What do birmabright, alnico and duralumin all have in common with one another? Several things, as it happens, the main one being that they're all aluminum alloys, which combine aluminum with a secondary or tertiary element.
While aluminum alloys are naturally resistant to corrosion, they're not invulnerable to it. This reality raises a key question: How can you determine birmabright's, duralumin's, or alnico's resilience and resistance to corrosion when extreme temperatures and moisture combine? The answers start through reliable CASS testing.
Whether a CASS test is entirely new to you or you've heard about it before but only in passing reference, the following will help you understand more about this unique testing procedure, why it's performed and how the test method is carried out.
What is a CASS test and what does CASS stand for?
Short for Copper Accelerated Salt Spray, a CASS test is an accelerated corrosion test that uses a combination of sodium chloride and water to, in effect, simulate the manifestations of corrosion, which helps determine the test subject's corrosion resistance. The main difference between a CASS test and a regular spray fog test is the composition of the test subject, which is usually aluminum or an aluminum alloy.
Aluminum is Earth's third-most abundant element."
According to ThoughtCo, aluminum is Earth's third-most abundant element. Pairing it with others is what creates alloys, which in addition to the aforementioned, include zamak (zinc, magnesium, copper), silumin (silicon) and titanal (zinc, magnesium, copper, zirconium). The combining factor helps aluminum perform better and be more adaptive in various environments.
In addition to evaluating the corrosion resistance of aluminum alloys, the CASS test is also used for verification purposes. More specifically, it assesses the presence of nickel and chrome when used as a means of plating. Products that often receive chrome plating include high-powered equipment, stainless steel and automobiles.
How is a CASS test conducted?
A CASS test has many similarities to the standard ASTM B117 salt spray test. For example, it typically uses the same break down in how many parts are water (95%) and how many parts are sodium chloride (5%). As Corrosionpedia explains, a CASS test also uses reagent grade copper chloride as an added ingredient, which serves as a dehydrating element. The reagent grade copper also affects the pH level of the solution, moving it from a neutral 6.5 to 7.2 to somewhere between 3.1 and 3.3, which makes the solution much more aggressive before spraying begins.
Upon atomizing the solution, the mist is dispensed evenly as an acid salt spray fog. The test itself occurs within a test cabinet, which allows for simulated service evaluations that can be customized and expedited.
"CASS tests generally last no fewer than 24 hours."
In terms of how long CASS tests last, they generally run no fewer than 24 hours, but they can be 48 hours or longer as well. However, the abrasiveness of the salt spray is such that the CASS test tends to be shorter than the ASTM B117.
What does NSS and AASS stand for in corrosion testing?
Salt spray tests are variations of one another, where a certain ingredient is added, eliminated or adjusted to affect the outcome. Short for neutral salt spray, NSS is another name for the ASTM B117 standard, the oldest and most well-known of the salt spray testing methodologies. AASS — or acetic acid salt spray — leverages acetic acid to assess the resilience of die casting, nickel-plated and chrome-plated materials. Known in the industry as ASTM G85, modified salt spray tests such as these are highly regulated and require precise temperature controls and pH levels, all of which are governed by ASTM International. How long these tests last in terms of duration and the temperature maintained in salt fog test cabinets depend on the type being used. For example, according to ASTM International, the acidified salt fog test (i.e. ASTM G85 Annex A2) is required to maintain a constant cabinet temperature of 120 degrees Fahrenheit, while the temperature specification acceptance for seawater acidified tests (i.e. ASTM G85 Annex A3) can be lowered below 120 degrees Fahrenheit if the test subject has an organic coating. Otherwise, it too maintains a temperature of 120 degrees Fahrenheit (49 degrees Celsius).
Why are salt spray tests even necessary?
It isn't so much a matter of if a metallic product will corrode, but when. A salt spray test may not be a perfect predictor of when corrosion will take place, but it can give you a general idea of what conditions will lead to observable wear and tear and offer insight on what coatings may be best to delay its development.
Again, while ASTM B117 tests remain the gold standard in corrosion assessment salt spray methodology, ASTM G85 and ASTM B368 are designed for products that are lined with chromium, nickel, zinc and/or cadmium plating. A classic example of a product that would undergo ASTM B368 testing, as opposed to ASTM B117, is stainless steel, depending on how much of a given element or alloy it contains in terms of percentages.
As an industry leader in environmental testing, Auto Technology is equipped with the training, tools, equipment and experience that can adequately assess how your products will last under various conditions. We leverage the very latest in research and development to produce results that you can trust. Please contact us today to learn more.