The rotating equipment business uses a great deal of 300 series stainless steel, and as a result we often experience several types of corrosion:
- General corrosion
- Galvanic corrosion
- Inter granular corrosion
- Chloride stress corrosion cracking
- Erosion- corrosion
- Concentrated cell or crevice corrosion
- Selective leaching
- Micro organisms
At the end of this aticle is a page titled, “The Galvanic Series Of Metals and alloys”. I’ll be referring to this chart during our discussion.
The basic resistance of stainless steel occurs because of its ability to form a protective coating on the metal surface. This coating is a “passive” film which resists further “oxidation” or rusting. The formation of this film is instantaneous in an oxidizing atmosphere such as air, water, or other fluids that contain oxygen. Once the layer has formed, we say that the metal has become “passivated” and the oxidation or “rusting” rate will slow down to less than 0.002″ per year (0,05 mm. per year).
Unlike aluminum or silver this passive film is invisible in stainless steel. It’s created when oxygen combines with the chrome in the stainless to form chrome oxide which is more commonly called “ceramic”. This protective oxide or ceramic coating is common to most corrosion resistant materials.
Halogen salts, especially chlorides easily penetrate this passive film and will allow corrosive attack to occur. The halogens are easy to recognize because they end in the letters “ine”. Listed in order of their activity they are:
- astatine (very unstable.)
These are the same chemicals that will penetrate Teflon and cause trouble with Teflon coated or encapsulated o-rings and/ or similar coated materials. Chlorides are one of the most common elements in nature and if that isn’t bad enough, they’re also soluble, active ions; the basis for good electrolytes, the best conditions for corrosion or chemical attack.
GENERAL OR OVERALL CORROSION.
This type of corrosion occurs when there is an overall breakdown of the passive film formed on the stainless steel. It’s the easiest to recognize as the entire surface of the metal shows a uniform “sponge like” appearance. The rate of attack is affected by the fluid concentration, temperature, fluid velocity and stress in the metal parts subject to attack. As a general rule the rate of attack will double with an eighteen degree Fahrenheit rise in temperature (10° C.) of either the product or the metal part.
If the rotating portion of the seal is rubbing against some stationary component, such as a protruding gasket or fitting, the protective oxide layer will be polished off and the heat generated will increase the corrosion as noted above. This explains why corrosion is often limited to only one portion of the mechanical seal, metal casing.
There are many good publications available to help you select the proper metal for any given mechanical seal application. As a general rule, if the wetted parts of the equipment are manufactured from iron, steel, stainless steel or bronze, and they are showing no signs of corrosion, grade 316 stainless is acceptable as long as you do not use stainless steel springs. (see chloride stress corrosion below)
If you put two dissimilar metals, or alloys in a common electrolyte, and connect them with a voltmeter, it will show an electric current flowing between the two. (This is how the battery in your automobile works). When the current flows, material will be removed from one of the metals or alloys ( the ANODIC one) and dissolve into the electrolyte. The other metal (the CATHODIC one) will be protected.
Move down to the end of this aticle and look at the Galvanic Series chart The further apart the materials are located on this chart, the more likely that the one on the ANODIC end will corrode if they are both immersed in any fluid considered to be an electrolyte.
Salt water, is one of the best!
A ship has lots of bronze fittings and a steel hull. Note that steel is located seven lines from the ANODIC end, and bronze is listed at twenty seven rows from the same end. Sea water is a perfect electrolyte, so the bronze fittings would immediately attack the steel hull unless something could be done to either protect the steel ,or give the bronze something else to attack.
The classic way to solve this problem is to attach sacrificial zinc pieces to the hull and let the bronze go after them. Again, looking at the chart, you’ll note that zinc is found on line three from the top of the chart. In other words the zinc is further away from the bronze than the iron, so the galvanic action takes place between the zinc and the bronze, rather than between the steel and the bronze. Zinc paint is used for the same reason.
Nickel base tungsten carbide contains active nickel. When this face material is used in dual seal applications it is common to circulate water or antifreeze between the seals (as mentioned in the beginning of this paper, water can be an excellent electrolyte because of the addition of chlorine and fluorine). You’ll note that active nickel is located twenty one rows from the top of the chart. Passivated 316 stainless steel is positioned nine rows from the bottom. This means that the stainless steel can attack the nickel in the tungsten carbide causing it to corrode.
The rate at which corrosion takes place is determined by :
- The distance separating the metals on the galvanic series chart
- The temperature and concentration of the electrolyte. The higher the temperature, the faster it happens. Any stray electrical currents in the electrolyte will increase the corrosion also.
- The relative size of the metal pieces. A large cross section piece will not be affected as much as a smaller one.
