Type of Corrosion
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Discuss the major types of corrosion. uniform Corrosion: uniform corrosion is characterized by corrosive attack proceeding evenly over the entire surface area, or a large fraction of the total area. General thinning takes place until failure. On the basis of tonnage wasted, this is the most important form of corrosion. (corrosion-doctors. org) This reminds me of some of the contractor spec’s I have seen for just this type of corrosion.
Many of the clarification mechanisms I have seen specs for require a safety factor that If a 1/4″ of the total cross section of each ember were to corrode away that the mechanism would still be strong enough to take the load. These mechanisms are usual in a very acidic, completely submerged environment, so this would make a lot of sense for a long salting system. Erosion Corrosion: Erosion corrosion is acceleration in the rate of corrosion attack in metal due to the relative motion of a corrosive fluid and a metal surface.
The Increased turbulence caused by plttlng on the Internal surfaces of a tube can result In rapidly Increasing erosion rates and eventually a leak. (corrosion-doctors. org) An example would be ater running over a dam, usually a movable gate. Another example would be an impeller in a clarification tank for mining purposes. have experience with these and the motion of the spinning impeller is the most damaging. I have seen the corrosion patterns in the metal from is spinning and turbulence.
Galvanic Corrosion: Galvanic corrosion (also called ‘ dissimilar metal corrosion’ or wrongly ‘electrolysis’) refers to corrosion damage Induced when two dissimilar materials are coupled In a corrosive electrolyte. (corrosion-doctors. org) I have seen this in aluminum hand rails for walkway bridges. Usually this happens when a contractor forgets to put the rubber spacer between the aluminum and steel. The aluminum last only about a year before the hand rail falls off, making for a hug problem.
Pitting Corrosion: Pitting corrosion is a localized form of corrosion by which cavities or “holes” are produced In the material. Pitting Is considered to be more dangerous than uniform corrosion damage because It Is more difficult to detect, predict and design against. (corrosion-doctors. org) I have mainly seen pitting corrosion in gear faces. Many of the gears I work with are very susceptible to different lubricants. The wrong lubricant and pressure right on the face of a gear makes pits that are usually very large. This leads to a sand paper like noise coming from the gear at low speed.
Crevice Crevice corrosion Is a localized form of corrosion usually associated with a stagnant solution on the micro-environmental level. Such stagnant microenvironments tend to occur in crevices (shielded areas) such as those formed under gaskets, washers, insulation material, fastener heads, surface deposits, dis-bonded coatings, threads, lap Joints and clamps. (corrosion-doctors. org) I have little experience of this type of orrosion; from the reference It looks like a nut is the last thing to corrode and the material around Is the one corroding away.
I would Imagine a high strength weld nut doing the same type of thing, or a material around a weld doing this. I also see this being attributed to a large spread between the materials in the galvanic scale. Filiform Corrosion: This type of corrosion occurs under painted or plated surfaces when moisture permeates the coating. (corrosion. ksc. nasa. gov) I have seen this under many paints and chrome plating. The paint looks like a blister, and in steels leads to an exfoliation type corrosion.
This goes to show that all paints are not created equal and can actually hurt you in the long run. Inter-granular Corrosion: The microstructure of metals and alloys is made up of grains, separated by grain boundaries. Inter-granular corrosion is localized attack along the grain boundaries, or immediately adjacent to grain boundaries, while the bulk of the grains remain largely unaffected. (corrosion-doctors. org) Most of my experience of this is from school. I have seen this in a pump shaft failure.
The interesting part is that when the crack starts if will move around the grains until it gets enough energy to go straight hrough the grains. Under a microscope it is very nice that you can see plain as day the crack moving around the grains. Exfoliation: Corrosion products building up along these grain boundaries exert pressure between the grains and the end result is a lifting or leafing effect. (corrosion- doctors. org) The best example of this is rust on an old truck. The rust leaves large leaf like chunks of rusty metal behind and then Just flakes off.
When you see it you can crunch all of the layers together with your hand. It is a major weakening of the structure, and is one of the worst I have seen. This is usually very easy to see and is n large areas. Cavitation: Cavitation occurs when a fluid’s operational pressure drops below its vapor pressure causing gas pockets and bubbles to form and collapse. This can occur in what can be a rather explosive and dramatic fashion. (corrosion-doctors. org) Cavitation-corrosion is particular form of erosion caused by the “implosion” of gas bubbles on a metal surface. cdcorrosion. com) I never knew about this type of corrosion.
After reading along in the webpage I discovered a lot of hydraulic systems see this at large pressures, temperatures and changes in these parameters. I once changed an ORF itting on a hydraulic test stand I worked with because it was leaking. The face where the O-ring was looked like it had blown out. This type of corrosion is defiantly what I saw, so now I know. Fretting Corrosion: Fretting corrosion refers to corrosion damage at the asperities of contact surfaces.
This damage is induced under load and in the presence of repeated relative surface motion, as induced for example by vibration. (corrosion-doctors. org) I have seen this in long walkway bridges that are not bolted down because they expand and contract about 2″ as the day goes on due to temperature. The bridge weighs a lot and the liding poinst makes large grooves on a steel to steel surface. The paint doesn’t last, and so rust sets in. I don’t know why a bearing system is not put in other than the cost is too great and the plates are so thick that corrosion would take years to eat through.