Noble metals, erosion-corrosion

Galvanic corrosion may also occur by transport of relatively noble metals, either as particulate or as ions, to the surface of an active metal. For example, ions of copper, perhaps resulting from corrosion or erosion-corrosion at an upstream site, may be carried by cooling water to the surfaces of aluminum, steel, or even stainless steel components. If the ions are reduced and deposit on the component surfaces, localized galvanic corrosion may result. 

Erosion caused by high exit velocities can also cause corrosion problems. Some metals do not corrode due to a self-regenerating protective surface film however, if this film is removed by erosion faster than it is formed, the metal corrodes rapidly. For lining either noble metals or ceramics should be considered in such situations. The preferred arrangement for flashing service is to use a reduced port angle valve discharging directly into a vessel or flash tank. 

A stainless steel pump impeller with an expected lifetime of two years failed in three weeks in a reducing solution. Metals that are soft are readily damaged or worn mechanically examples are copper and lead. Even the noble or precious metals, such as silver, gold, platinum, are subject to erosion-corrosion.16,31... 

Magnesium is thermodynamically one of the less noble metals, and it can protect most other metals when used as sacrificial anodes (see Section 10.4). In the atmosphere the metal is covered by an oxide film. Therefore it resists rural atmospheres but is subject to pitting in marine atmospheres. Magnesium alloys are also liable to SCC and erosion corrosion, and are attacked by most acids. Mg alloys are used in automobile engines, aircraft, missiles and various movable and portable equipment, in all cases primarily because of their low density (1.76 g/cm ). 

Although copper and brass typically have good corrosion resistance in aqneous solntions, they may be subject to corrosion in plant environments, depending upon the process stream. The presence of sulfides and ammonia componnds in wastewater can lead to dissolntion of cuprous compounds. Further, if copper is coupled to a less noble metal like steel or alnminnm, galvanic corrosion of the less noble metal may resnlt. Because copper is a fairly soft material, it is also subject to erosion. This type of corrosion is accelerated by high fluid velocities, high temperatures, and abrasive particulate matter. 

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