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Corrosion Control

All metals and alloys are susceptible to corrosion in some environments and, therefore, no single metal or alloy is suitable for all applications. For example, gold, which historically is known for its excellent resistance to the atmosphere, will corrode if exposed to mercury at ambient temperature. On the other hand, iron is relatively inert to mercury but corrodes readily in the atmosphere.

Fortunately, one or more materials will perform satisfactorily in a given environment. The stainless steels are versatile in that they are resistant to corrosion in a wide range of environments.

 

The Problem of Corrosion

Selecting a material with inadequate corrosion resistance for a particular application can be a costly mistake. Direct and indirect economic losses which can result from corrosion include expenses due to:

1. Replacement of corroded equipment.

2. Overdesign to allow for corrosion.

3. Shutdown of equipment because of a corrosion failure.

4. Loss of a product, such as a container that corroded through.

5. Contamination of a product.

6 Loss of efficiency. For example, corrosion product lowers heat transfer rate in heat exchangers.

Some of these indirect losses, such as loss due to shutdown of equipment, can cost many times more than the difference between buying a material that would have performed satisfactorily and one that did not. Be sure to consider potential indirect losses due to corrosion when making a material selection.

Corrosion can also constitute a significant safety hazard, for example, in containers for toxic products (poisonous gases, etc.) and critical parts in transportation media.

 

The Special Case of Stainless Steel

The fundamental resistance of stainless steel to corrosion occurs because of its ability to form a protective coating on its surface. This coating is a passive film which is resistant to further oxidation or other forms of chemical attack. This passive film may be monomolecular in thickness, usually invisible, but generally protective in oxidizing environments such as air and nitric acid. The passive film will, however, tend to lose its protectiveness in reducing environments such as hydrochloric acid. Whether an environment is oxidizing or reducing is not always a function of its oxygen content. For example, different aqueous solutions can oxidize the surface of a metal to different degrees independent of their oxygen content. Also, the oxidizing power of the given solution may change with concentration, temperature and impurity content.

Chromium is the most important element in maintaining the passive film. With free chromium (not present as carbides or other compounds) in excess of about 11%, steels do not typically form red rust, and so they are called "stainless." Increasing the chromium content of the stainless steel invariably broadens the range of environments which are sufficiently oxidizing to maintain a passive film. Alloying additions of nickel and molybdenum also expand the range of passivity.

Fundamental to most types of corrosion to which stainless steels are subject is that halogen salts, primarily chlorides, easily penetrate the passive film and allow corrosive attack to occur. Chlorides are abundant in nature and are used extensively for de-icing, cooking, etc. Chlorides are soluble, active ions and the basis for good electrolytes—good conditions for chemical attack or corrosion.

 

Types of Corrosion

Corrosion can be divided into two basic types:

1. General Corrosion in which the metal corrodes at a uniform rate over the entire surface; and

2. Localized Corrosion in which only a small area of the metal surface is affected but the rate of corrosion in this small area is relatively high. These types of localized corrosion are discussed in detail in Carpenter's booklet, "Alloys for Corrosive Environments."

a. Intergranular Corrosion

b. Pitting Corrosion

c. Crevice Corrosion

d. Galvanic Corrosion

e. Stress-Corrosion Cracking

 

Corrosion Testing 

Selection of appropriate corrosion tests requires consideration of the potential forms of corrosion, details of the service environment and the material composition and fabrication. Some of the factors affecting corrosion are presented above.

Corrosion evaluation methods can be divided into simulated service and accelerated tests. In a simulated service test, both environment and material condition are similar to that in service. Long-term exposures can be required for a proper evaluation. Accelerated tests are designed to detect the susceptibility of a material to one or more forms of corrosion in a relatively short period of time.

 

Intergranular Corrosion 

The standard tests for intergranular attack are generally viewed as accelerated techniques and often are used to verify that the material received a good anneal. The ASTM standards are listed in Table 1. Each ASTM designation is applicable to different alloys or material conditions: A 262 for austenitic stainless steels, A 763 for ferritic stainless steels and G 28 for wrought, nickel-rich, chromium-bearing alloys.

