ASTM G150 Standard Test Method for Electrochemical Critical Pitting Temperature Testing of Stainless Steels and Related Alloys
ASTM G150 standard, which is designed in line with the ISO, presents a rapid, electrochemical approach to finding the critical pitting temperature (CPT) of stainless steels or other alloys (in a specified chloride atmosphere). This is a test that provides a method of comparing pitting resistance in a controlled manner and proves particularly handy in alloy development and in quality control of corrosion-resistant stainless products.
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ASTM G150 Introduction
Corrosion in the stainless steels and other alloys tends to begin at the sites of discontinuities of the passive film and propagate into stable pits, especially in the presence of chloride-containing environments. Under the test, the lowest temperature at which propagating pitting is observed under a constant anodic potential in a standard electrolyte is called the critical pitting temperature or CPT. The common test procedure ASTM G150 offers an electrochemical procedure to calculate CPT in a reproducible manner, which offers alloy developers and corrosion engineers an analogue measure of pitting resistance. The process is applicable to wrought or cast stainless steels (plate, sheet, tubing, bar, forgings, welds) possessing the corrosion-resistance of foundry-grade UNS S31600 down to UNS S31254 (6% Mo) and can be further widened to other alloys on a case-by-case basis by varying the parameters.
ASTM G150 Test Method
Immersion and stabilization of the specimen
The prepared specimen is immersed in 1 M sodium chloride (NaCl) solution, which is maintained near 0 °C, and allowed to stabilize in the electrolyte in order for a uniform passive surface to be established before applying the potential.
Temperature ramp and potential application
The sample is usually exposed to a constant anodic potential, usually in the order of +700 mV vs SCE, for approximately 60 seconds before the temperature ramp starts. Subsequently, the temperature is raised with a controlled heating rate, typically around 1 °C per minute, while current density is recorded.
Determination of critical pitting temperature (CPT)
The CPT is defined as the temperature at which current density exceeds 100 µA/cm² and remains above the value for at least 60 seconds under the applied potential. The test concludes with a visual confirmation of stable pitting without the incidence of crevice corrosion.
ASTM G150 Equipment and Sample Preparation
Specimen geometry and test area
The specimen shall have a minimum exposed area of 1 cm². Any cut or sheared edges shall be removed by grinding unless the test is intended to study edge effects.
Surface preparation and finish
The surface should be clean and smooth, compatible with the specimen holder. Cleaning before testing consists of degreasing, rinsing in purified water, followed by an organic solvent rinse, and air drying to remove contaminants.
Test environment and cell conditions
The electrochemical test cell shall be designed for operation from 0 °C to 100 °C with a precision of ±0.4 °C and a ramp rate control of 1 ± 0.3 °C/min. The cell design shall be such that crevice corrosion is avoided and there is uniform heating.
ASTM G150 Test Results and Interpretation
ASTM G150 test result is given in degrees Celsius in the form of critical pitting temperature (CPT). It is the temperature at which stable pitting occurs under the mentioned environmental conditions and anodic potential. An increase in CPT shows an increase in resistance to localized pitting corrosion. It is mainly a comparative test, which is not to be applied to determine the exact service performance, but to rank alloys.
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ASTM G150 Related Articles
A number of related articles build up the concept of localized corrosion testing. One is concerned with the effect of alloy composition, in particular, molybdenum and nitrogen levels, on CPT behavior. The other one is focused on the effect of surface finishing and heat treatment on the pitting resistance of duplex and austenitic grades. Another study uses the methodology to measure the critical crevice temperature (CCT), which provides information on the susceptibility of crevice corrosion in comparable electrochemical control.
Materials Commonly Tested with ASTM G150
Most frequently used is in austenitic stainless steels type 316 and high-molybdenum stainless steels like UNS S31254 and duplex and super-duplex steels or alloys that are intended to be used in a chloride environment.
Applications of ASTM G150 in Industry
ASTM G150 testing finds extensive application in the development and qualification of new stainless steel alloys, in process control to produce corrosion-resistant components, and in testing materials to use in chemical processing, in marine, and desalination service, where exposure to chlorides is a crucial factor.
Safety and Best Practices in ASTM G150
Handling of hot solutions of electrolyte should be done carefully, well-ventilated, and no crevices should be formed between the specimen and the holder. Laboratory safety measures must be adhered to during all testing, which includes protective eyewear, equipment, and gloves.
Importance of ASTM G150
ASTM G150 is a consistent, reliable electrochemical measure of the pitting resistance of stainless steels and similar alloys. It is significant because it enhances the choice of materials, maximizing the alloy design, and increasing the performance of corrosion assurance in industries where pitting corrosion has a direct impact on the long-term service integrity.
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Where can I get the astm g150 tested?
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