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Test plates are hung in the chamber for testing. Photo courtesy of Q-Lab. PRODUCTS FINISHING — pfonline.com 21 CORROSION AND SALT SPRAY 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. Neutral Salt Spray Testing NSS testing uses the oxygen-concentration cell corrosion mechanism to accelerate corrosion for use in performance analysis on a variety of substrates and coatings. Adding sodium chloride (NaCl) at a concentration of 5 percent keeps the corroded metal ions in solution so that they can act as conductors to enhance the corrosive effect. Salt helps to extend the life of each corrosion cell because it allows more metal to be in solution. The salt is actually increasing the solubility point of certain elements, like metal ions. Temperature is elevated during the NSS test as well in order to increase the speed of the electrochemical reactions attempting to take place. Parts are inclined to prevent the droplets of water being formed from becoming overly satu- rated. If the droplets fill up with metal ions and reach their saturation point, they will cease corroding the metal, which would defeat the whole purpose of the experiment. A typical salt-spray chamber has some basic components: an air saturation tower that stabilizes the salt concentration, a reservoir for the solution itself, an atomization nozzle for the creation of the fog, supporting mechanisms to hold the parts, a method for distributing heat inside the chamber and a temperature controller. If you have control over the way your parts are manu- factured, you can do a lot to ensure the plating process runs smoothly and the coatings will be very corro- sion resistant. For instance, an area of a part that has been stressed due to a crimping, machining or stamping opera- tion is more likely to become corroded when compared with an area that has main- tained its composition without significant stress. Once the microscopic grain or micro-crystalline struc- ture has been tampered with due to any type of force, a corrosive environment is poised to preferentially attack areas that have been modified, over regions that have sustained less deformation. Welds are a good example of this problem. Welders often use fluxes, or intermediate glues, which unfortu- nately can act as poultice—a moist conglomeration of conduc- tive material contained in a soft, but self-contained slurry. The poultice is often composed of sodium, calcium, chloride, sulfate and other corrosive ionic agents that add to the conductivity of the electrolytic cell being formed. Welds also tend to be porous, which means they can trap chemicals from the processing solutions causing bleed out and other phenomena, and they splatter metal fragments that can also negatively impact salt-spray results. Stress can also be created in the deposit itself by the inclusion of carbon, sulfur and other elements that are usually provided by proprietary additives like brighteners and carriers. The more stress your Controller Compressed Air-In Solution To Pump Vent Lid Solution Reservoir Specimens Chamber Heater Fog Nozzle Bubble Tower Pump A typical salt-spray chamber has an air saturation tower that stabilizes the salt concen- tration, a reservoir for the solution itself, an atomization nozzle for the creation of the fog, supporting mechanisms to hold the parts, a method for distributing heat inside the chamber and a temperature controller. Photo courtesy of Q-Lab.