 

Pitting and Crevice Corrosion

ASTM G 48 describes accelerated tests for pitting and crevice corrosion in ferric chloride or ferric chloride-hydrochloric acid. Samples (with or without crevices) may be exposed at one constant temperature and evaluated by weight loss and appearance. Alternatively, the critical temperature for attack may be determined by exposing several sets of specimens at increasing temperatures and recording the temperature at which attack occurs. Critical pitting temperature can also be determined electrochemically using ASTM G 150.

Cracking

The boiling magnesium chloride test of ASTM G36 has been used extensively to evaluate resistance to stress-corrosion cracking at elevated temperature, but this test is much more severe than most service environments. An alternative environment, which may be more useful to predict service experience, is found in ASTM G 123 and consists of boiling 25% NaCl acidified to pH 1.5 with phosphoric acid. 

Cracking that occurs at lower temperatures can be studied using the salt spray test of ASTM B117 at 95°F (35°C). If hydrogen sulfide is present, sulfide-stress-cracking resistance can be evaluated using NACE TM0177 which involves exposing stressed samples to an acidified H2S environment.

Cracking is possible in other than chloride environments. For example, sensitized Type 304 can be cracked in polythionic acid, produced when hydrogen sulfide and sulfur dioxide are bubbled through water. The evaluation test is found in ASTM G 35.

Several methods are available to externally stress samples for exposure to corrosive environments. Sample configurations include U-bends (ASTM G 30), bent beams (ASTM G 39), C-rings (ASTM G 38), and tensile samples (ASTM G 49). C-rings and tensile samples can be notched to change the stress state and increase the likelihood that failure will occur in a predetermined area. Some notched samples can be fatigue pre-cracked to study crack propagation. Examples of such specimens are wedge open load, compact tension, cantilever beam and double cantilever beam. In addition, slow strain rate tests, which evaluate stress corrosion resistance by slowly pulling a specimen to failure in a corrosive environment, are found in ASTM G 129.

Test samples for the evaluation of weldments are described in ASTM G 58. These include samples using the residual stresses from welding as well as externally stressed or pre-cracked specimens.

 

Corrosion in Atmospheres

Three tests have been widely used for stainless steels. All are performed in controlled-atmosphere chambers. The mildest, 100% humidity at 95°F, simulates storage or use in many damp environments. The 5% salt spray (sodium chloride) of ASTM B 117 is more aggressive and has been used to simulate exposure to road salt or marine environments. The Copper-Accelerated Acetic Acid-Salt Spray test (ASTM B 368) is an even more severe test in which 5% sodium chloride with a copper II chloride addition is acidified using acetic acid. This test and the Salt Spray test are not suggested for all grades of stainless steels. 

 

Table 1 – ASTM Intergranular Corrosion Tests

Alloy System

ASTM

Standard

Test Media

Test

Duration

Austenitic stainless steels

 

A 262-A

 

A 262-B

 

A 262-C

 

A 262-E

Oxalic acid etch

 

Ferric sulfate-sulfuric acid

 

Nitric acid (Huey test)

 

Copper sulfate - 16% sulfuric acid

(copper contact)

Etch test

 

120 hours

 

240 hours

 

24 hours

 

Wrought nickel-rich, chromium-

bearing alloys

G 28-A

 

G 28-B

Ferric sulfate - sulfuric acid

 

Mixed acid-oxidating salt

24/120 hours

 

24 hours

 

Ferritic stainless steels

A 763-W

 

A 763-X

 

A 763-Y

 

A 763-Z

Oxalic acid etch

 

Ferric sulfate - sulfuric acid

 

Copper sulfate - 50% sulfuric acid

 

Copper sulfate - 16% sulfuric acid

(copper contact)

Etch test

 

24/120 hours

 

96/120 hours

 

24 hours

 

Importance of Cleaning and Passivating

The corrosion-resisting qualities of stainless steels are inherent in the metal itself. However, contamination of the surface by adhering dirt or scale can have a deleterious effect. For this reason, surfaces must be free of scale, lubricants, foreign particles, and coatings applied for drawing and heading. After fabrication of parts, cleaning and/or passivation should be considered.  

Passivation maximizes the inherent corrosion resistance of stainless steel. Perhaps the best test to confirm that passivation has been effective is a 24-hour exposure to 100% humidity at 95°F.

June 2006

Carpenter Technology Corporation Carpenter Technology Corporation

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