When metal is exposed to salt water, variations in electric potential arise on the metal's surface, resulting in the creation of countless tiny corrosion cells. This process destroys spots (the anodes) on the metal surface where metallic ions enter the electrolyte. Those areas are then unavailable for further reaction with other substances in the environment so they quickly become covered by a layer of inert material (passive protection). The more frequent these anode spots are destroyed, the less likely it is that the metal will fail during use.
In general, salt has two effects on metals: it promotes corrosion and alters the mechanical properties of the metal.
Salt can promote corrosion by several mechanisms. It can increase the rate at which electrons flow from a metal object into surrounding water, causing oxidation-relating enzymes to produce more oxidizing agents such as oxygen molecules. This increased oxidation causes the metal to lose more electrons and thus become more negatively charged. Since positive charges are needed to create a path for current to flow, this means there are fewer negative charges available to create anodic sites on the metal's surface. Thus, it becomes more vulnerable to corrosion.
At high concentrations, salt can also act as a cathodic agent. This means it will cause the metal to lose electrons rather than gain them, which would otherwise happen in a normal electrochemical reaction.
Salts have a number of effects on corrosion rates. To begin, salt is hygroscopic, which means it collects water from the air. Third, the chloride ions in salt have the ability to dissolve the protective oxide coating that develops on the surface of several metals. Salt exposure, for example, damages aluminum's ordinarily protective oxide coating. Finally, when salt melts or evaporates, it leaves behind a residue of sodium and chlorine atoms that can react with organic materials such as wood pulp or paper products to form chlorides.
Salt has many beneficial uses too. It helps plants by acting as a natural fertilizer, it reduces erosion, and it makes roads safer. However, like any other material, salt should be used properly by limiting its exposure to environmental conditions that may cause it to corrode.
Salt water is an electrolyte that carries ions and so accelerates corrosion. As an electrolyte (any material containing free ions that permits the substance to conduct electricity), salt solution helps iron to lose electrons more quickly, speeding up the rusting process. If you leave a metal object in saltwater for a long time, it will eventually grow a thick protective layer of iron oxide.
If you spray saltwater on your metal objects, the oxygen in the water will speed up the oxidation process. After about six months, all the saltwater will be absorbed by the object's surface and the iron will be left with only a thin layer of iron oxide, which will wear off after several more months.
The best way to prevent salt from affecting your metal objects is not to expose them to any water other than fresh tap water. Even then, you should avoid leaving objects in saltwater for longer than is necessary.
However, if this is not possible you can try cleaning affected areas with a mild soap and water solution or baking soda and vinegar mixture. Do not use household cleaners with abrasives such as sandpaper, as this will damage the coating on your object.
In conclusion, salt does affect rusting but only if it is left on the object for too long.
When salts and other inorganic compounds dissolve in water, they form small electrically charged particles known as ions. Ions improve the capacity of water to carry electricity. Because dissolved salts and other inorganic compounds conduct electricity, conductivity rises with salinity. The more frequent the electrical shocks, the more salt will be removed from the water.
Conductivity is a measure of how easily electrons flow through a solution. Salt increases conductivity because it attracts electrons from any impurities in the water column and forces them into adjacent molecules. This creates new pathways for electrons to take so that when an electric current is passed through the solution, it flows more freely.
Conductivity is measured in microsiemens per centimeter (mS/cm). The higher the number, the more saline the water is. Pure water has no conductivity because there are no impurities to attract electrons from inside the water molecule and force them into adjacent molecules. Water with low concentrations of salts has lower conductivity than water with high concentrations because even though there are more ions present, they are spread out over a larger area. Water with high levels of calcium or magnesium can also have lower conductivities if there are not enough positive charges present to attract electrons from any impurities in the water column.
Conductivity is useful for measuring water quality because it provides a direct measurement of the amount of salt present in the water.
Factors Influencing Metal Corrosion Despite the fact that stainless steel, aluminum, copper, and galvanized steel are corrosion-resistant, they nevertheless react to saline air and oxygen. The oxygen in salty air contributes significantly to metal corrosion. When the metal reacts with oxygen, it oxidizes. Oxidation is the process by which molecules gain electrons. This process leaves the metal more vulnerable to other forms of attack.
Stainless steel, for example, loses its corrosion resistance when exposed to salt water because saltwater contains substances that can erode stainless steel. The same thing happens if you park your car in a high-crime area or near a highway. The heat from the motor and the traffic may cause the stainless steel wheels to rust.
If you drive a car with steel wheels but otherwise have all-stainless-steel body work, then you are being exposed to extremely corrosive conditions. The surface area of the steel in contact with air and water will quickly be eaten away. Even though you cannot see it, the risk is still there. You might think the only place for metal wheels to rust is under the vehicle, but this is not true. Even if the tires are filled with oil, they will likely still rust underneath if they are not protected.
Aluminum, copper, and zinc all corrode when exposed to salt water.