SUBJECT: Corrossion problems associated with stainless steel 4-1

The rotating equipment business uses a great deal of 300 series stainless steel, and as a result we often experience several types of corrosion:

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:

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)

GALVANIC CORROSION

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!

Example #1.

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.

Example #2

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 :

PITTING

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.

INTERGRANULAR CORROSION

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:

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:

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.

EROSION CORROSION

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.

FRETTING CORROSION

This type of corrosion is easily seen on the pump shaft or sleeve. You'll see the damage on the shaft under:

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:

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:

SELECTIVE LEACHING

The process fluid selectively removes elements from the piping or any other part that might be exposed to the liquid flow. The mechanism is:

MICRO ORGANISMS

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:

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 ALLOYS

CORRODED END ( ANODIC OR LEAST NOBLE)

MAGNESIUM
MAGNESIUM ALLOYS
ZINC
ALUMINUM 5052, 3004, 3003, 1100, 6053
CADMIUM
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)
LEAD-TIN SOLDERS
LEAD
TIN
INCONEL 600 (ACTIVE)
NICKEL (ACTIVE)
60 NI-15 CR (ACTIVE)
80 NI-20 CR (ACTIVE)
HASTELLOY B (ACTIVE)
BRASSES
COPPER (CA102)
MANGANESE BRONZE (CA 675), TIN BRONZE (CA903, 905)
SILICONE BRONZE
NICKEL SILVER
COPPER - NICKEL ALLOY 90-10
COPPER - NICKEL ALLOY 80-20
430 STAINLESS STEEL
NICKEL, ALUMINUM, BRONZE (CA 630, 632)
MONEL 400, K500
SILVER SOLDER
NICKEL (PASSIVE)
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)
SILVER
TITANIUM (PASS.) HASTELLOY C & C276 (PASSIVE), INCONEL 625(PASS.)
GRAPHITE
ZIRCONIUM
GOLD
PLATINUM

PROTECTED END (CATHODIC OR MOST NOBLE)

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