In underground or immersion situations, cathodic protection is also a good choice. [6] X Research source
Any method of protection that prevents ion flow between the metals can potentially halt galvanic corrosion. Giving the metals a protective coating can help prevent electrolytes from the environment from creating an electrical conducting path between the two metals, while electrochemical protection processes like galvanization and anodizing also work well. It’s also possible to thwart galvanic corrosion by electrically insulating the areas of the metals that come into contact with each other. Additionally, the use of cathodic protection or a sacrificial anode can protect important metals from galvanic corrosion. See below for more information.
Exposure to an environment high in chlorides (like, for example, salt water) is known to accelerate the pitting process.
Crevice corrosion is of special concern when dealing with metals like aluminum which have a protective, passive outer layer, as the mechanism of crevice corrosion can contribute to the breakdown of this layer.
Preventing SCC is partly a design issue. For instance, by choosing a material that is SCC-resistant in the environment in which the metal will operate and ensuring that the metal material is properly stress-tested can help prevent SCC. Additionally, the process of annealing a metal can eliminate residual stresses from its manufacture. SCC is known to be exacerbated by high temperatures and the presence of liquid containing dissolved chlorides.
However, paint itself is vulnerable to degradation. Reapply paint whenever it becomes chipped, worn or damaged. If paint degrades to the point that the underlying metal becomes exposed, be sure to inspect for corrosion or damage on the exposed metal. There are a variety of methods for applying paint to metal surfaces. Metalworkers often use several of these methods in conjunction to ensure that the entire metal object receives a thorough coating. Below is a sampling of methods with comments on their usages: Brush - used for hard-to-reach spaces. Roller - used for covering large areas. Cheap and convenient. Air spray - used for covering large areas. Quicker but less efficient than rollers (paint wastage is high). Airless spray/Electrostatic airless spray - used for covering large areas. Quick and allows for variable levels of thick/thin consistency. Less wasteful than ordinary air spray. Equipment is expensive.
Because lubricants don’t dry in place like paints, they degrade over time and require occasional re-application. Reapply lubricants to metal parts periodically to ensure they remain effective as protective sealants.
Dirt, grime, and other debris interferes with paint and lubricants by keeping the paint or lubricant from adhering directly to the metal surface. For instance, if you paint over a sheet of steel with a few stray metal shavings on it, the paint will set on the shavings, leaving blank spaces on the underlying metal. If and when the shavings fall off, the exposed spot will be vulnerable to corrosion. If painting or lubricating a metal surface with some existing corrosion, your goal should be to make the surface as smooth and regular as possible to ensure the best possible adherence of the sealant to the metal. Use a wire brush, sandpaper, and/or chemical rust removers to remove as much loose corrosion as possible.
In addition to watching for exposure to moisture during use, be sure to store the metal items indoors in a clean, dry place. For large objects that won’t fit in a cupboard or closet, cover the object with a tarp or cloth. This helps keep out moisture from the air and prevents dust from accumulating on the surface.
This process involves handling industrial chemicals, some of which are hazardous at room temperature, at extremely hot temperatures and thus should not be attempted by anyone other than trained professionals. Below are the basic steps of the hot-dip galvanization process for steel: The steel is cleaned with a caustic solution to remove dirt, grease, paint, etc. , then thoroughly rinsed. The steel is pickled in acid to remove mill scale, then rinsed. A material called a flux is applied to the steel and allowed to dry. This helps the final zinc coating adhere to the steel. The steel is dipped in a vat of molten zinc and allowed to heat to the temperature of the zinc. The steel is cooled in a “quench tank” containing water.
Sacrificial anodes are made from several different types of reactive metal. Zinc, aluminum, and magnesium are three of the most common metals used for this purpose. Because of the chemical properties of these materials, zinc and aluminum are often used for metal objects in saltwater, whereas magnesium is more suitable for fresh water purposes. The reason a sacrificial anode works has to do with the chemistry of the corrosion process itself. When a metal object corrodes, areas that chemically resemble the anodes and cathodes in an electrochemical cell naturally form. Electrons flow from the most anode parts of the metal surface into surrounding electrolytes. Since sacrificial anodes are very reactive compared to the metal of the object being protected, the object itself becomes very cathodic in comparison and, thus, electrons flow out of the sacrificial anode, causing it to corrode but sparing the rest of the metal.
Note that the type of current used for impressed current protection systems is usually direct current (DC). Usually, corrosion-preventing impressed current is generated by burying two metal anodes in the soil near the metal object to be protected. Current is sent through an insulated wire to the anodes, which then flows through the soil and into the metal object. Current passes through the metal object and returns to the source of the current (generator, rectifier, etc. ) through an insulated wire. [8] X Trustworthy Source United States Environmental Protection Agency Independent U. S. government agency responsible for promoting safe environmental practices Go to source
The chemical process behind anodization involves the fact that many metals, like aluminum, naturally form chemical products called oxides when they come into contact with oxygen in the air. This results in the metal normally having a thin outer oxide layer which protects (to varying degree, depending on the metal) against further corrosion. The electric current used in the anodizing process essentially creates a much thicker buildup of this oxide on the surface of the metal than would normally occur, providing great protection from corrosion. There are several different ways to anodize metals. Below are the basic steps of one anodizing process. [9] X Research source See How to Anodize Aluminum for more information. The aluminum is cleaned and de-greased. The aluminum’s surface impurities are removed with a de-smut solution. The aluminum is lowered into an acid bath at a constant current and temperature (for instance, 12 amps/sq ft and 70-72 degrees F (21-22 degrees C). The aluminum is removed and rinsed. The aluminum is optionally submerged in dye at 100-140 degrees F (38-60 degrees C). The aluminum is sealed by placing it in boiling water for 20-30 minutes.
One well-known example of a metal that exhibits passivation is stainless steel. Stainless steel is an alloy of ordinary steel and chromium that is effectively corrosion-proof in most conditions without requiring any other protection. For most day-to-day uses, corrosion isn’t usually a concern with stainless steel. However, it bears mentioning that in certain conditions, stainless steel is not 100% corrosion-proof - notably, in salt water. Similarly, many passive metals become non-passive under certain extreme conditions and thus may not be suited for all uses.
