Nickel Manganese

Embrittlement of nickel by incorporated sulfur when heated above 200 °C (390 °F) can arise by formation of brittle grain boundary films. In electrodeposits, the sulfur incorporation can result from the use of organic addition agents put into the solution in order to control internal stress in the plating. In these circumstances, manganese ions can be added to the solution so as to allow deposition of a nickel-manganese alloy resistant to sulfur embrittlement.

Manganese ions are also added in the absence of sulfur-bearing addition agents in the solution in order to produce nickel-manganese alloys that are stronger than plain nickel. There are both actual and potential applications of nickel-manganese alloy deposition in electroforming.

Advantages. The ability to neutralize the harmful effect of sulfur is not shared by iron or cobalt. Harder, stronger nickel alloys can be attained with manganese than with cobalt.

Disadvantages. Manganese does not codeposit as readily as iron or cobalt with nickel, and so nickel-manganese alloys contain much less manganese for a given concentration in solution of the second metal. Nickel-manganese alloys containing a useful amount of manganese tend to have high tensile internal stress and to be brittle.

Process Variables and Properties of the Alloys from Solutions with Stress Reducer. The percentage of manganese in the alloy at a fixed level of manganese in solution rises as deposition current density is increased, whether the solution is Watts-type, conventional sulfamate, or 600 g/L nickel sulfamate Ni-Speed (Ref 20). Manganese content rises from 0.02 to 0.03% at 4.3 A/dm2 (40 A/ft2) to 0.17 to 0.22% at 12.9 A/dm2 (120 A/ft2). These data were obtained in a solution that contained 15 g/L Mn and 0.25 g/L soluble saccharin to control deposit stress. Although sulfur uptake in the deposit also rises with increasing current density, by a half in the Watts and Ni-Speed solutions, the ratio of manganese content to sulfur content rises much more, from 1.1 to 6.2. Measurements of deposit ductility after heat treatment at various temperatures show that a higher ratio is required to avoid embrittlement for higher temperatures: 1.1 for 200 °C (390 °F) and 5.1 for 500 °C (930 °F) (Ref 20).

Alloy Hardness. As-deposited hardness of alloys plated with 15 g/L Mn and 0.25 g/L saccharin rises with an increase in percentage manganese in the alloy, from 287 HV at 0.02% Mn to 420 HV at 0.27% Mn when the base solution is Watts, and from 338 HV at 0.04% Mn to 445 HV at 0.23% Mn when the base solution is conventional sulfamate (Ref 21). Heat treatment for 22 h at 200 °C (390 °F) increases room-temperature hardness about 10% for all alloys with 0.1% Mn or more. Similar periods of heat treatment at 300 °C (570 °F) drastically reduce hardness to about 170 HV for all alloys irrespective of manganese content. Further increase in temperature to 400 °C (750 °F) or 500 °C (930 °F) has little further effect.

Internal stress in deposits from the 15 g/L Mn plus 0.25 g/L saccharin system is compressive for alloys deposited at up to 10.8 A/dm2 (100 A/ft2), whether the base solution is Watts, conventional sulfamate, or 600 g/L nickel sulfamate. Values lie in the range of 43 to 62 MN/m2 (6200 to 9000 psi) for Watts, 62 to 91 MN/m2 (9000 to 13,200 psi) for 600 g/L nickel sulfamate, and 94 to 100 MN/m2 (13,600 to 14,500 psi) for conventional sulfamate (Ref 20). At the highest current density, 12.9 A/dm2 (120 A/ft2), the Watts deposit remains in its compressive range while the conventional and 600 g/L nickel sulfamate deposits show small tensile stress values, respectively 17 MN/m2 (2500 psi) and 36 MN/m2 (5200 psi). These low-stress deposits produced at 12.9 A/dm2 (120 A/ft2), with hardness values 420 to 443 HV, are harder than any low-stress alloys produced with the nickel-cobalt system.

Properties of Alloys from Solutions without Stress Reducer. Alloys deposited from straight nickel plating solutions with only manganese additions can have tensile strengths exceeding 1000 MN/m2 (145,000 psi) and yield strengths up to 800 to 950 MN/m2 (116,000 to 138,000 psi) (Ref 22). However, ductility is low when more than a small percentage of manganese is alloyed with the nickel, deposit internal stress is tensile, and the alloys may crack during formation. Nonetheless, such alloys have been used as rigid electroforms (Ref 23). Heat treatment of the alloys increases ductility and relieves stress, the values depending on heat-treatment temperature and manganese content, and deposit properties can be further modified using pulse plating during deposition of the alloys (Ref 22).

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