94 THE DISTRIBUTOR’S LINK ROB LaPOINTE FASTENER SCIENCE: SALT SPRAY TESTING PER ASTM B117 from page 8 FIGURE 2 STEEL TOOLS RUST WHEN LEFT OUT IN THE RAIN On Mars, the red rust color of the soil is actually from red rust. The extensive deposit of rusted iron on Mars is a leading indicator of global oceans that, at one time, covered the planet. The water molecule (H 2 O) has two hydrogen atoms connected to one oxygen atom in its most common state. The bonds that hold the atoms together to form the water molecule are a result of shared (covalent) electrons. Oxygen has additional electrons beyond the ones it’s sharing with the hydrogen atom that are squeezed together and repel each other. This electron repulsion is what causes the shape of the atomic cluster to bend. It is also what causes the molecule to have an overall electrically polar quality. This means that one side of the molecule is electrically positive, and the other side is electrically negative due to an uneven charge distribution (see Figure 3). This polar nature allows water to dislocate electrons (negatively charged) from iron atoms which leaves the remainder of the iron atom positively charged. This process allows the solid iron to go into solution with water as the solvent. The positive iron ion clusters around the negative side of the water molecule. In the process of dissolution, the iron takes hydrogen away from the water and leaves hydroxides (negatively charged molecules) in the solution. In the second step of the process, iron reacts with these hydroxides to produce a green colored rust. At the same time, iron continues to react with hydrogen and oxygen from the water to form red rust, which is iron hydroxide. This compound dehydrates to form the familiar dry red powder we call rust. Rust doesn’t come from the water alone, but results as a reaction of the iron in the metal with water. For every new rust molecule that is formed, iron must be removed from the metal and therefore decomposes the structure of the metal allowing the once solid material to blow away like dust in the wind. You’re welcome for the music that is now playing in your head. If you’re not hearing anything, Google “dust in the wind – song.” Shown in Figure 2, carbon steel rusts readily when exposed to water, but what about stainless steel? Stainless steels are ferrous (iron based) just like carbon steels. Additional chromium content is what makes stainless steel iron rust resistant. With more chromium, oxygen in the atmosphere reacts with the chromium to make chromium oxide which inhibits water from interacting with the surface and rusting its iron. The process call passivation emboldens this chromium oxide layer and simultaneously removes iron from the surface layer. With depleted iron on the surface and a robust layer of chromium oxide, stainless steels are strongly rust resistant. FIGURE 3 WATER MOLECULE SHOWING ITS POLAR NATURE What Is The Role Of Salt In Rusting? The presence of salt in the water accelerates the process of rusting. Remember that rusting is caused by the formation of charged atoms (ions) which motivates electron transfer between atoms. Salt is an ionic compound, meaning that the sodium and chlorine atoms are held together by electrical attraction caused by the transfer of an electron from the sodium atom to the chlorine atom. CONTINUED ON PAGE 142
In the Winter 2023 issue of 6 DISTR
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