1. 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 member 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 an 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 pitting 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 water running over a dam, usually a movable gate. Another example would be an impeller in a clarification tank for mining purposes. I 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 Corrosion:
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 corrosion; 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 through 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 in 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 fitting 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 sliding 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.

Environmentally Induced Cracking: i) Stress corrosion cracking: * This is usually caused by a constant tension stress in something like a pinned connection. It is similar to the inter-granular corrosion but is introduced or accelerated be stress.

ii) Corrosion fatigue cracking: * Similar to the SCC but is a cycling loading condition. These are usually trans-granular with no branches in the crack. (corrosion-doctors.org)

iii) Hydrogen induced cracking: * There are several categories of hydrogen phenomena that are localized in nature. Atomic hydrogen, and not the molecule, is the smallest atom and it is small enough to diffuse readily through a metallic structure. When the crystal lattice is in contact or is saturated with atomic hydrogen, the mechanical properties of many metals and alloys are diminished. (corrosion-doctors.org) I have no experience with this type of cracking. It does sound very bad that it can change mechanical properties. This looks very hard to detect, and contribute a failure too.

iv) Hydrogen blistering: * Atomic hydrogen diffusing through metals may collect at internal defects like inclusions and laminations and form molecular hydrogen. High pressures may be built up at such locations due to continued absorption of hydrogen leading to blister formation, growth and eventual bursting of the blister. Such hydrogen induced blister cracking has been observed in steels, aluminum alloys, titanium alloys and nuclear structural materials. (http://en.wikipedia.org/wiki/Hydrogen_damage#Blistering) I again have never seen this. I see a major problem is that the blister could push in to a passing part causing damage in multiple components.

Corrosion fatigued:
Corrosion fatigue is a special case of stress corrosion caused by the combined effects of cyclic stress and corrosion. No metal is immune from some reduction of its resistance to cyclic stressing if the metal is in a corrosive environment. Damage from corrosion fatigue is greater than the sum of the damage from both cyclic stresses and corrosion. (corrosion.ksc.nasa.gov) This is usually shown by ‘beach’ marks in the fracture surface. You can also see individual crack progression in the parts. I have seen this on a Brocken shaft that was turning very slowly under large stress and contaminated oil.

Dealloying:
Dealloying is a rare form of corrosion found in copper alloys, gray cast iron, and some other alloys. Dealloying occurs when the alloy loses the active component of the metal and retains the more corrosion resistant component in a porous "sponge" on the metal surface. (corrosion.ksc.nasa.gov) Have never seen this in its raw form but is similar to pitting corrosion. Another source says: Dealloying or selective leaching refers to the selective removal of one element from an alloy by corrosion processes. A common example is the dezincification of un-stabilized brass, whereby a weakened, porous copper structure is produced. (corrosion-doctors.org) I would be interested to see an example in my hands. This is tough to wrap my head around, because just one element in being corroded out of a material.

Bio-corrosion:
Kinetics of corrosion processes of metals, mineral and polymeric materials can be influenced by biofilms. Products of their metabolic activities including enzymes, exopolymers, organic and inorganic acids, as well as volatile compounds. (.besg.group.shef.ac.uk/Research_Area/Applied_Biofilm/Biocorrosion.htm) I have seen this in the waste water filtration industry. They use many little organisms in the water to kill, eat, or hold onto the different bacteria or ‘stuff.’ This makes a corral looking stuff on the steel that eats right through it. I haven’t looked at it close. Usually because it look gross and I know what else is in the water.

2. What surface characteristics of surfaces play a role in corrosion? How can surface be characterized and what factors should be considered?

Surface characteristics: I. Mechanical: * Finish, smoother usually equals tougher for corrosion to latch onto * The stress should be low if concerned about SCC * Surface finish combined with lubrication that sits on the surface can lead to corrosion. * Hardness

II. Chemical: * What elements in the material or electrolyte may be harmful to each other, or may react with each other * Treated by some kind of chemical like salt bath nitriding to help limit corrosion

III. Electrical: * Does an electrical current promote corrosion, or change the material make up. * Coatings attached by electrolysis, that could be reversed be another opposite current.

IV. Thermal: * Elevated temperatures generally speed up corrosion processes. Is this speed too fast for the design?

V. Environmental: * Probably the most important. * Moisture * Temperature, Change in * Climate * What’s in the electrolyte, or what is the electrolyte

3. Corrosion failures are classified by such terms as section reduction, change of appearance etc. Suggest other ways to characterize corrosion failures. * Mechanical properties changing, removal of just one element. * Distortion * Material missing * Something growing on it * Local, or even corrosion

4. What role, if any, does stress play in corrosion failures and describe the mechanisms of corrosion that have stress dependency. * All of the corrosion described such as SCC, and SFC. * Usually if stress introduces of changes the surface so that corrosion is easier to start. * Cracks!

