Stainless Steel Corrosion Resistance Testing

Stainless steel corrosion resistance testing mainly conducts scientific testing and analysis on the pitting potential measurements, critical pitting temperature determinations, intergranular corrosion sensitivity testing, crevice corrosion testing, stress corrosion cracking testing and uniform corrosion rate determinations, objectively assessing the product quality grade and issuing a detailed test report, providing a basis for manufacturers' quality control and downstream users' product acceptance.

Testing Items

Pitting potential measurements: Through the potentiodynamic scanning method, the pitting potentials (Eb or Ep) of the stainless steels are determined in the chloride ion-containing solutions, and this is the key indicator of evaluating their pitting resistance capabilities. The more positive the potentials, the stronger the capabilities of the materials to resist the pitting initiation in the specific environments. The testing is usually carried out in the simulated seawater (such as the 3.5% NaCl solutions).

Critical pitting temperature determinations: Through the methods such as the ASTM G48, in the ferric chloride solutions of the fixed concentrations, the temperatures are gradually increased until the pitting is observed, and this temperature is the CPT. The higher the CPT, the better the pitting resistance properties of the materials, often used for the screening of the high-alloy stainless steels.

Intergranular corrosion sensitivity testing: Assessing the corrosion tendencies caused by the chromium-depleted zones formed by the chromium carbide precipitation at the grain boundaries after the stainless steels are treated in the sensitization temperature ranges. The common methods include the sulfuric acid-copper sulfate corrosion testing (such as the GB/T 4334 Method E) and nitric acid method (ASTM A262 Method C), etc., and the qualifications are determined through the bending or metallographic observations.

Crevice corrosion testing: Simulating the working conditions with the crevices such as the flanges and gaskets, assessing the local corrosion of the stainless steels in the enclosed zones inside the crevices caused by the oxygen concentration differences and acidification. The common methods include the multi-crevice specimen immersion testing of the ASTM G48 standard and electrochemical crevice corrosion testing.

Stress corrosion cracking testing: Assessing the cracking sensitivities of the stainless steels under the combined actions of the tensile stresses and specific corrosive media (such as the chlorides and alkaline solutions). The common methods include the U-bend specimen immersion testing (such as the GB/T 17898), constant load or slow strain rate tensile testing (SSRT).

Uniform corrosion rate determinations: Through the immersion weight loss method (such as the GB/T 10124), determining the overall corrosion rates of the stainless steels in the specific acid and alkaline media, with the units often being mm/a. This is the basic data of the material selection, used to assess the overall corrosion resistance lives of the materials.

Electrochemical impedance spectroscopy analysis: Through the application of the small-amplitude sinusoidal wave perturbations, measuring the impedance responses of the stainless steel/solution interfaces. The EIS can be used to study the growth, stabilities and destruction processes of the passivation films, obtaining the parameters such as the film resistances and capacitances, and is a non-destructive interface state monitoring means.

Testing Scope

Austenitic stainless steels: Such as the 304, 316 and 316L series. The testing emphases lie in their pitting, crevice corrosion and stress corrosion cracking properties in the chloride ion-containing environments, especially the effects of the molybdenum (Mo) element contents on the pitting resistance capabilities.

Ferritic stainless steels: Such as the 430 and 444. The testing needs to focus on their intergranular corrosion sensitivities (especially the welded heat-affected zones) and uniform corrosion behaviors in the organic acids and weakly acidic environments. Their chloride stress corrosion resistance properties are usually better than those of the austenitic steels.

Martensitic stainless steels: Such as the 410 and 420. Due to their high hardnesses but relatively low corrosion resistance, the testing mainly revolves around their uniform corrosion and rust resistance capabilities in the weak corrosive media (such as the atmospheres and water) and specific functional environments (such as the cutters and bearings).

Duplex stainless steels: Such as the 2205 and 2507, with both the austenitic and ferritic structures. The testing core is to verify their excellent chloride pitting, crevice corrosion and stress corrosion cracking resistance properties, and the CPT determinations and chloride SCC testing are usually required.

Precipitation hardening stainless steels: Such as the 17-4PH and 15-5PH. The testing needs to comprehensively consider the balance between their mechanical properties after the aging treatments and corrosion resistance, emphatically assessing the pitting and stress corrosion behaviors in the marine atmospheres or moderate corrosion environments.

Welded joints and heat-affected zones: The welding processes may change the local structures and compositions of the materials, forming the corrosion weak links. The testing scope needs to specially cover the weld metals, fusion lines and heat-affected zones, carrying out the intergranular corrosion and selective corrosion testing.

Surface-treated stainless steels: Including the products after the surface treatments such as the passivation, electrolytic polishing and coatings. The testing needs to assess the effects of the treatment processes on the surface passivation film qualities, as well as the capabilities to maintain the corrosion resistance in the actual service environments.

Testing Methods

Chemical immersion method: The specimens are placed in the corrosive media of the specific compositions, temperatures and concentrations, and after the specified times, the corrosion resistance is evaluated through the mass losses, macro/micro morphology observations and mechanical property changes. This is the most basic and widely applied laboratory method, with the complete standard systems.

