OXIDATION OF COPPER

11.10 OXIDATION OF COPPER

Copper oxidizes in air at low temperatures ( < 260 ° C) in accord with the two - stage logarithmic equation, forming a fi lm of Cu 2 O. The rate varies with crystal face,

OXIDATION OF COPPER

decreasing in the order (100) > (111) > (110). Heat treatment of polycrystalline copper with hydrogen at 300 – 450 ° C decreases the oxidation rate in oxygen at 200 ° C because submicroscopic surface facets are formed by adsorbed hydrogen, presumably favoring the (111) orientation. On the other hand, heat treatment with nitrogen or helium increases the rate because adsorbed oxygen (traces from gas or metal) favors submicroscopic facets of predominantly (100) orientation

[40] . Between about 260 ° C and 1025 ° C, the Cu 2 O fi lm is overlaid by a superfi cial fi lm of CuO. Oxidation changes from logarithmic to parabolic behavior above 400 – 500 ° C. Only Cu 2 O forms in air above 1025 ° C. Copper oxidizes at a rate slightly higher than that for iron, and much more rapidly than that for nickel or the heat - resistant Cr – Fe alloys. This is shown by the following temperatures [41] ,

below which the scaling losses in air are less than approximately 2 – 4 g m −2 h −1 : Cu, 450 ° C; Fe, 500 ° C; Ni, 800 ° C; 8 – 10% Cr – Fe (0.1% C), 750 ° C; 25 – 30% Cr – Fe (0.1% C), 1050 – 10 ° C.

Alloying elements that are particularly effective for improving oxidation resistance at high temperatures are aluminum, beryllium, and magnesium; for example, at 256 ° C, a 2% Be – Cu alloy oxidizes in 1 h at 1/14 the rate of copper

[42] . Maximum improvement by aluminum additions occurs at about 8% [43] .

11.10.1 Internal Oxidation

When alloyed with small percentages of certain metals (e.g., aluminum, beryllium, iron, silicon, manganese, tin, titanium, and zinc), copper oxidizes with precipita- tion of oxide particles within the body of the metal as well as forming an outer oxide scale. Oxidation within the metal is called subscale formation or internal oxidation. Similar behavior is found for many silver alloys, but without formation of an outer scale. Internal oxidation is not observed, in general, with cadmium - , lead - , tin - , or zinc - based alloys. A few exceptions have been noted, such as for alloys of sodium – lead, aluminum – tin, and magnesium – tin [44] . Internal oxidation is usually not pronounced for any of the iron alloys.

The mechanism is apparently one of oxygen diffusing into the alloy and reacting with alloying componens of higher oxygen affi nity than that of the base metal before the alloying components can diffuse to the surface. Above a critical concentration of the alloying component, a compact protective layer of the component oxide tends to be formed at the external surface which thereafter suppresses internal oxidation. In accord with a diffusion - controlled mechanism, the depth of subscale grows in accord with the parabolic equation [44] . The subject was reviewed by Rapp [45] .

11.10.2 Reaction with Hydrogen ( “Hydrogen Disease ”)

The tendency of copper to dissolve oxygen when the metal is heated in air leads to rupture of the metal along grain boundaries by formation of steam when the metal is subsequently heated in hydrogen. Cast tough - pitch copper containing

OXIDATION

free Cu 2 O is very sensitive to this type of damage. Instances are on record where damage has been caused by hydrogen at temperatures as low as 400 ° C (750 ° F). Oxygen - free coppers are not susceptible, but they may become moderately so, however, should they be heated at any time in oxygen or in air.

Silver similarly dissolves oxygen when heated at elevated temperatures in air, and it becomes blistered or loses ductility if later heated in hydrogen above 500 ° C (925 ° F). The mechanism is the same as that applying to copper. Oxygen -

free silver heated in hydrogen for 1 h at 850 ° C (1550 ° F) is not embrittled or damaged. However, when it is heated immediately afterward in air at the same temperature, loss of ductility occurs that is similar to, but not as severe as, the characteristic loss when silver containing oxygen is heated in hydrogen [46] . Some dissolved hydrogen undoubtedly escapes before oxygen can diffuse into the silver, hence diminishing subsequent damage. Gold and platinum dissolve little or no oxygen and, consequently, are not subject to similar damage when heated in hydrogen.