34

Electrodeposition of Alloys Containing Molybdenum and Nickel, Cobalt, or Iron

34.1 Introduction

THE CONSTRUCTION OF mechanisms that must operate under load at elevated temperatures requires metals that have structural strength under these con-ditions. The alloys of only two metalstungsten and molybdenumare economically feasible for these applications. However, since this country is dependent on foreign sources for tungsten, whereas it has plentiful deposits of molybdenum ores, considerable current interest has been directed toward molybdenum. The United States government has been sponsoring com-prehensive researches dealing with all phases of the metallurgy, properties, and structural applications of molybdenum and its alloys.

The properties of molybdenum are similar to those of tungsten but not as outstanding. For practical applications its most important properties are its high melting point of 2610C (compared to 3410C for tungsten) and its relatively high mechanical strength at elevated temperatures. At 870C the hardness of rolled molybdenum is about 140 VHN and the hardness of the alloy containing 0.45% titanium is about 200 VHN. The latter alloy has a tensile strength of about 5000 kg/cm

2 (70,000 lb/in.

2) at this temperature as compared to about

1000 kg/cm2 (15,000 lb/in.

2) for the so-called superstrength alloys now in current

use. Molybdenum has a considerable advantage over tungsten for structural purposes as it is much more ductile.

The one drawback to the use of molybdenum at elevated temperatures is its ease of oxidation. Above 800C molybdenum reacts with oxygen of the air to form M0O3, which being volatile escapes as it is formed and does not protect molybdenum from further oxidation. For example, at about 980C a strip of molybdenum 1.4 mm thick loses 60% of its weight in 30 minutes. Intensive efforts are being made to develop coatings for protecting molybdenum from oxidation at elevated temperatures.

Another property of molybdenum of practical interest is its low coefficient of linear thermal expansion, 4.82 X 10"

6/C between 0 and 100C. The

coefficient is not quite as low as that of tungsten. As compared with other metals, molybdenum has a relatively high Young's modulus of elasticity, and good electrical and thermal conductivity.

Molybdenum has good resistance to chemical attack. It is not attacked appreciably by any of the common mineral acids, with the exception of nitric acid which rapidly dissolves it. It is attacked by boiling concentrated sulfuric acid and by strong acids in the presence of oxidizing agents. It is resistant to attack by many molten metals. Solutions of caustic soda do not attack molybdenum, but fused alkalies in the presence of oxygen dissolve the metal. Salt water tarnishes

4 1 3 .