bimetallic corrosion or dissimilar metal corrosion) is an electrochemical process in which one metal corrodes preferentially when it is in electrical contact with another, in the presence of an electrolyte. A similar galvanic reaction is exploited in primary cells to generate a useful electrical voltage to power portable devices. Crevice corrosion occurs within crevices and other shielded areas on metal surfaces exposed to corrosives
- Intergranular Corrosion. Grain boundary effects are of little or no consequence in most applications or uses of metals.
- Erosion corrosion is the acceleration or increase in rate of deterioration or attack on a metal because of relative movement between a corrosive fluid and the metal surface
3 Pit corrosion experience
Author experienced a pit corrosion at STG lube oil cooler bottom cap, here like to share the process and root cause.
One day floor after the SGEC commissioning 1 years, operator reported to me the STG (510MW) Toshiba steam turbine lube oil cooler has a leakage from the bottom figure 1 and figure 2, to be author surprised is the lube oil cooler water side pressure is not high, and lube oil cooler only operation less than 1 years, and the thickness at least 10 mm, why it is leakage, does there is manufacture default on the cooler, closely look at the cooler the leakage is not on the welding area or the welding effect zone. Plant transfers the cooler to standby cooler operation then to
4 Pit corrosion Root Cause
Figure 1 STG lube oil cooler leakage
Figure 2 lube oil cooler leakage
From the picture is showing the slit a sludge deposit on the surface to form under deposit pit corrosion the chemical reaction with the painting which cause the painting was damaged. According to the lists of type of the corrosion, this is under sludge deposit then developed to pit corrosion. The driving power for pitting corrosion is the depassivation of a small area, which becomes anodic (oxidation reaction) while an unknown but potentially vast area becomes cathodic (reduction reaction), leading to very localized galvanic corrosion. The corrosion penetrates the mass of the metal, with a limited diffusion of ions. Metal oxidation results in localized acidity that is maintained by the spatial separation of the cathodic and anodic half-reactions, which creates a potential gradient and electro migration of aggressive anions into the pit.
Pitting can be initiated by a small surface defect, being a scratch or a local change in the alloy composition (or local impurities, e.g. metallic sulfide inclusions such as MnS or NiS), or a damage to the protective coating.
Figure 3 Dismantle the bottom flange to see inside
Figure 4 some sludge build on the inside of the bottom
5 how to fix it
Figure 5 under deposit pit corrosion
Figure 6 sand blasted bottom to show the corrosion
Figure 7 the review of the entire bottom
For this cause, plant sent the end cap to the contractor shop to cleaning and sandblast all surface (figure 7) then welding repair the pit area, repainting all the surface to prevent future corrosion , installed small piece of the sacrifice zine anode block which can last one year, then set up yearly inspection the end cap. In the meaning time to set up use the ultrasonic to scan the end cap yearly , also can add the corrosion inhibitor to the cooling tower water but it is expensive so install sacrifice anode it best selection .
6 Conclusion:
Pit corrosion is fast process under the suitable condition, the lube oil operation less than 1 year. The pit corrosion penetration the entire wall thickness and cause the leakage. Pit corrosion is often difficult to detect and so is extremely insidious, as it causes little loss of material with the small effect on its surface, while it damages the deep structures of the metal.