Understanding Corrosion and Salt Spray
How it’s produced, NSS testing and how to get the best results possible.
“Why did these parts fail at 96 hours? The last ones went for over 120 hours without any issues.”
I am sure these words have escaped your lips at least once in your career as an electroplater in some form or fashion after reading the most recent report from your neutral salt spray (NSS) chamber operator.
To be less confused and frustrated by the results the plater receives from any accelerated corrosion testing facility, you must possess a basic understanding of corrosion. What it is; how it is produced; what NSS testing is; how you can produce, handle and package a product to give you the best results possible; and how you can engage your NSS operator when the root cause for any part failure seems to be generating out of the test itself, and not from the parts or the plating process itself.
To prevent corrosion, it’s imperative to know that corrosion is the process by which a metal in a solid state—such as zinc metal (Zn⁰)—is chemically changed due to a loss of electrons, turning solid metal into something different, often the cation Zn⁺².
There are a number of different corrosion reactions and types, but the one we focus on here is called oxygen-concentration cell corrosion, because that is the one employed when NSS tests are conducted. For instance, if we were to measure the concentration of oxygen directly in the middle of a drop of water on a piece of steel, we would find that it is a lot less than the concentration of oxygen at the very edge of the drop.
The different concentrations of oxygen in this droplet set up a corrosion cell wherein the oxygen-deficient area in the middle of the drop becomes the anode (or the corroding area just like you might witness in a plating tank) corroding away the iron in the steel and also becoming more acidic. The exterior of the drop (the cathode, or the oxygen-rich area) becomes more alkaline, thus precipitating out iron hydroxide in the form of red rust. This is because the cations (or positively charged particles of iron) are reduced or brought to a neutral state on the surface of the steel with the transfer of electrons. Anything that restricts the access of oxygen to a metal surface can develop what are termed differential aeration cells. Examples of restrictions would include anything from dust particles to a simple plastic washer.