What is Stainless Steel?

Abrasion, fretting and erosion sometimes are classified as corrosion mechanisms, but technically they are a mechanical metal removal process as compared to a chemical removal process. Chemical reaction may accompany the mechanical removal process to speed up the dissolution, but the chemical reaction will fit into the two basic forms.

Some authorities list nine other types of corrosion, but the other types generally are modifications of one of the existing corrosion forms. A correct alloy choice for one type of corrosion mechanism may be entirely the wrong choice for another. Therefore, a proper diagnosis of the failure is essential to make the correct material choice.

Within these two basic classifications there are five types of corrosion:

1. General or uniform corrosion
2. Intergranular corrosion
3. Galvanic corrosion, including pitting and crevice corrosion
4. Stress corrosion cracking
5. Microbiologically induced corrosion (MIC).

Many times, a metal starts to corrode by one mechanism, for example pitting corrosion, and then fails by a second mechanism, stress corrosion cracking.

BASIC CORROSION RESISTANCE

A metal derives its corrosion resistance by forming a protective oxide film on the surface. Metals may be classified in two categories-active and passive, depending on the nature of the oxide film. With active film metals, the oxide film continuously grows until it reaches a limiting thickness then sloughs off, continues to grow, sloughs off-repeating this process until the metal is completely consumed.

• Examples of metals with active oxides are iron, copper and zinc.
• Examples of metals with passive films are stainless steel, titanium, gold, platinum, and silver.

Passive film metals form an extremely thin oxide layer, in the order of 10-100 atoms thick, then stop growing. This film remains stable until something upsets the equilibrium.

GENERAL OR UNIFORM CORROSION

Uniform corrosion occurs over large areas of the metal surface. This is the most common form of corrosion with steel and copper. It is the easiest form of corrosion to measure, and service lifetime is easy to calculate. This is the only form of corrosion that may be accurately calculated for lifetime before failure and the only corrosion mechanism in which increased section thickness gives longer life.

This type of corrosion is measured by corrosion rate, usually reported as mpy (mils per year), mm/y (millimeters per year), ipm (inches per month), or mg/ sdm/yr (milligrams per square decimeter per year). This type of corrosion may be minimized in the active metals by painting the surface, and unexpected failures can be avoided by periodic inspections.

Acid cleaning of metals is an exaggerated example of general corrosion. Every time a copper or carbon steel surface is acid cleaned, the metal walls are thinned due to uniform corrosion. Stainless steel is subject to general corrosion in many acids and some salt solutions. They are not subject to general corrosion in water; therefore, no data is available. Uniform corrosion can be reduced or even prevented by proper selection of materials that are resistant to the corrosive environment.

Certain elements make the alloy more resistant to different media.

• High chromium content imparts oxidation resistance. Therefore, look for high chromium for use with nitric acid, the higher the better. High chromium is useful for high temperature oxidation resistance; so, any stainless steel is better than carbon steel in elevated temperature applications.
• High copper content in stainless steel imparts resistance to sulfuric acid, as with Carpenter 20Cb-3‚® stainless steel.
• High nickel content gives resistance to reducing acids and produces a tightly adhering oxide film in high temperature oxidation.

A useful tool in determining corrosion resistance is the “Y” of corrosion shown in the graphic below. This chart divides the alloys into three classes:

1. Those resistant to oxidizing acids on the left
2. Those resistant to reducing acids on the right
3. Those resistant to a mixture of the two in the center

Oxidizing acids are those acids that oxidize the metals they come in contact with, and are themselves, reduced in the process. Reducing simply dissolves the metal without a change in valence or a release of hydrogen in the process. Corrosion resistance increases as you move up the chart. This chart indicates relative corrosion resistance.

A variation of galvanic corrosion can occur with passive film metals. If the alloy loses the passive film in one spot, then it becomes active in that area. Now the metal has both passive and active sites on the same surface.

This is the mechanism for pitting and crevice corrosion. Table 2 is a list of materials and their relative position in the galvanic series.