- Many metal seal components are isolated from each other by the use of rubber o-rings or similar materials and designs. Shaft movement that causes fretting of the 316 stainless steel rubs off the passivated layer and exposes the active stainless to the electrolyte until the metal part becomes passivated once more. This is one of the reasons we see corrosion under o-rings, and Teflon wedges. In the following paragraph I’ll be discussing another cause of corrosion under rubber parts.
This is an accelerated form of chemical attack in which the rate of corrosion is greater in some areas than others. It occurs when the corrosive environment penetrates the passivated film in only a few areas as opposed to the overall surface. As stated earlier, halogens will penetrate passivated stainless steel. Referring to the galvanic chart you’ll note that passivated 316 stainless steel is located nine lines from the bottom and active 316 stainless steel is located thirteen lines from the top. Pit type corrosion is therefore simple galvanic corrosion, occuring as the small active area is being attacked by the large passivated area. This difference in relative areas accelerates the corrosion, causing the pits to penetrate deeper. The electrolyte fills the pits and prevents the oxygen from passivating the active metal so the problem gets even worse. This type of corrosion is often called “Concentrated cell corrosion”. You’ll also see it under rubber parts that keep oxygen away from the active metal parts, retarding the metal’s ability to form the passivated layer.
All austenitic stainless steels (the 300 series, the types that “work harden”) contain a small amount of carbon in solution in the austenite. Carbon is precipitated out at the grain boundaries, of the steel, in the temperature range of 1050° F. (565° C) to 1600° F. (870° C.). This is a typical temperature range during the welding of stainless steel.
This carbon combines with the chrome in the stainless steel to form chromium carbide, starving the adjacent areas of the chrome they need for corrosion protection. In the presence of some strong corrosives an electrochemical action is initiated between the chrome rich and chrome poor areas with the areas low in chrome becoming attacked. The grain boundaries are then dissolved and become non existent. There are three ways to combat this:
- Anneal the stainless after it has been heated in this sensitive range. This means bringing it up to the proper annealing temperature and then quickly cooling it down through the sensitive temperature range to prevent the carbides from forming.
- When possible use low carbon content stainless if you intend to do any welding on it. A carbon content of less than 0.3% will not precipitate into a continuous film of chrome carbide at the grain boundaries. 316L is as good example of a low carbon stainless steel.
- Alloy the metal with a strong carbide former. The best is columbium, but sometimes titanium is used. The carbon will now form columbium carbide rather than going after the chrome to form chrome carbide. The material is now said to be “stabilized”
CHLORIDE STRESS CORROSION.
If the metal piece is under tensile stress, either because of operation or residual stress left during manufacture, the pits mentioned in a previous paragraph will deepen even more. Since the piece is under tensile stress cracking will occur in the stressed piece. Usually there will be more than one crack present causing the pattern to resemble a spider’s web. Chloride stress cracking is a serious problem in industry and not often recognized by the people involved. In the seal business it is a serious problem if you use stainless steel springs or stainless steel bellows in your seals. This is the main reason that Hastelloy C is recommended for spring material. Here are some additional thoughts about chloride stress cracking that you’ll want to consider:
- Chlorides are the big problem when using the 300 series grades of stainless steel. The 300 series is the one most commonly used in the process industry because of its good corrosion resistant proprieties. Outside of water, chloride is the most common chemical found in nature and remember that the most common water treatment is the addition of chlorine.
- Beware of insulating, or painting stainless steel pipe. Most insulation contains chlorides and piping is frequently under tensile stress. The worst condition would be insulated, steam traced, stainless steel piping.
If it’s necessary to insulate stainless steel pipe, a special chloride free insulation can be purchased, or the pipe can be coated with a protective film prior to insulating.
- Stress cracking can be minimized by annealing the metal, after manufacture, to remove residual manufactured stresses.
- Never replace a carbon steel bolt with a stainless steel one unless you’re sure there are no chlorides present. Bolts can be under severe tensile stress.
- No one knows the threshold values for stress cracking to occur. We only know that you need tensile stress, chlorides, temperature and the 300 series of stainless steel. We do not know how much chloride, stress or temperature.
- Until I figured out what was happening I had trouble breaking stainless steel fishing hooks in the warm water where I live in Florida.
- Many cleaning solutions and solvents contain chlorinated hydrocarbons. Be careful using them on or near stainless steel. Sodium hypochlorite, chlorethene. methylene chloride and trichlorethane are just a few in common use. The most common cleaner used with dye checking material is trichloroethane, explaining the reason we sometimes experience cracks after we weld stainless steel and dye check it to inspect the quality of the weld.
This is an accelerated attack resulting from the combination of mechanical and chemical wear. The liquid velocities in some pumps prevents the protective oxide passive layer from forming on the metal surface. The suspended solids also remove some of the passivated layer increasing the galvanic action. You see this type of corrosion very frequently at the eye of the pump impeller.
