BRONZE, OR COPPER-TIN, ALLOYS BASIC INFORMATION AND TUTORIALS

Bronze is an alloy consisting principally of copper and tin and sometimes small proportions of zinc, phosphorus, lead, manganese, silicon, aluminum, magnesium, etc. The useful range of composition is from 3% to 25% tin and 95% to 75% copper.

Bronze castings have a tensile strength of 195 to 345 MPa (28,000 to 50,000 lb/in2), with a maximum at about 18% of tin content. The crushing strength ranges from about 290 MPa (42,000 lb/in2) for pure copper to 1035 MPa (150,000 lb/in2) with 25% tin content.

Cast bronzes containing about 4% to 5% tin are the most ductile, elongating about 14% in 5 in. Gunmetal contains about 10% tin and is one of the strongest bronzes.

Bell metal contains about 20% tin. Copper-tin-zinc alloy castings containing 75% to 85% copper, 17% to 5% zinc, and 8% to 10% tin have a tensile strength of 240 to 275 Mpa (35,000 to 40,000 lb/in2), with 20% to 30% elongation.

Government bronze contains 88% copper, 10% tin, and 2% zinc; it has a tensile strength of 205 to 240 MPa (30,000 to 35,000 lb/in2), yield strength of about 50% of the ultimate, and about 14% to 16% elongation in 2 in; the ductility is much increased by annealing for ½ h at 700 to 800°C, but the tensile strength is not materially affected.

Phosphor bronze is made with phosphorus as a deoxidizer; for malleable products such as wire, the tin should not exceed 4% or 5%, and the phosphorus should not exceed 0.1%. United States Navy bronze contains 85% to 90% copper, 6% to 11% tin, and less than 4% zinc, 0.06% iron, 0.2% lead, and 0.5% phosphorus; the minimum tensile strength is 310 MPa (45,000 lb/in2), and elongation at least 20% in 2 in.

Lead bronzes are used for bearing metals for heavy duty; an ordinary composition is 80% copper, 10% tin, and 10% lead, with less than 1% phosphorus. Steam or valve bronze contains approximately 85% copper, 6.5% tin, 1.5% lead, and 4% zinc; the tensile strength is 235 Mpa (34,000 lb/in2), minimum, and elongation 22% minimum in 2 in (ASTM Specification B61). The bronzes have a great many industrial applications where their combination of tensile properties and corrosion resistance is especially useful.

CORROSION OF IRON AND STEEL AND PREVENTION BASIC INFORMATION AND TUTORIALS

What causes corrosion on Iron and Steel?

Principles of Corrosion.

Corrosion may take place by direct chemical attack or by electrochemical (galvanic) attack; the latter is by far the most common mechanism. When two dissimilar metals that are in electrical contact are connected by an electrolyte, an electromotive potential is developed, and a current flows.

The magnitude of the current depends on the conductivity of the electrolyte, the presence of high resistance “passivating” films on the electrode surfaces, the relative areas of electrodes, and the strength of the potential difference. The metal that serves as the anode undergoes oxidation and goes into solution (corrodes).

When different metals are ranked according to their tendency to go into solution, the galvanic series, or electromotive series, is obtained. Metals at the bottom will corrode when in contact with those at the top; the greater the separation, the greater the attack is likely to be.

Table 4-14 is such a ranking, based on tests by the International Nickel Company, in which the electrolyte was seawater.

The nature of the electrolyte may affect the order to some extent. It also should be recognized that very subtle differences in the nature of the metal may result in the formation of anode-cathode galvanic cells: slight differences in composition of the electrolyte at different locations on the metal surface, minor segregation of impurities in the metal, variations in the degree of cold deformation undergone by the metal, etc.

It is possible for anode-cathode couples to exist very close to each other on a metal surface. The electrolyte is a solution of ions; a film of condensed moisture will serve.

Corrosion Prevention.

An understanding of the mechanism of corrosion suggests possible ways of minimizing corrosion effects. Some of these include:

(1) avoidance of metal combinations that are not compatible,

(2) electrical insulation between dissimilar metals that have to be used together,

(3) use of a sacrificial anode placed in contact with a structure to be protected (this is an expensive technique but can be justified in order to protect such structures as buried pipelines and ship hulls),

(4) use of an impressed emf from an external power source to buck out the corrosion current (called cathodic protection),

(5) avoiding the presence of an electrolyte—especially those with high conductivities, and

(6) application of a protective coating to either the anode or the cathode.