1. Passivation

  Passivation is to use a certain solution to treat the surface of stainless steel to form a corrosion-resistant passivation film on the surface of the stainless steel, and to dissolve surface iron and other metal impurities to improve corrosion resistance. After pickling, stainless steel should be passivated according to the following process 3~61.

(1) Nitric acid 300~500g/L, 30~60min at room temperature.

(2) Nitric acid 300~500g/L, sodium dichromate 20~30gL at room temperature for 30~60min.

In order to avoid the residual passivation liquid on the stainless steel surface causing future corrosion, it should be neutralized in 30~50g/L sodium carbonate solution at room temperature for 1 minute after passivation.


2. Polishing: There are three commonly used polishing types: mechanical polishing, chemical polishing, and electrochemical polishing.

2.1 Mechanical polishing refers to polishing and batch polishing using polishing wheels, polishing belts, etc. Batch polishing uses the abrasives in the polishing agent to polish the surface of the parts to smooth the surface and achieve a polishing effect. After polishing, a mirror-like finish with a surface roughness of less than 0.4m can be obtained. Parts with simple shapes can be polished with a hard polishing wheel or polishing belt, and parts with complex shapes can be polished with a soft polishing wheel. Use the batch finishing method for small parts in large batches. Batch polishing methods include roller polishing, vibration machine vibration polishing, centrifuge centrifugal polishing and rotating polishing. Mechanical polishing requires very little surface grinding. Difficult to polish rough surfaces. At this time, it is necessary to grind in advance with a polishing wheel and a polishing paste. Coarse, medium and fine grinding. The surface roughness after fine grinding can reach 0.4m. In order to meet some other requirements, such as scale removal, burr removal, welding slag removal, matting, etc., surface treatments such as sandblasting, shot peening, and wire wheel brushing are sometimes performed using stainless steel wire. The wheel polished surface can better avoid iron contamination.

2.2 Chemical polishing is to immerse the parts in an appropriate solution, because the solution dissolves the convex parts of the surface faster than the concave parts, thereby smoothing the surface and achieving the purpose of polishing. Generally speaking, chemical polishing has poor polishing ability and can only slightly improve the brightness. But it is more labor-intensive and time-saving than machine polishing, and it can polish the inner surface of small parts. Recently, it has also been reported that the surface of l88 austenitic stainless steel can be polished to mirror brightness by adding brightener. The chemical polishing process is shown in Table 2, but the following points should be noted.

(1) Formula 2 has fast polishing speed and low smoothness, so it is suitable for pre-polishing.

(2) The active surface produced after chemical polishing must be passivated to ensure corrosion resistance.

(3) For large quantities of small parts such as vascular stents and screws, mechanical stirring should be used to polish them evenly.

(4) When polishing large-area surfaces of products such as stainless steel composite plates, special attention should be paid to keeping the polished surface moist. And it should be fully washed after polishing to prevent uneven surface brightness.

2.3 Electrochemical polishing can improve the reflective properties of parts; improve corrosion resistance, reduce the surface hardness of machined parts, and reduce the friction coefficient by reducing surface roughness. Electrochemical polishing can also be used to remove burrs from parts. Compared with mechanical polishing, electrochemical polishing has the following characteristics:

(1) Mechanical polishing will produce a surface hardened layer and abrasive inclusions that reduce the corrosion resistance of stainless steel, while electrochemical polishing will produce a passivated surface and increase the corrosion resistance of stainless steel.

(2) Electrochemical polishing has certain requirements for the base material. For example, when the metallographic structure is uneven, uneven polishing will occur and deep scratches on the surface cannot be polished flat. Mechanical polishing, on the other hand, has much lower requirements on the substrate.