Although chromium is more negative than iron in the electrochemical sequence, it has strong passivation properties. A passivation film is easily formed on the surface of the chromium layer in the atmosphere. The surface potential of the passivation film immediately after passivation becomes positive, and the electrode behavior is equivalent to that of silver. Therefore, the chrome plating on the iron is an anodized coating. As with other cathodic coatings, the substrate can be protected only if the coating is dense and free of voids. The fatal weakness of chromium plating is the presence of pores and cracks, and the brittleness of the chromium layer is relatively large. When the local area is compressed or impacted, the coating is prone to cracking. In this case, the steel matrix encounters moisture in wet weather and passes through the pores of the chromium layer, seeping into the surface of the matrix metal, so that the chromium and ferroelectric couples form a micro battery. At this time, the iron in the chromium and ferroelectric couple is actually The potential is lower than that of chromium, which acts as the anode corrosion, and the corrosion product is rust, and overflows through the pores of the chromium to form a flood point. As the time continues, the flooded point expands and increases, and the result spreads over the entire surface of the workpiece.
This phenomenon is more common in the edges of sheet metal parts and stamping parts. Because these parts are sheared, the structure is loose and there is no oxide scale. Excessive corrosion is easily caused when the acid is etched, and the cover of the flash edge and the chromium layer are relatively thin. The porosity is relatively high, and reddish-brown rust is more likely to appear.
In order to reduce the occurrence of the above phenomena, the following measures can be taken:
(1) Improve the surface roughness of the substrate;
(2) Strengthen the plating process;
(3) increase the thickness of chrome plating under possible conditions;
(4) Single layer chromium is not used except as necessary;
(5) Appropriately increase the concentration of the solution as allowed by the process conditions;
(6) After plating, wash it with hot water and quickly dry;
(7) Take oil seal anti-corrosion