Different kinds of corrosion Flashcards
what is crevice corrosion?
when a small volume of stagnant (O2 deficient) water becomes trapped in a crevice, making the crevice annodic. the realativley small area being anodic to the much larger cathodic area results in an inequal anode cathode ratio
what is pitting corrosion?
an intense form of localized corrosion caused by surface irregularity or scratches causing an inequal anode cathode ratio. they are important to us because they cause stress concentration sites subject to catastrophic failure
where are likely places for pitting corrosion to occur?
inside stanchion bases, between shafts and bearings, in stern tubes, under barnacles and on keel bolts
what is stainless steel made of?
iron and chromium
what happens when stainless steel is deprived of O2?
the protective chromium layer breaks down. this allows pitting corrosion to occur
what is poultice corrosion?
corrosion that occurs where soft, salt water saturated, usually organic material is in contact with metal for long periods of time
what is MIC?
when microorganisms (like aerobic and annerobic bacteria) from colonies on a metal surface they can facilitate corrosion.
what is dealloying or dezincification?
the preferencial disintigration or one particular component of an alloy
why is brass a bad option below sea level?
because it is an alloy of copper and zinc and dealloying will occur
what is erosion corrosion?
the mechanically assisted attack in which velocity or abrasion by a flowing medium is a major factor in deterioration
where do you often see erosion corrosion?
pipes , valves, heat exchangers etc
what is cavitation corrosion?
an aggressive form of erosion corrosion caused by the formation and collapse of vapor bubbles in a liquid against metal. these bubbles produce explosive shockwaves and the high pressure this ceates tends to enter cracks and pores on the metals surface. what happens next is very similar to pitting corrosion except the surface tends to be more coarse.
where do you often see cavitation corrosion?
mostly on propellers, where it appears as sever pockmarking at the trailing edge of the blade. it can also appear on pump impellers and any surface in contact with high velocity liquids.
what is stress corrosion cracking?
the cracking of metal resulting from the combination of corrosion and tensile stress. SCC typically consists of a network of fine cracks with multiple branches
if SCC is not seen visibly, how else can you find it?
use a low power magnifying glass or penetrating dyes