This type of corrosion is easily seen on the pump shaft or sleeve. You’ll see the damage on the shaft under:
- The grease or lip seal that is supposed to protect the bearings.
- The packing used to seal the fluid.
- The dynamic Teflon or elastomer used in most original equipment seals.
- The vibration damper used in rotating metal bellows seals.
- The rubber boot used in low cost seals, if it did not attach to the shaft properly.
As mentioned earlier, 300 series stainless steel passivates its self by forming a protective chrome oxide layer when ever it is exposed to free oxygen. This oxide layer is very hard and when it imbeds into a soft elastomer it will cut and damage the shaft or sleeve rubbing against it. The mechanism works like this:
- Oxygen passivates the active stainless steel forming a protective ceramic layer.
- The seal or packing removes the oxide layer as the shaft or sleeve rubs against it.
- The ceramic passivated layer sticks into the soft elastomer turning it into a “grinding surface”.
- The oxide reforms when the active metal is exposed and the process starts all over again.
- A visible groove is cut into the shaft, or sleeve that will cause seal leakage and “hang up”.
CONCENTRATED CELL OR CREVICE CORROSION
This corrosion occurs any time liquid flow is kept away from the attacked surface. It is common between nut and bolt surfaces, under O-rings and gaskets, and between the clamps and stainless steel shafts we find in many split seal applications. Salt water applications are the most severe problem because of the salt water low PH (8.0&endash;9.0) and its high chloride content. Here is the mechanism:
- Chlorides pit the passivated stainless steel surface.
- The low PH salt water attacks the active layer that is exposed
- Because of the lack of fluid flow over the attacked surface, oxygen is not available to passivate the stainless steel.
- Corrosion continues unhampered under the rubber and tight fitting clamp.
- The inside of the o-ring groove experiences the same corrosion as the shaft or sleeve.
The process fluid selectively removes elements from the piping or any other part that might be exposed to the liquid flow. The mechanism is:
- Metals are removed from the liquid during a de-ionization or de-mineralizing process.
- The liquid tries to replace the missing elements as it flows through the system.
- The un-dissolved metals often coat them selves on the mechanical seal faces or the sliding components and cause a premature seal failure.
- Heat accelerates the process.
These organisms are commonly used in sewage treatment, oil spills and other cleaning processes. Although there are many different uses for these “bugs”, one common one is for them to eat the carbon you find in waste and other hydrocarbons, and convert it to carbon dioxide. The “bugs” fall into three categories:
- Aerobic, the kind that need oxygen.
- Anaerobic, the kind that do not need oxygen.
- Facultative, the type that goes both ways.
If the protective oxide layer is removed from stainless steel because of rubbing or damage, the “bugs” can penetrate through the damaged area and attack the carbon in the metal. Once in, the attack can continue on in a manner similar to that which happens when rust starts to spread under the paint on an automobile.GALVANIC SERIES OF METALS AND ALLOYSCORRODED END ( ANODIC OR LEAST NOBLE)MAGNESIUM
ALUMINUM 5052, 3004, 3003, 1100, 6053
ALUMINUM 2117, 2017, 2024
MILD STEEL (1018), WROUGHT IRON
CAST IRON, LOW ALLOY HIGH STRENGTH STEEL
CHROME IRON (ACTIVE)
STAINLESS STEEL, 430 SERIES (ACTIVE)
302, 303, 304, 321, 347, 410,416, STAINLESS STEEL (ACTIVE)
NI – RESIST
316, 317, STAINLESS STEEL (ACTIVE)
CARPENTER 20CB-3 STAINLESS (ACTIVE)
ALUMINUM BRONZE (CA 687)
HASTELLOY C (ACTIVE) INCONEL 625 (ACTIVE) TITANIUM (ACTIVE)
INCONEL 600 (ACTIVE)
60 NI-15 CR (ACTIVE)
80 NI-20 CR (ACTIVE)
HASTELLOY B (ACTIVE)
MANGANESE BRONZE (CA 675), TIN BRONZE (CA903, 905)
COPPER – NICKEL ALLOY 90-10
COPPER – NICKEL ALLOY 80-20
430 STAINLESS STEEL
NICKEL, ALUMINUM, BRONZE (CA 630, 632)
MONEL 400, K500
60 NI- 15 CR (PASSIVE)
INCONEL 600 (PASSIVE)
80 NI- 20 CR (PASSIVE)
CHROME IRON (PASSIVE)
302, 303, 304, 321, 347, STAINLESS STEEL (PASSIVE)
316, 317, STAINLESS STEEL (PASSIVE)
CARPENTER 20 CB-3 STAINLESS (PASSIVE), INCOLOY 825
NICKEL – MOLYBDEUM – CHROMIUM – IRON ALLOY (PASSIVE)
TITANIUM (PASS.) HASTELLOY C & C276 (PASSIVE), INCONEL 625(PASS.)