PITTING CORROSION

Pitting corrosion is a form of galvanic corrosion in which the chromium in the passive layer is dissolved leaving only the corrosion prone iron. The voltage difference between the passive and active layer on an austenitic stainless steel is +0.78 volts. Acid chlorides are the most common cause of pitting in stainless steel.

Chlorides react with chromium to form the very soluble chromium chloride (CrCl3). Thus, chromium is removed from the passive layer leaving only the active iron. As the chromium is dissolved, the electrically driven chlorides bore into the stainless steel creating a spherical, smooth wall pit. The residual solution in the pit is ferric chloride (FeCl3), which is very corrosive to stainless steel.

This is the reason ferric chloride is used in so many of the corrosion tests for stainless steel. When molybdenum and/or nitrogen is used as an alloying element in stainless steel, the pitting corrosion resistance improves.

In an attempt to quantify the effect of alloying elements, a relationship of the various elements responsible for corrosion resistance was developed. The resulting equation is called the Pitting Resistance Equivalent Number, or PREN. A PREN of 32 is considered the minimum for seawater pitting resistance.

Three factors influence pitting corrosion:

1. Chloride content
2. pH
3. Temperature

In general, the higher the temperature and chloride content and the lower the pH, the greater the probability of pitting. For a given chloride content, a higher temperature and lower pH encourage pitting. Conversely, a lower temperature and a higher pH reduce pitting. The worst conditions occur with acid chlorides, and less dangerous conditions occur with alkaline or high pH chlorides.

Pitting can occur rapidly once it starts.

For example, under the right conditions of chloride content, pH and temperature, a type 304 tube with a .035” (0.89mm) wall thickness will pit through in less than 8 hours. Increasing the molybdenum in the alloy produces greater resistance to pitting. Therefore high molybdenum – high chromium alloys generally provide the best pitting resistance. Figure 3 shows the relationship of pitting, molybdenum content, pH, and chloride content.

Table 3 lists alloys within the molybdenum contents shown on the graph. The molybdenum line represents the threshold at which pitting starts. Above the line pitting can occur rapidly while below the line pitting corrosion will not take place. This chart is very helpful in determining the amount of chloride and pH that can be tolerated for a given alloy class.

CREVICE CORROSION

Crevice corrosion is another form of galvanic corrosion, which occurs when the corroding metal is in close contact with anything that makes a tight crevice. Crevice corrosion is usually the first to occur and is predictable as to when and where it will take place.

Like pitting, a conductive solution must be present; and, the presence of chlorides makes the reaction proceed at a fast rate. Crevice corrosion depends on the environmental temperature, alloy content and metallurgical category of the alloy. Also, there is a relationship between the tightness of the crevice and the onset time and severity of corrosion.

There is a “critical crevice corrosion temperature” (CCCT) below which corrosion will not occur. Figure 4 is a plot of the PREN versus CCCT and metallurgical category. Table 4 lists the PREN for some of the more common alloys. These values are based on the lower composition value for each alloy addition; therefore, the results are conservative.

The greater the difference between the CCCT and the operating temperature, the greater the probability that crevice corrosion will occur.

This chart is very useful in determining the effect of temperature on corrosion by indicating the approximate temperature at which pitting corrosion begins. The effect of temperature on pitting corrosion is not as clear as that for crevice corrosion, but by adding approximately 100° F (60° C) to the CCCT, the approximate temperature at which pitting starts can be determined.

INTERGRANULAR CORROSION

All metals are composed of small grains that are normally oriented in a random fashion. These grains are each composed of orderly arrays of atoms with the same spacing between the atoms in every grain. Because of the random orientation of the grains, there is a mismatch between the atomic layers where the grains meet.

This mismatch is called a “grain boundary.” In a typical stainless steel product, there are about 1,000 grain boundaries that intersect a one-inch (25 mm) line drawn on the surface.

Grain boundaries are regions of high-energy concentration. Therefore, chemical or metallurgical reactions usually occur at grain boundaries before they occur within the grains. The most common reaction is formation of chromium carbide in the heat-affected zone (HAZ) during welding.