Electrochemical testing method: Using the electrochemical workstations, through the potential or current controls, the corrosion tendencies and rates of the stainless steels are quickly and quantitatively assessed. Mainly including the potentiodynamic polarization curve method (measuring the pitting potentials and repassivation potentials), cyclic polarization curve method and potentiostatic/galvanostatic immersion method.

Salt spray testing: Simulating the marine or salt-containing atmospheric environments, the specimens are placed in the sealed chambers, continuously or alternately spraying the sodium chloride solution mists. Mainly assessing the surface rust resistance capabilities and passivation film durabilities of the stainless steels. The common standards include the GB/T 10125 (Neutral salt spray) and ASTM B117.

On-site coupon testing: The standard specimens are directly placed in the real industrial environments, marine environments or chemical equipment for the long-term exposures. This method has the results closest to the actual service conditions, and is the key basis of verifying the laboratory data and material selections, but the cycles are relatively long.

Microscope analysis auxiliary method: After the corrosion testing, the stereomicroscopes, metallographic microscopes, scanning electron microscopes and energy dispersive spectrometers must be used to observe and analyze the corrosion morphologies, types and product compositions, to accurately judge the corrosion mechanisms, such as distinguishing the pitting, crevice corrosion or intergranular corrosion.

Non-destructive testing technologies: For the in-service equipment, the in-situ or online monitoring technologies such as the electrochemical noise, AC impedance and potential distribution measurements can be adopted, assessing the local corrosion activities and passivation film states of the stainless steel structures, realizing the corrosion early warnings.

Laboratory accelerated corrosion testing: In order to obtain the comparison data in the relatively short times, the testing with the intensified corrosion conditions is often adopted, such as the ferric chloride pitting testing, copper shot-sulfuric acid intergranular corrosion testing and boiling acid testing. The results need to be carefully extrapolated to the actual long-term service environments.

Testing Standards

Domestic Standards:

1. GB/T 10125-2021, Corrosion tests in artificial atmospheres — Salt spray tests

2. GB/T 4334, Corrosion of metals and alloys — Test methods for intergranular corrosion of austenitic and austenitic-ferritic (duplex) stainless steels

3. GB/T 17898, Test method for stress corrosion cracking of stainless steels

4. GB/T 10124, Metals and alloys — Test method for uniform corrosion of metallic materials in the laboratory by total immersion

International Standards:

1. ASTM G48, Standard test methods for pitting and crevice corrosion resistance of stainless steels and related alloys by use of ferric chloride solution

2. ASTM A262, Standard practices for detecting susceptibility to intergranular attack in austenitic stainless steels

3. ASTM B117, Standard practice for operating salt spray (fog) apparatus

4. ISO 9227, Corrosion tests in artificial atmospheres — Salt spray tests

Note: The applicable standard can be selected according to the product use and customer trade requirements; standards agreed in the contract take precedence over general standards.

Testing Instruments and Equipment

Electrochemical workstation: The core equipment of the corrosion resistance testing, used for all the electrochemical testing. Usually a three-electrode system, including the working electrodes (specimens), reference electrodes (such as the saturated calomel electrodes) and auxiliary electrodes (platinum sheets or graphite), with the various functions such as the potential scanning and impedance measurements.

Corrosion testing chambers: Including the salt spray testing chambers, damp heat testing chambers and sulfur dioxide corrosion testing chambers, used to simulate and intensify the specific atmospheric or industrial environments, providing the controllable temperature, humidity and medium concentration conditions, for the long-term or accelerated corrosion testing.

Analytical balance: The precision needs to reach 0.1mg, used for the precise weighing of the specimen masses before and after the corrosion testing, calculating the weight loss rates of the uniform corrosion, and is the basic equipment for obtaining the quantitative data of the chemical immersion method.

Stereomicroscope and metallographic microscope: Used for the low-magnification and high-magnification observations of the macro and micro morphologies of the corrosion specimens, judging the corrosion types (such as the pitting densities and depths), measuring the corrosion pit sizes, and observing the relationships between the metallographic structures and corrosion morphologies.

Scanning electron microscope and energy dispersive spectrometer: The SEM can provide the high-resolution corrosion morphology images, observing the three-dimensional morphologies of the corrosion pits and crack propagation paths. The EDS can carry out the micro-region composition analyses of the corrosion products, inclusions or chromium-depleted zones, and is a powerful tool for the studies of the corrosion mechanisms.

Constant temperature devices: Including the constant temperature water baths, oil baths or environmental chambers, used to precisely control the temperatures of the corrosion testing media. Many corrosion testing (such as the CPT determinations and boiling acid testing) have the extremely high requirements for the temperature control precisions, needing to reach ±1°C or even higher.

Specimen preparation equipment: Including the cutting machines, mounting machines, grinding and polishing machines and electrolytic polishing equipment. The standardized specimen preparation is the premise of obtaining the reliable, repeatable testing results, requiring the consistent specimen surface states (such as the roughness and cleanliness), without the structure changes caused by the processing.