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 Corrosion

CORROSION:       One of the most damaging and costly naturally occurring events seen today. Corrosion is an all-too-common result of electrochemical reactions between materials and substances in their environment.  https://www.electrochem.org/corrosion-science

GALVANIC CORROSION:       Refers to corrosion damage induced when two dissimilar materials are coupled in a corrosive electrolyte (saltwater). It occurs when two (or more) dissimilar metals are brought into electrical contact under water. When a galvanic couple forms, one of the metals in the couple becomes the anode and corrodes faster than it would all by itself, while the other becomes the cathode and corrodes slower than it would alone.  http://www.boatus.com/boattech/articles/marine-corrosion.asp

ELECTROLYSIS:       More commonly called "stray current" corrosion, adds an external electrical source to the corrosion equation, rapidly accelerating the reaction. It occurs when metal with an electrical current flowing into it is immersed in water that is grounded (which would include any lake, river, or ocean). This can happen if a short develops between an external current source (almost always the 12-volt electrical system on your boat or someone else's) and some part of the electrical system that is tied into the boat's underwater metals. The stray current will exit the boat from an underwater metal fitting.  https://www.boatus.com/seaworthy/magazine/2015/july/marine-corrosion-101.asp

DEALLOYING:       Dealloying or selective leaching refers to the selective removal of one element from an alloy by corrosion processes. A common example is the dezincification of un-stabilized brass, whereby a weakened, porous copper structure is produced. "Discoloration" or a "pinkish hue" in brass or bronze is a sign of dealloying.  https://www.nace.org/Dealloying/

BONDING & HALOING:      At the first sign of haloing (burnt paint around thru-hull fittings), the first step is to inspect all bonding system connections to ensure they are clean, tight and corrosion free. Next would be to have a corrosion survey conducted on the vessel to determine if proper galvanic protection is being provided. If diver’s find paint haloing they will note specific details & take photographs of the severity. https://www.passagemaker.com/technical/-through-hull-fittings-and-canaries-in-coal-mines

PROPELLER CAVITATION:       As the propeller turns it absorbs the torque developed by the engine at given revolutions i.e., the delivered horsepower – and converts that to the thrust which, in turn, pushes the vessel through the water. According to Bernoulli’s law the passage of a hydrofoil (propeller blade section) through the water causes a positive pressure on the face of the blade and a negative pressure on its back. The negative pressure causes any gas in the water to evolve into bubbles similar to those found when opening a carbonated beverage. These bubbles collapse and can cause hammer like impact loads on the propeller blades, resulting in the observed damage to the propeller blade surfaces. We typically see cavitation pitting at the base of the propeller flukes and even on the sides of rudders. This pitting is concentrated to one specific location, this is how we determine if its corrosion vs. cavitation.  https://www.iims.org.uk/introduction-propeller-cavitation/

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