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Porosity and Corrosion Resistance of Electroless Nickel Coating

Porosity of electroless nickel plating 

Electroless nickel coating belongs to cathodic coating, and porosity is a very important quality index, which is not only related to the corrosion resistance of the coating, but also related to its density and toughness. One of the reasons for the good corrosion resistance of electroless nickel plating is its low porosity. The thicker the coating is, the smaller the porosity is, and the porosity of electroplated nickel coating with the same thickness is much lower than that of electroless nickel plating. The porosity of 15 microns thick nickel plating and 12 microns thick copper plating is only equivalent to 7 microns electroless nickel plating. The low porosity of electroless nickel plating is related to its dense amorphous or microcrystalline structure. The general acidic plating bath is basically non-porous when the thickness reaches 15 microns. The most common type of electroless nickel plating is pinhole-shaped porosity. In order to prevent it, it is only necessary to strengthen the flow of solution or the relative movement between workpiece and plating solution except adding surfactant. The round hole on the falling dust surface on the upper part of the plating part is caused by fine suspended particles. Sand hole refers to a deep hole connected to the substrate all the way. The difference between sand hole and air hole lies in its asymmetry and no tailing. The reason is that the surface of the substrate is unqualified (sundries in the plating bath will also cause holes), and it may also be enlarged by bright electroless plating. Other granular tumors, surrounded by sand holes and mistaken for holes, are actually caused by rapid sinking or other solid particles. 

The surface porosity of the coating is related to the following factors: 

Usually, the porosity of alkaline plating bath is larger than that of acidic plating bath, and the coating obtained by using hydrazine as reducing agent is higher than that obtained by using sodium hypophosphite. The type of additive also has an effect, the porosity increases with the rapid plating speed, and the impurities and suspended solids in the plating process not only increase the surface roughness, but also increase the porosity; 

Surface roughness of plated parts the smoother and smoother the surface of plated parts, the smaller the porosity. Sandblasting surface generally needs a coating larger than 30 microns to be non-porous. If the coating with the same thickness is plated twice, the porosity can also be reduced. For example, the porosity of an acidic plating solution with a thickness of 10 microns is 4-5 points/square centimeter at one time, and it is reduced to 1.9-2.5 points/square centimeter at two times; 

Heat treatment is helpful to reduce the porosity of the coating. Electroless nickel coating is a cathodic coating. If there are many holes, it will form a corrosion battery with large cathode and small anode, which will accelerate the corrosion of the substrate. When porosity is used for coating quality inspection, it is only used for relative comparison, because different porosity determination methods have different results. Before the coating is used, it is best to do tests according to the working conditions. The porosity is determined by salt spray test in addition to sticking method. Reliable results can be obtained by sulfur dioxide test with different concentrations. 

Erosion of electroless nickel plating 

Although electroless nickel coating has strong corrosion resistance, in actual production, due to technological reasons, sometimes it can not completely achieve the ideal performance. Therefore, we are more concerned about how to improve or ensure its corrosion resistance. The factors affecting the performance of electroless nickel plating are nothing more than substrate quality, pretreatment, plating bath, process, post-treatment and inspection between various processes. 

Substrate quality is the basic guarantee of coating holding capacity, but it is often ignored by people. Before plating, we must know the composition, state, metallurgical quality and processing process of workpiece materials. Improper processing will cause stress and microcracks. There should be no defects such as folding, welding chips, burrs, holes and sand holes on the workpiece. Inclusions in steel, especially sulfide, not only affect the corrosion resistance of the coating, but also poison the plating bath; 

It is difficult to clean the original lubricating oil or corrosion inhibitor on the substrate before plating. Poor treatment will not only reduce the adhesion of the coating, but also produce pinholes and reduce the corrosion resistance. In addition to chemical cleaning method, electrolytic cleaning method is often recommended for carbon steel substrate, but this cleaning method obviously affects the corrosion resistance of the coating. For example, anode cleaning in alkaline medium will increase the porosity of coating; Although cathode cleaning has low porosity, it may cause hydrogen embrittlement. Using alkaline rust remover, if the direction is changed and cleaned periodically and repeatedly, that is, soaking at 85 ℃ for 10 minutes and then cleaning anode for 2 minutes, the effect is not as good as cleaning cathode after soaking. If the iron matrix is activated with hydrochloric acid, the corrosion rate will increase. If the iron matrix is activated with dilute sulfuric acid, the effect will be better, and the shorter the time, the better. Corrosion inhibitor must not be used in activation solution. Lead in the substrate will make the coating appear micropores, reduce the adhesion and poison the plating solution. This kind of substrate should not be activated with sulfuric acid, and 25% HBF4 should be used, and flash plating can also be done first. 

Post-treatment the post-treatment of electroless nickel plating is nothing more than heating to remove hydrogen, improve hardness, improve wear resistance or adhesion, or further treatment to improve corrosion resistance. The coating to be treated after plating only accounts for a small part of the total coating. Experiments show that the heat treatment method of 200 ℃ for one hour is also beneficial to improve the pitting corrosion resistance of the coating. Hydrogen was removed by heating at low temperature for a short time, and the amorphous structure of the coating continued to be maintained. At the same time, the maximum relaxation occurred, which reduced its volume, increased its density and decreased its porosity. Chromate sealing treatment is the simplest and most effective method to improve the anti-discoloration ability of electroless nickel coating, prolong the salt spray test time and corrosion resistance. 

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