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Corrosion Sl Part Two
- 1. Module 2 “The Forms of Corrosion”
- 6. New—Old Pipe Cell Old Pipe (cathode) New Pipe (anode) Old Pipe (cathode)
- 19. Copper Pitting
- 26. Crevice Corrosion Background M+Cl- + H2O = MOH + H+Cl- Corrosion Inside Crevice (Anode) Chemical Reaction with Salt NaCl
- 29. Results of Statistical Analysis
- 30. Another Example of Crevice Corrosion
- 34. Stress Corrosion Cracking Susceptibility
- 37. Stainless vs. Other Alloys Stress Cracking Lab Data
- 39. Stress Corrosion Cracking 100x mag.
- 40. Area A Area B Area C Carbon steel Carbon steel stress relieve Nickel based alloys Carbon Steel Caustic Soda Service Chart Sodium hydroxide concentration 0 100 180 F Stress cracking line 120 F
- 49. Corrosion Galvanic
- 50. Galvanic Series More Reactive Less Reactive Potassium Magnesium Beryllium Aluminum Zinc Chromium Iron Nickel Tin Copper Silver Platinum Gold
- 58. Intergranular Corrosion 500 x mag. Dark areas are corroded
- 69. Corrosion of Stainless Steel in Sulfur vapor 1300 F Lab Data 314 16.9 mpy mils per year 310 18.9 309 22.3 304 27.0 302B 29.8 316 31.1 321 54.8 Haynes alloy 556 6
Hinweis der Redaktion
- In module 2 we will take a look at what kinds of corrosion occur on different kinds of metals for example carbon steel vs. stainless steel. The types of corrosion damage will be related in pictures so that you can see what it looks like visually. You will be able to classify the corrosion this way . The remedies for each form of corrosion will be introduced. The remedies are the thought process for deciding direction in eliminating a corrosion issue.
- As you can see there are several forms of corrosion. The most common ones for Colgate are general, crevice, pitting and stress corrosion cracking. General being mostly seen in carbon steel equipment such as tanks. Crevice, pitting and stress corrosion are localized forms of corrosion and is generally seen in stainless steel equipment.
- General corrosion can be monitored using ultrasonics. Unexpected failure therefore is not common. Decreasing wall thickness can be monitored with time.
- The typical bright red orange corrosion of carbon steel in the outdoor environment is shown here. Another word used for it is hematite which is a Iron oxide.
- Mills scales is what is formed on the steel at the mill during processing. It is a tight adherent scale that forms during hot fabrication or heat treatment of metals. In a piping system if you weld in a new piece of pipe it is the anode. Oxidation occurs more readily on new pipe than old pipe because the old pipe already has an oxidation product on the surface. If a material is bent or welded it is at higher energy state therefore more ready to corrode to a lower energy state.
- This shows the reason why a new pipe section could fail sooner than the old pipe. It becomes a smaller anode in a larger cathode pipe.
- Additional stress can come from mechanical working or welding. Bending, hammering, is are examples of mechanical.
- Conductance is a measure of the ease of electron flow through an electrolyte or a solid metal. Resistance to electrical flow is measured in ohms.
- The gases present in the electrolyte and their concentration can cause localize corrosion cells. The main one being oxygen.
- You can create a potential difference between high and low concentrations of metallic ions against a metallic surface. The areas in high concentration of metallic ions become cathodic to the lower concentration anodic areas. Therefore a corrosion cell is formed.
- Typical coatings are paints and linings are thicker. The decision to use a coating or a lining depends on the operating conditions; i.e. chemistry and temperature. If the general corrosion rate is greater than 20 mils per year then alternative metals are generally used over steel , such as stainless steel. By adjusting the pH of the solution we can decrease the corrosion rate. Corrosion inhibitors slow either the anodic or cathodic reaction. Cathodic protection is using a sacrificial metal to protect the metal you do not want to corrode. Anodic protection is using reverse current through a rectifier etc. to cause electron flow in the opposite direction than what normally occur during corrosion.
- It is important to note here that structures can be compromised by corrosion. The shell of this tower became so thin at the point of the red arrow that support beams had to be installed to take the load off this section. The solution was to fillet weld new carbon steel plate over the corroded areas. It was too hot in general to solve the problem with a coating or lining.
- There are quite a few new alloys out there but the best still for sulfuric acid is Alloy 20. Sulfuric acid can be oxidizing or reducing depending on the concentration. And Alloy 20 has the right composition to handle both except the mid range about 75% sulfuric which generally needs other alternatives than metal.
- We use nickel alloys for high temperature caustic for example because of cracking issues with carbon steel.
- This slide is a statement to point out that the atmosphere can be corrosive and should be accounted for in plant site selection etc.
- Handouts for reference in further reading.
- Areas where we would see the most localized corrosion would be in our bleach and soap making business. The bleach and soap contain high amounts of salt. It is important to note that pitting and crevice corrosion are the most common forms of corrosion on stainless steels.
- As you can see from the photograph, pitting is very localized in nature and the rest of the surface is not attacked. This type of corrosion could lead to product contamination. Microbes such as sulfate reducing bacteria can and do cause this type of corrosion.
