Corrosion Testing
Corrosion testing plays a crucial role in understanding and evaluating the performance of metals when exposed to various environmental conditions. Metals are essential materials in countless industries, ranging from infrastructure and construction to aerospace and automotive. However, the destructive nature of corrosion poses a significant threat to the integrity, reliability, and longevity of metal structures and components. To mitigate these risks, corrosion testing methods have been developed to assess the susceptibility of metals to corrosion and determine their resistance against different corrosive agents. By subjecting metals to controlled conditions and monitoring their behaviour, corrosion testing enables researchers, engineers, and manufacturers to make informed decisions about material selection, design optimization, and maintenance strategies.
10% oxalic acid etch testing for stainless steel
The oxalic etch test serves as a rapid screening method for evaluating specimens, offering a faster alternative to the Strauss and Streicher tests. This test involves subjecting a polished sample to a 10% oxalic acid (H2C2O4) environment at room temperature, with an applied anodic current of 1A/cm2 for 1.5 minutes. Microscopic analysis is then conducted to determine if further testing is required or if the specimen is not sensitized. The presence of grain boundaries appearing as ditches in the micrograph indicates the need for additional testing.
For extra-low carbon grades and stabilized grades, the oxalic etch test is performed following sensitizing heat treatments conducted at temperatures ranging from 1200 to 1250°F (650 to 675°C) to induce maximum carbide precipitation. Prior to the oxalic acid etch test, these sensitizing treatments must be applied, typically involving a 1-hour treatment at 1250°F (675°C).
Each specific practice specification includes a table that outlines the acceptable and non-acceptable etch structures for a given stainless steel grade in that particular test. If a specimen exhibits an acceptable etch structure, it does not need to undergo the hot acid test. However, if the etch structure is non-acceptable, the specimen must be tested using the specified hot acid solution.
It is important to note that the oxalic acid etch test is applicable only to the grades listed in the individual hot acid tests. This test categorizes specimens as either acceptable or suspect. The oxalic acid etch test can be used to screen specimens intended for testing in various practices, including the ferric sulfate-sulfuric acid test (Practice B), nitric acid test (Practice C), copper-copper sulfate-16% sulfuric acid test (Practice E), and copper-copper sulfate-50% sulfuric acid test (Practice F).
Streicher Test (ASTM A262 Practice B)
The Streicher test is commonly used for stainless steel and nickel alloys. In this method, the specimen is boiled in a Ferric Sulfate - Sulfuric Acid solution for a duration of 24 to 120 hours. The extent of corrosion is determined by measuring the mass loss of the specimen.
Huey Test (ASTM A262 Practice C)
Similar to the Streicher test, the Huey test method utilizes a nitric acid solution. The specimen is subjected to five 48-hour boiling intervals. At each interval, the sample's weight is measured to assess mass loss and the degree of susceptibility to intergranular attack. The Huey test is particularly suitable for detecting chromium depletions and corrosion in intermetallic phases.
Strauss Test (ASTM A262 Practice E)
The Strauss test is exclusively employed to determine corrosion in areas high in chromium carbide formations. It is not as effective in identifying intergranular attack caused by other means. In this method, Strauss test specimens are boiled in a Copper Sulfate - Sulfuric Acid mixture for 15 hours. A bend test followed by visual or micro-examination is performed to determine whether the test specimens pass or fail.
Copper Sulfate Test (ASTM A262 Practice F)
The Copper Sulfate test is a 120-hour boiling test designed for "as received" specimens of stainless steel. It is particularly effective in determining susceptibility to intergranular attack in low carbon steels.
Pitting Test
Pitting tests are employed to assess the likelihood of localized corrosion, characterized by the formation of small holes or pits on the surface of a metal. These tests are conducted following a specific standard, such as ASTM G48, using different metal and solution combinations. The selection of the metal and solution serves as a reference point to ensure consistency of data and accuracy of results.
Understanding Pitting Test
Pitting tests are crucial for evaluating the resistance of metals to pitting and crevice corrosion, which can pose significant risks by compromising structural integrity. Pitting and crevice corrosion occur when corrosive agents, such as low pH acids, water, and salt solutions, accumulate in specific areas.
The resistance of a metal to pitting and crevice corrosion can be determined empirically and ranked based on a critical crevice temperature (CTT), as outlined in the ASTM Standard G48-03. This standard, titled "Standard Test Methods for Pitting and Crevice Corrosion of Stainless Steels and Alloys by Use of FeCl3," provides guidance on conducting pitting tests and recommends six different methods for implementation. Each method serves a specific purpose:
Method A - Used to assess the relative pitting resistance of stainless steels and nickel-base, chromium-bearing alloys. Method B - Employed to determine the resistance of stainless steel alloys against pitting and crevice corrosion. Methods C, D, E, and F - Applied to rank different alloys based on the minimum (critical) temperature required to initiate corrosion. These methods are specifically relevant for stainless steels, nickel-base alloys, and chromium-bearing alloys tested in a standard ferric chloride solution.