If you're in the automotive industry, stainless steel or virtually any other sector where metal produced is exposed to the elements, corrosion resistance is crucial to ongoing performance. How do you recreate conditions so you can see if metals will effectively withstand driving rain, searing sun and steady snowfall?
Corrosion resistance testing procedures make this possible. Whether it's the salt spray test, humidity or copper sulfate, corrosion tests examine the durability of metal products, equipment and parts to assess the degree to which they'll rust, typically appearing as orangey-brown or red rust. In a matter of hours, you can get a general idea of what the resulting wear and tear would look like when the metal is exposed to outdoor environments for years or more.
It raises the question: What are salt spray test hours equivalent to in years? Before we get into that and why this isn't easy to answer, let's explore a little more into how a salt spray test actually works.
How is salt spray testing carried out?
In a controlled environment, such as an environmental test chamber, the subject in question is coated with a fine mist, which is made up of sodium chloride, the chemical equivalent of salt. This mixture is released from an air-pressurized nozzle and is sprayed continuously. The actual length of time can vary, from a few minutes, to hours, to several days. The resulting pitting and overall damage are designed to show what the test piece would look like over an extended period of time, only not to the same extreme on a minute-to-minute basis. There are several modified tests that can be done but the most common is ASTM G85. Common metals that undergo such ASTM testing include chromium, steel and zinc. Chromium is a hard white metal used in stainless steel. Other types of testing ASTM procedures include salt fog, seawater acidified and electrolyte salt fog/dry.
In a real-world setting, it's unlikely that any metal would be exposed to this level of harshness for so short a period of time, but it's done as a means to expedite the oxidation process instead of waiting for Mother Nature to take its course.
What is a neutral salt spray test?
Sometimes, the salt spray test used depends on the metal that's being evaluated for corrosion. A classic example is steel. While technically an alloy, the neutral salt spray test is the most common method use for testing steel and is different from ASTM G85 in terms of its pH or acidity. The pH level for neutral salt spray typically ranges between 6.5 and 7.2 and provides a more accurate representation of what steel would look like after extended exposure to damp and wet conditions, albeit over a much briefer period.
This brings us back to the main question at issue: What are salt spray hours equivalent to in years? The truth is, there is no way of knowing this. Several studies have taken a crack at trying to determine what the translation is, and all have come away with the same finding: salt spray tests do not perfectly correlate with real world environments.
There are a few reasons why this is the case, but above all else, it's due to the environment being naturally variable. Any given second of the day brings certain wind, rain, humidity and sunlight levels that make it impossible to get a perfect correlation of what the results from a salt spray test over hours actually look like in real time. However, the accelerated weathering that salt spray testing provides remains the best way to get a high-level assessment of the corrosion resistance of your materials and can provide insight into what you need to do to make them stronger.
Auto Technology is a nationwide industry leader in top quality environmental testing. Contact us today to schedule some time in any one of our corrosion evaluation chambers or purchase one of your own.