Monday, December 12, 2016

Bearing Design Guide: Chapter Twenty-One: Corrosion Resistance Of Some Bronzes

We are going to continue with our chapter, but first a little announcement...

And now onto...Bearing Design Guide: Chapter Twenty-One: Corrosion Resistance Of Some Bronzes

          Corrosion is defined as the eroding of a metal as a result of a reaction with its environment, or exposure to various liquids or gases.

          Some metals and alloys are naturally resistant to certain corrosive environments. The product of a corrosive film which forms when metals are subjected to corrosive attack protects them from speedy damage by virtue of this protective oxide film.

          Corrosion can occur in bronzes as a result of slow dissolution of copper and copper alloys either because no protective film is formed or because as fast as a film is formed, it enters into solution in the corroding medium.

          Outdoors, copper and copper alloys develop a relatively protective skin of sulfides, oxides or soot. The sulfides form as a result of a reaction with sulfuric acid in the atmosphere and oxides as a result of a reaction with oxygen in the air. These reactions speed up in humid and rainy climates. They would cease entirely in the absence of water.

          Galvanic corrosion is caused by compounds which are electrical conductors when in solution in water and are known as electrolytes. The ions in these electrolytes are ever ready to conduct electricity if an anode (the positive ion) and cathode (the negative ion) of any type are present.

           Solutions of carbon dioxide, sulfur dioxide, oxygen, chlorides and fluorides are condensed or precipitated from the atmosphere. When a metal is to be used where there is an electrolytes as in sea water, the coupling of metals must be close together in the EMF series; to reduce the tendency to corrode the least nobler (cathodic) material.

          Galvanic corrosion often can be prevented by separating the less noble material by insulating it with rubber or synthetic resins.

           Although aluminum alone as a base material possesses good corrosion resistance to dry atmospheres, it actually corrodes very rapidly until a surface film forms. The surface film arrests further action to sea water, many fresh waters, chemicals and foods.

          This oxide film is extremely thin and, when damaged or scratched, corrosion will reform another thin film. Aluminum, therefore, depends on the resistance of the formed oxide film to attack rather than to the base metal.

          If aluminum is coupled with a copper-based alloy as bronze in a wet atmosphere, such as in marine environments, corrosion of the aluminum will continue unabated and would be an unfavorable galvanic couple. However, in a dry atmosphere, there is no precipitable galvanic action encountered.

          The high-leaded tin bronzes, leaded tin bronzes and tin bronzes have poor resistance to most acids but good resistance to sea water, fresh water, gasolines, fuel oils, alcohols, Freons and many other mediums. But it is recommended that the copper alloys be tin-coated or plated.

          For the most corrosive resistant alloys, the aluminum bronze alloys offer the greatest protection. The aluminum bronze alloys are used for their strength primarily and extensively used in general outdoors, marine service and exposure to many acids.

Galvanic Corrosion On Propeller

          A brief list follows showing the acceptable and non-acceptable exposure to various corrosive medium by aluminum bronzes.

          In the case of manganese bronzes, which contain less than 80% copper, zinc is selectively removed from the alloy by most acids when diluted in water. Manganese bronzes can be used for marine applications, water pump rotors and in sea water.

That's it for today.  Until next time my metal loving friends...

Next Up: Chapter 22: Soldering, Brazing and Welding Of Bronze Alloys


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