Electroless nickel plating
Electroless nickel is deposited as wear or corrosion protection is usually on metallic materials.
Contrast to galvanic nickel
The difference with galvanic nickel is, inter alia, that for deposition no external electrical power is about a rectifier is used, but for the deposition (reduction) of the nickel ions are generated electrons required by chemical oxidation reaction in the bath itself. This gives the chemical nickel plating contours loyal coatings whose dimensions may be at a tolerance of ± 2 microns to ± 3 microns in the range of 8 microns to 80 microns. However, from 50 microns with stresses in the layer must be expected.
Non- conductive base body
Due to the electroless plating, it is possible, also electrically non- conductive body to be coated, for example of plastics such as polyamide. At the easiest way to ABS (acrylonitrile- butadiene -styrene ) coating adheres well: After etching the ABS with a chromic acid solution, the nickel deposits after seeding with a noble metal ( palladium) in the fine holes from which the chromic acid by the removal of butadiene forms. The coating interlocks in the plastic. A seeding is necessary in non-conductive base body, since the electrolyte deposition ( almost) begins only on bare metal surfaces. Otherwise, the electrolyte would degrade themselves.
Layer properties
It is in this coating is a nickel -phosphorus alloy, which is mainly used in functional areas. One can control the film properties on the deposited layer in the phosphorus. A distinction, medium (9-12 %) and low (3-7 %) phosphorus content between a high (10-14 %).
The corrosion protection of the layer is primarily based on a high phosphorus content and the deposition of a non-porous layer, which is always from the base material and its processing depending, for example: polishing, grinding, turning, milling. Pre-machining of the material, in turn, affects the adhesive strength of the coating. The layer thicknesses, depending on the base material and its processing is usually at least 30 microns - 50 microns.
Depositing hardness increases with decreasing phosphorus content and can be raised by heat-treating the layer at a maximum of 400 ° C and one hour dwell time at values of 800 to 1100 HV 0.1. The layer thicknesses are here depending on the application between 10 microns and 50 microns.
The adhesion of the layer depends mainly on the base material, and the pretreatment of the material. Also the bond strength can be improved by a heat treatment, this one uses lower temperatures with higher retention times.
The look of the film is from the pre-processing of the base material on which the layer is deposited, depending on: blasted surfaces remain matt, polished surfaces stay shiny. The appearance of the coat is not, as with galvanic coatings on additives in the electrolyte (eg brightener ) optically adjusted. In addition: In the matter have been developed for this purpose ways to reach targeted layer properties with selected components, such as can look and grain boundary density can be influenced.
Due to the high cost of these coating layers rarely higher than 50 microns are deposited. Deposition of 10 micron electroless nickel takes around 1 hour
Installations for surface coating
To prevent Mitbeschichtung of the system components such as tanks, pumps, etc. Ensure these plant parts are often made from stainless steel and artificial passivated as well as a small positive voltage is applied ( sump guard ), so that eventually deposited nickel dissolves immediately. The counter electrode is usually an passivated stainless steel cathode is then deposited on the as in a pure nickel electroplating processes. If an incorrect approach or imported alien substances or destruction of the passivation of the system about by mechanical damage, however, a spontaneous decomposition of the electrolyte under heavy metal deposition can still happen. The facilities and bathrooms where electroless nickel coating, are more expensive than the galvanic method. The electrolyte " bleed out ", the nickel ions are present in the bath preparation. The stabilizers, the temperature and pH of the bath must be kept constant within certain tolerances; This is realized via Nachdosierpumpen and periodic monitoring of values . After about 1-2 weeks of nickel in the bath is exhausted and it must be recognized a new bathroom. The bath age is measured by so-called MTOs (metal turnover ). The costly and bath control the deposition rate is a major reason for the much higher price than is the case with electroplated coatings.