Page 81 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

MECHANICALLY ASSISTED CORROSION 59

roughening the surface to increase friction (72); (iv) increasing the hardness of one or
both of the contacting surfaces, surface hardening such as carburizing and nitriding
or applying protective coatings by electrodeposition, plasma spraying, vapor depo-
sition, or anodizing of aluminum alloys. Hard materials are more resistant than soft
materials. A soft surface can yield by shearing instead of sliding (8) at the interface;
(v) increasing the load to reduce the slip between mating surfaces (9, 17); (vi) using
dissimilar metals that give wear resistance when coupled. Steels may be coupled with
silicon bronze and stellite alloys. Surface treatment or adding a coating is useful in
preventing wear (60).

1.7.27 Testing

The measurement of corrosion, wear, corrosion-wear interactions, and erosion–
corrosion interactions involves a multistep process. Each aspect of the interaction
must be measured separately. The resulting data must be combined to assess the
synergistic effects and arrive at a total picture of the damage process. Measurement
of the combined action of wear and corrosion is not easy. The ASTM, G119 (30)
standard test applies to liquid systems or slurries and can be adapted to dry corrosion
and wear combination as well (22).

1.7.28 Measurement of Wear and Corrosion

Jet and whirling arm tests are used to assess erosion (22). In the whirling arm
test, the impact velocity is known, and the entire face of the sample is eroded
resulting in a more uniform surface. The machining test is commonly used in
high-temperature abrasive tests, as the machining process results in high tem-
peratures. The high-temperature ring-on-disk test is commonly used for abrasive
elevated-temperature tests (22).

1.7.29 Galling Stress
Wear galling is a good measure of wear resistance of a given material pair. Galling
results in a groove or a score mark, leading to a mound of metal. Galling data show
that identical metals do poorly than dissimilar metal couples with respect to galling
wear resistance. Stainless steels when coupled together exhibit poor galling resistance
compared to other steels by a factor of 2 or more (60).

1.7.30 CF
CF involves cracking in materials under the combined action of a fluctuating or cyclic
stress and a corrosive medium. The damage because of CF is generally greater than
the sum of the damage by corrosion and fatigue acting separately.
CF is involved in the case of the shaft of a ship propeller, slightly above the water-
line, which can normally function until a leak occurs, resulting in the water impinging
on the shaft in the area of maximum alternating stress. Failure of the shaft in a few