- Common in sulfates, moist chlorine, and chlorides.
- Depending on when the pitting issue is determined will dictate the remedy. For example if after a launch you find you have a pitting issue, one of the quickest ways to slow it down is through adjusting the pH in the alkaline direction. Or if stagnant pockets are designed into the process , the best thing is to eliminate them or speed up , more agitation, added to the process.
- Three effects are shown here Chloride concentration has a significant impact on the pitting potential Temperature has more of an effect on lower salt concentrations in that it raises the pitting susceptibility. As the temperature increases there is less differentiation between different metals and their associated pitting potentials.
- Pitting is a concern mainly in areas where you have tubes that are thin walled. Tubes can be less than 1mm thick and the time perforate in a pitting environment can be a matter of weeks. In this case super fatted soap lowers the available NaOH making pitting easier to happen.
- The purpose of this exercise is to give participants working knowledge on how to resolve a type of localized corrosion phenomenon on a low alloy stainless steel spring.
- This is an example of corrosion that could be seen by the consumer.
- Very common form of corrosion and it is what caused the corrosion of soft-soap pump springs. Cleaning of vessels and piping is one way of preventing crevice corrosion on stainless steel. The deposits on the surface act as crevice corrosion initiators.
- A electron micrograph is a way to look at surfaces under very high magnification.
- This slide shows data produced in Piscataway.
- Interesting to note here the change in days for the onset of corrosion to begin from 25 to 45+ at 2.5% NaCl and pH.
- Piping connections joined by flanges is one of the most common areas where crevice corrosion initiates.
- 1. Crevice Corrosion 2. change the formula, lower the salt concentration below 1%. Raising the pH would help also. Adding an inhibitor such as sodium sulfate. Or changing the alloy for the spring material. Use a higher alloy for the spring
- Packing materials that are best are those that do on adsorb moisture such as Teflon.
- Cracking failures are dangerous. It could lead to a safety incident or disruption of plant production.
- The important point here is that 304 and 316 SS are susceptible to cracking corrosion. Mostly in chlorides and hydroxides.
- This is a very important slide and should be taken very seriously. When operating processes with salt above 140 F , do not use 304 or 316 SS. Use alloys with at least 25 Nickel and 4% Molybdenum if the pH is 7 or below.
- The important point here that corrosion can cause metal to become brittle and break into small pieces. These pieces could get into our products and cause a significant business issue.
- This shows the effect of adding nickel to an alloy to reduce the risk of stress corrosion cracking. Duplex alloys being on the outside danger region to the left of the green curve.
- Stress cracking can cause a number of problems. Leaking from the process to the jacket side for example. It can eventually cause a plant system to just fall apart. The metal becomes brittle and cannot be rewelded.
- Note the effect of corrosion here . It attacks the metal in a very localized fashion but eventually weakens the structure significantly.
- This show the cracking potential of sodium hydroxide. At elevated temperatures or concentrations of NaOH it is wise to consult on materials of construction.
- Example of tank issues with salt containing materials.
- There could be a significant benefit to using a duplex alloy over 316 SS or a higher alloyed material with more nickel which is more costly. For example 2205 SS is the same basic cost as 316L SS and is not susceptible to stress corrosion cracking.
- The reason for requiring a very alloyed material such as Inconel 625 is that Chlorate is an oxidizing salt that is very corrosive from a pitting standpoint as well as a stress corrosion standpoint. So you need over 30% Nickel and a minimum of 6% molybdenum.
- Fatigue cracking has occurred in sulfur burning-SO2 piping.
- Surface finish and using more corrosion resistant alloys are the easiest remedies for fatigue.
- Found in Oil refineries.
- Whenever you build a plant out of more than one material of construction you have the potential for galvanic corrosion.
- Corrosion driven by the potential difference between copper and aluminum. Aluminum being anodic to copper and therefore corroded in this picture.
- This chart shows the large number of elements apart that copper and aluminum are. It is also a basic chart and it is mostly used for teaching purposes.
- A unique example how galvanic corrosion caused a considerable amount of problems for Colgate in bottle making.
- Self explanatory slide.
- In Colgate areas where erosion can be a concern are piping and pumps. Piping carrying high amounts of solids or abrasives for example.
- The shows the elbow effect.. Particles crashing into the pipe wall just after the bend causing erosion of the metal at that point.
- In the soap barrel there is pressure to move the soap out, this fact, and the fact the soap has salt and sometimes abrasive, the worm wears or erodes with time. The clearance changing from 1 to 3 mm over several years between the barrel and the worm.
- Erosion is best controlled in the design.
- Most common when using high carbon stainless steels.
- You can see here that this corrosion is also quite serious. The grains of the metal are eaten away allowing it to basically crumble.
- Fretting corrosion is a combined wear and corrosion process in which material is removed from contacting surfaces when motion between the two surfaces is restricted or in other words machine components that are considered fixed and not expected to wear.
- This basically shows that like materials do not have high fretting corrosion resistance. Like materials attract and unlike material repel each other which is good when designing for fretting corrosion resistance.
- High temperature in general starts to occur around 750 degrees Fahrenheit.
- Sulfidation 310 or 314 SS.