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In any given working environment, the lifetime and performance of components is influenced by the phenomena it will encounter, such as corrosion, wear, fatigue, thermal effects, etc. An optimal solution requires a deep understanding of the purpose and the environment to which the component will be exposed.

Whether on the surface or beyond, Oerlikon Metco provides economic solutions to enhance components’ lifetime, performance and reliability. The functional solutions here are some of the many where Oerlikon Metco’s application-tailored methodology has successfully solved the design challenges faced by our customers.

The damaging effects of corrosion cost businesses and industry approximately US$ 2.5 trillion annually (impact.nace.org, 2016). Corrosion is an electrochemical process that occurs between a metal and the surrounding environment. This chemical reaction produces oxides and other undesirable compounds.

The corrosion process consists of three components:

Anode: a metal subjected to the effects of corrosion through oxidation
Electrolyte: acts as the corrosive medium transporting the ions essential for the corrosion mechanism
Cathode: completes the electrical cell through reduction

Corrosion comes in many different forms and can be difficult to manage. For many applications the environment itself is the cause of the corrosion. In other applications the cause may be due to the process or transport of corrosive media. Often high heat can exacerbate the effects of corrosion. When combined with wear mechanisms complex tribochemical effects can occur, which can be even more problematic to combat.

Oerlikon Metco has been providing successful corrosion control solutions to industry for over 85 years. Using processes such as thermal spray, PTA, laser cladding and weld hard facing in combination with a wide choice of materials, we give industry the results it needs to control and prevent corrosive mechanisms. Some of the benefits being:
  • Longer service life for components and systems
  • Improved efficiency over the life of components and systems
  • Reduced cost of replacement
  • Reduced impact on the environment in the form of scrap reduction
For engineers and designers a surface solution can lead to the freedom to choose a structural material that has great properties for the part or system, such as cost, machinability, strength, stress / strain resistance, but may not be corrosion resistant.
 

General Corrosion

General (or uniform) corrosion is evidenced by materials that form a uniform oxide layer over large surface areas, or even the entire exposed surface. Depending on the service environment, some substrates may form a stable, well-adhered oxide layer or passive film. In these situations the rate of corrosion may be very gradual. On other substrates, the oxide layer may not be stable and be continually replaced resulting in a faster, more problematic volume loss. This can also be the case if the oxide layer is disturbed, such as in combination with abrasion or erosion, even when those wear mechanisms are relatively small.

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Pitting Corrosion

Pitting is characterized by craters on a surface, or for thinner parts, holes that may go through the part. Pits are a localized form of corrosion that can be difficult to detect because they can be quite small or covered with corrosive by-products. By the time the pits can be seen using visual inspection, the part or structure may be beyond repair. Pits can manifest in many different shapes, from broad and shallow to narrow and deep. In some instances, a pit may have a narrow throat near the surface but expand subsurface.

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Crevice Corrosion

Crevice corrosion can occur where two or more metallic components leave a narrow gap. Common areas for this type of corrosion are on the faces between bolted or riveted structures, the surfaces formed by fasteners and the structure, under gaskets, overlapping faces, etc. Corrosion results from the build-up of aggressive ions in the crevice or oxygen starvation within the crevice. This gives rise to a differential in polarity between the surface and the crevice, whereby the crevice becomes anodic compared to the surface. This type of corrosion can be extremely aggressive and can lead to failure of multiple components in the system.

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Galvanic Corrosion

Galvanic (or dissimilar metal) corrosion occurs as a result of two metals of differing galvanic potential in close proximity to one another in an electrolyte, such as water, saline solutions or acids. The component that is more anodic will corrode faster than it would if not in a galvanic couple.

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Erosion-Corrosion

Erosion-corrosion combines erosive stress with corrosion thereby accelerating the wear rate. The motion of the erosive media brings the corrosive elements to the metal surface, where mechanisms such as pitting is aggravated by the fluidic motion.

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High Temperature Corrosion

At elevated temperatures, an electrolyte is not required to drive the corrosion reaction. Instead, ions within hot gases attack the substrate directly. In high-temperature oxidation, it is oxygen that causes the attack through the formation of an oxide scale on the surface of the component. Initially, the scale may be relatively stable, but as time goes on, the scale continues to grow. Stresses build within the scale and eventually, the stresses become high enough to cause the scale to crack and spall. The process then repeats to the point of failure.

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CMAS Attack

CMAS (calcia-magnesia-alumina-silicate) is debris that exists in the atmosphere as a result of volcanic activity and other natural and industrial processes. In systems that cycle from very high temperatures to ambient temperatures, CMAS is known to attack EBCs (environmental barrier coatings) and TBCs (thermal barrier coatings), both of which are used in the hot sections of modern gas turbine engines. The CMAS particulate is ingested into the engine, where it becomes molten in the hot section areas where they solidify on cooling to form deposits on turbine components. However, the CMAS and the ceramics used for EBCs and TBCs have very dissimilar thermal expansion coefficients causing the EBC or TBC to crack, spall, and fail.

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