Chemical polishing processing before plating

In electroplating production line, a series of pretreatment before electroplating workpiece are in order to get a better coating, the chemical polishing processing before the whole, also is very important in the following we came to know about the chemical polishing processing before plating.

Chemical polishing is to point to in a suitable solution, do not use external power supply, rely on chemical etching effect for polishing workpiece. In chemical polishing process, the metal microscopic surface formed the uneven passivation membrane, or due to form the similar produced in the process of electrolytic polishing of thick mucous membrane, so that the surface of the dissolution rate of micro bulge significantly greater than the micro recessed parts, thus reducing the surface micro roughness of the parts, the parts surface is bright and smooth. Chemical polishing are widely used in the polishing of stainless steel, copper and aluminum alloy, also used for some parts to do decoration processing. Chemical polishing can be used as processing before plating, also can be supplemented by the necessary protective measures after polishing and use directly.

Compared with electrolytic polishing, chemical polishing, don’t need the power supply and conductive hanging fixture can be of various sizes of parts of complex shape polishing, high production efficiency. The concentration of solution is its weakness, short service life of adjustment and regeneration is difficult, often precipitate some harmful gases. Chemical polishing of the polishing quality is worse than electrolytic polishing, this is mainly because in chemical polishing, uneven due to the quality of the material, can cause the local potential height is differ, produce local polar of Yin and Yang, the formation of local micro battery short circuit, result in partial dissolution of anode. In the electrolytic polishing, due to the action and potential can completely eliminate the local cathodic area, thus thoroughly electrolysis, thus more effective.

(1) the polishing liquid composition and other factors.

(1) of polishing solution: in order to guarantee the effect of chemical polishing, must to dissolve the metal surface, and form the liquid film on the surface or solid membrane. Chemical polishing liquid generally includes the basic component of etchant, oxidant, additives and water. Among them, the etchant is the main component, the main parts in solution dissolve; Antioxidants and additive inhibits corrosion process, the reaction is advantageous to the polishing direction; Water regulating effect to the solution concentration, facilitate the spread of the reaction products.

Used for metal dissolution composition is commonly acid, which is use morer H2S04, HN03, HC1, H3PO. And HF acid and amphoteric metal of aluminum as NaOH are available, and also due to a high concentration of phosphoric acid and sulfuric acid in the acid has higher viscosity, can form liquid membrane diffusion layer, so this kind of composition has two functions. It is mainly used in chemical polishing liquid composition of phosphoric acid and sulfuric acid. In order to improve the viscosity, the diffusion layer is easy to form can also be added into gelatin or glycerine can increase the viscosity of the additives. In order to promote the formation of a solid film, will be expected to join is given priority to with nitric acid and chromic acid strong oxidizer.

(2) the polishing time: chemical polishing is a best polishing time range. If the time is too short, can only get no pear skinning surface luster; If time is too long, not only solution loss increase, and that there will be a stain on the surface of the processing or spots. And this time by material, polishing liquid and polishing temperature range, the influence of such factors as usually hard to predict, can only be determined through experiment. Chemical polishing tend to produce hydrogen gas at the same time, this is when polishing with hydrogen embrittlement sensitivity materials must pay attention to the problem. While polishing liquid temperature as high as 100 ~

200 cc, will also occur annealing effect. In order to keep and hydrogen annealing effects to a minimum, must be in optimal temperature range selection and polishing time as short as possible.

(3) polishing temperature, chemical polishing, the dissolution rate increases significantly with the increase of the polishing liquid temperature. In addition, strong oxidizing acid at high temperature oxidation becomes very significant. In chemical polishing, due to the acid dissolution and oxidation will occur at the same time, therefore, in most cases are the polishing liquid is heated to high temperatures for polishing.

Need to provide the temperature for polishing metal such as steel, nickel, lead, if the temperature is lower than a certain value, will lose the corrosion surface smooth, therefore, there is a form burnish the critical point, above the critical point of temperature range polishing effect is better. And the temperature range in different liquid composition varies, if above this temperature range, can form pitting, local stains or spots, reduce the polishing effect. In addition, the higher the temperature, the material of dissolved loss.

(2) chemical polishing of copper and its alloys. Copper and copper single-phase alloy in sulfuric acid or a nitrate, acetic acid phosphate type a nitric acid, chromium acid solution in chemical polishing

In use process need to often added nitric acid, precipitation is less, if the nitrogen dioxide when polishing parts surface is dark yellow, can according to the configuration of a third added nitric acid. To prevent excessive water into the slot, parts should be dry before polishing.

(3) aluminium and aluminium alloy chemical polishing. In order to obtain and shining surface, must be strictly controlled polishing liquid nitrate content. Nitrate content is too low, the polishing speed slowly and after polishing surface gloss is poor; Excessive amounts of nitrate, prone to squamous corrosion. Phosphoric acid concentration is low, cannot obtain the shining surface, in order to prevent diluted solution, the parts surface should be dry before polishing. Acetic acid and sulphuric acid can inhibit the pitting corrosion, making the polished surface evenly, careful. Ammonium sulfate and urea can reduce nitrogen oxide precipitate, and help to improve the polishing quality. A small amount of copper ions can prevent corrosion, so as to improve the uniformity of the polished surface, but the content is too high will reduce the polishing of the surface of the reflective ability. Chromium anhydride can improve the polishing quality of aluminum zinc copper alloy, zinc, copper wire with high strength aluminum alloy in polishing liquid does not contain chromic anhydride, difficult to get the surface of the light.

After chemical polishing of parts, generally should be at room temperature in the middle of 400-500 g/L of nitric acid or in 100 ~ 200 g/L of chromic anhydride in the solution, dipping a few seconds to tens of seconds, removed from the surface in contact with copper.