Sur-Tech A/S Surface Technology

Transcription

1 Sur-Tech A/S Surface Technology ELECTROLESS NICKEL A wear resistant and corrosion protective coating DS/ ISO 9001 ISO Quality Assurance System Environmental Management System Sur-Tech A/S Bygmarken Farum Phone: st@sur-tech.dk

2 What is electroless nickel? Electroless nickel plating is a process in which nickel is precipitated without the use of an extern electrical circuit. The needed energy to make the precipitation comes from a chemical reducing additive that is added frequently to the bath along with nickel salts. The coating is precipitated in a uniform layer thickness independent of the geometric shape of the production parts, and may be applied in a thickness up to 100µm. Electroless nickel plating coatings are precipitated auto catalytically, meaning that the precipitation rate is constant as long as the control of the process is correct. The coating consists of an alloy of nickel and phosphorus (approx. 10% P). The contents of phosphorus and the quality of the coating in general are also dependent on the chemistry of the bath, together with the chosen pretreatment of the actual material. A few types of electroless nickel plating may be considered as amorphous metals (metal glasses) as the plating is microcrystalline. Applications of electroless nickel There is a great potential for the use of electroless nickel coatings within the electronic industry and the apparatus industry, machine industry etc. Especially electroless nickel plating has become a great success on various tools and tool parts to minimize wear and corrosion and has to a great part exchanged hard chromium. Hydraulic pistons can be plated with electroless nickel, and if maximum wear resistance and corrosion protection is required an additional hard chromium coating can be applied. Electroless nickel plating of aluminum and brass is a new potential application if corrosion protection and increased wear resistance is required on complex geometries. An electroless nickel plating can be used if soldering is necessary on aluminum or magnesium surfaces. Within the off shore sector electroless nickel has been important for many years for corrosion protection of oil valves and other components in the marine environment

3 How to choose electroless nickel Before electroless nickel is chosen it is important to become aware which material characteristics the new surface is going to posses. When an extended corrosion resistant surface is required, a coating having an amorphous structure should be chosen (metal glass), in which the phosphorous contents typically is % P. The coating typically has compressive stresses in the surface reducing the risk of crack formation. It is important that the pretreatment processes are sufficient and that the surface roughness is not higher than 1µm (R a -value) or contains pores and particles. When a high wear resistance is required, a coating having slightly lower P-contents can be chosen a crystalline coating that afterwards can be hardened at o C for 1-3 hours. The coating thereby obtains a hardness of about 1000 HV. During the hardening process nickel phosphides may precipitate a dispersion hardening similar to age hardening of aluminum. This process besides increased hardness also results in a small volume change (contraction), which may be a course to crack formations in the surface. In this case the corrosion resistance will be reduced considerably. It seems to be a problem to obtain both advantages (corrosion resistance and wear resistance) with electroless nickel at the same time. However it is possible to apply a thin pre-coating of ductile metal prior to the electroless nickel coating. This solution has shown good results in certain applications. However it ought to be tested and evaluated prior to any application. Materials suitable for electroless nickel plating Many different materials may be coated with electroless nickel. However, mostly steel, alloyed steel, brass, copper, aluminum and magnesium are plated with electroless nickel, even certain ceramics and plastics can be coated too. The treatment of the latter mentioned materials is very special and is carried out only for very special products. By the plating of steel, especially tool steel, it is very important to know the actual steel type and the hardening parameters. There is a risk that especially hard steel in connection with plating can - 2 -

4 become brittle because of hydrogen diffusion into the steel matrix. This problem can be avoided by making a so-called hydrogen de-embrittlement (baking) immediately after the plating operations. Hydrogen embrittlement is a phenomenon occurring when atomic hydrogen (formed during the electroless nickel precipitation) penetrates into the base material and forms molecular hydrogen. Thereby a very high pressure (higher than the tensile strength in the steel) is created, and cracks may form. The problem is avoided making a suitable heat treatment immediately after the plating. What is the corrosion resistance of electroless nickel? Defining the corrosion resistance of electroless nickel it must be specified whether it is the coating s own corrosion resistance or it is the corrosion protection of the under laying material. Concerning the corrosion protection ability, this is dependant solely on the coating s pore density, which again is dependant on the chosen process and the quality of the base material, the pretreatment and maybe a treatment afterwards. The corrosion resistance of the coating itself is on the contrary dependant on the chosen process alone and the quality assurance in this connection. Choosing the right process, the corrosion resistance measured in a standard salt spray test can be as much as 1000 hours for a nickel layer of 20 µm. If, on the contrary, neither the pretreatment nor the chosen process are wrong the salt spray test may result in a corrosion resistance of only 20 hours for the same layer thickness. It is therefore recommended that any user of electroless nickel is aware of these conditions before any decisions are taken. The corrosion resistance of the electroless nickel layer, having an amorphous structure, is excellent and may in several situations be better than chromium alloyed steel. The cause of this is that amorphous coatings have better corrosion resistance than corresponding polycrystalline materials, which have a large number of grains and phase borders. In the following table approximate values for the corrosion rates measured in µm per year are given

5 Approximate values for the corrosion resistance of electroless nickel coatings in µm per year Distilled water 95 o C / O 2 saturated 0 Sea water 3.5% sodium chloride / 95 o C 0 Nitric acid (1%) 25 Hydrochloric acid (2%) 27 Sulfuric acid (65%) 9 Phosphoric acid (85%) 3 Acetic acid (99%) 0.8 Citric acid (saturated) 7 Ferric chloride (1%) 200 Sodium hydroxide (45%) 0 Sodium hydroxide (50% / 95 o C) 0.2 Ammonia (25%) 16 Ammonium sulphate (saturated) 3 Ammonium nitrate (20%) 15 Sodium carbonate (saturated) 1-4 -

6 Quality assurance on electroless nickel coatings Quality assurance is important when the coating is used in equipment requiring a reliable corrosion protection and it is important that the most correct and informative tests are used to ensure the quality. We can offer the elaboration of a complete quality control system that suits the requirements to a uniform quality. Figure 1: Hardening of electroless nickel depending on temperature and time. Hardness is given in HV - 5 -