These carbides, formed along the grain boundaries, are called “sensitization.” Because the carbides require more chromium than is locally available, the carbon pulls chromium from the area around the carbon. This leaves a low chromium grain boundary zone and creates a new low chromium alloy in that region. Now there is a mismatch in galvanic potential between the base metal and the grain boundary; so, galvanic corrosion begins. The grain boundaries corrode, allowing the central grain and the chromium carbides to drop out as if particles of rusty sand. The surface of the metal develops a “sugary” appearance as illustrated in Figure 5.

The only solution is to use a “stabilized” grade, one in which titanium, columbium (niobium) or both are added to react with the carbon forming stable grains of titanium or niobium carbide thus stabilizing the alloy.

The type 304 equivalent stabilized with titanium is type 321, and the type 304 equivalent stabilized with niobium is type 347. Stabilized grades should be used whenever the steel is held for long periods in the temperature range of 800° to 1500° F (425° to 800° C). Sigma or “chi” phase may be minimized by avoiding the temperatures where they form, or by using alloys high in nickel and nitrogen.

Other compounds are:

• Delta ferrite
• Sigma phase (a chromium-iron compound)
• Chi phase (a chromium-iron-molybdenum compound)
• Several other compounds that are found less often

Special mention should be made concerning delta ferrite. All stainless steels are compounded to have a certain amount of delta ferrite in the microstructure to minimize micro cracking during cooling of the weld. The Welding Research Council recommends a range of 2-5%, with most welds measuring at 2%.

However, when delta ferrite is exposed to high chloride waters including many hot water systems-the chloride begins to attack the delta ferrite corroding it preferentially and leakage occurs.

STRESS CORROSION CRACKING

Stress corrosion cracking (SCC) is one of the most common and dangerous forms of corrosion. Usually it is associated with other types of corrosion that create a stress concentrator that leads to cracking failure.

Nickel containing stainless steel is especially susceptible to chloride induced SCC. Figure 7 indicates the maximum susceptibility is in the nickel range of about 5-35% and that pure ferritics, such as Types 430, 439, and 409 are immune. The point of maximum susceptibility occurs between 7-20% nickel. This makes types 304/304L, 316/316L, 321, 347, etc., very prone to such failure.

Stress corrosion cracking (SCC) has three components:

1. Alloy composition
2. Environment
3. The presence of tensile stress

All metals are susceptible to stress corrosion cracking, as Table 5 indicates.

This is a characteristic that distinguishes SCC from other types of cracking. Using microprobe analysis, or electron dispersive spectroscopy (EDS), on the crack surface to look for the presence of chlorine, we can observe conclusive evidence that SCC has occurred.

MICROBIOLOGICALLY INFLUENCED CORROSION

Microbiologically influenced corrosion (MIC) is a recently discovered phenomenon. Actually, it is not a separate corrosion mechanism, rather a different agent that causes corrosion of metals. It is not limited to stainless steel as Table 6 indicates. Some form of bacteria action attacks most metals. The mechanism is usually general or crevice corrosion under the bacteria colonies as seen in Figure 9.

In some cases, the metabolic byproducts react with the environmental solution to create a very corrosive media. An example is the reaction of chlorine in water with the manganese dioxide byproduct from gallionella bacteria on the surface of the stainless steel. This reaction generates hydrochloric acid, which causes rapid pitting of many common grades of stainless steel.

One of the most common forms of MIC is the metabolic byproduct of the sulfur-fixing bacteria that produces sulfurous or sulfuric acid.

These bacteria cause rapid corrosion of the lower alloy stainless steels, like Types 304L or 316L, resulting in through wall crevice corrosion under the bacteria colonies.

Other than the use of bactericides, such as chlorine or ozone, the usual solution to this type of corrosion is to use a 6% molybdenum alloy such as AL-6XN®‚ (a superaustenitic stainless steel) or the highly alloyed nickel alloys. Therefore, if MIC corrosion is taking place, it is best to use one of these alloys.