5. Based on the question 4, can you visualize corrosion failures originating subsurface? How could this occur? * Origin would be at the point of highest stress * Cracks start at highest stress, so corrosion can then start in a crack creating two failure modes.

6. How would you suggest that corrosion degradation can be classified and related to component monitoring through non-destructive testing (NDT)? * Size of flaw * Area in which we are looking for a flaw, large or small. * Ways to find flaw, Ultasound, Eddey current, radiography, ect. * Who is certified to look for certain things (size). * How much will this cost?

7. Coatings are frequently used to prevent corrosion. What are the characteristics of coating would be important to prevent corrosion. * Complete coating * Impermeable * Galvanic series, Zinc * Resistance to the electrolyte * Thickness

8. Discuss the concept of inter-granular corrosion. What characteristics of material are important to the rate of progression of inter-granular corrosion? * This is where a crack is going around the grain sizes. This is described in more detail above. A large grain will hinder the propagation of a crack. Also things like work hardening a material, artificially creating large grain bounders will hinder the crack. * Size of the crack is importance because at some point it will become trans-granular and the crack will move fast. 9. In a general sense, discuss the roles of temperature and time in the progression of corrosion. * Elevated temperature usually promotes corrosion. Especially in the Bio-corrosion. Temperature also promotes the chemical changes that usually occur with either oxygen or hydrogen in a material.

10. What forms of corrosion could lead to cracking in engineering materials. * The kinds that like to actually remove material or change material characteristics. Combined with some type of stress this will create cracking on a large scale.

11. Discuss the following three corrosion tests: (astm.org) * ASTM G 48 i. Lab testing that is used to rank stainless alloys and their related alloys to its resistance to pitting or crevice corrosion. A ferric chloride solution is used to create the corrosive environment. Mainly used for 304 SS. This test focuses on the examination and examination of the corrosion.

* ASTM F 746 ii. Similar to the test above, but is used for mainly 316 SS and for the surgical implant industry.

* ASTM G 61 iii. This is for testing iron, cobalt, and nickel alloys. This method is given by the potential at which the anodic current increases rapidly. This test is not intended to correlate in a quantitative manner with the rate of propagation that one might observe in service when localized corrosion occurs. This method used conducting cyclic potentiodynamic polarization measurements, which I do not get.

12. How would you suggest corrosion failure mechanisms be incorporated in a “damage tolerant design”? * I think that is I was writing a spec for a damage tolerant design I would not be very concerned by which mechanism of corrosion is affecting my design. I am more concerned with the end result, i.e. how much of the design has been corroding away, and how strong is it after a certain amount of corrosion. I may focus on a mechanism if it seems like it will occur more easily. SCC in a pinned member or exfoliation in a piece of steel in a water tank. I would still focus my time in the end result and to avoid failure. There are too many possible mechanisms to try and write out in a design. Just write ‘corrosion cannot do this…..’ but with a little more professional wording. Initial flaw size stuff

13. Do you believe corrosion phenomenon play a role in establishing product reliability and quality? * Yes, simply put. * Reliability is completely based on how a design may corrode and how fast this may happen. I don’t quite know how I would put a timeline on some of these corrosion mechanisms, and I would find that very interesting. * Quality I see as how well a design was made. i.e. surface finish, coatings, installation etc. This will affect promote corrosion if it’s done incorrectly. Also material quality, in chemical makeup and grain size could affect corrosion and how it may become a problem.

14. How could you suggest that the education of mechanical engineers to be modified in order to introduce more of corrosion? * I can only speak of the University of Utah, but it is slim to none the amount undergraduates get. Only a short 2 credit class is offered. Not until you are a graduate student that you are really shown corrosion. Just last week I turned over a decision at work due to my fear of corrosion and potential failure in the next 20 years. This all came from the knowledge from this class, so I know it can be very useful. 15. What role do corrosion phenomena play in manufacturing? * Most of my experience in this comes from rust starting before a coating can be applied. Usually within just a few days, that is very hard to remove on a large scale. Also, as discussed above the quality of the manufacturing process will affect corrosion. Handling of parts in the fabrication house, no scratches, dents, using carbon tools on stainless steel, can all affect corrosion.

16. Make a cause and effect diagram for corrosion
See attached

References
.besg.group.shef.ac.uk/Research_Area/Applied_Biofilm/Biocorrosion.htm. (n.d.). astm.org. (n.d.). cdcorrosion.com. (n.d.). corrosion.ksc.nasa.gov. (n.d.). corrosion.ksc.nasa.gov/filicor.htm. (n.d.). corrosion-doctors.org. (n.d.). http://en.wikipedia.org/wiki/Hydrogen_damage#Blistering. (n.d.).