Chromium coatings duplex

Standards have been established for the application of chromium and nickel coatings for various atmospheres. These coatings consist of a thin topcoat of chromium and combinations of microdiscontinuous chromium and duplex or triplex nickel coatings. These will provide excellent corrosion... 

However, some efforts [27] have been made to develop noncoated duplex-phase stainless steel with high chromium content, such as 2205 steel, for application as metal plate materials and getting rid of the expensive coating. The passive layer in the steel was formed quickly in the fuel cell working environment, but no details of in sifu fesfing were reported. In addition, higher electrical interface contacf resisfance (ICR) of the passive layer in the duplex-phase stainless steel is an issue for fhe application. 

A well-designed duplex coating is an effective and safe barrier interposed between the construction materials and the environment, and it is effective for a time roughly proportional to the weight of the anodic layer. Its use is not limited to the bright chromium finishes, but it can be extremely appropriate if the corrosion products of the base material are undesirable and if, furthermore, the contact between the material of construction and the environment must be avoided. 

To solve this problem, a duplex nickel coating was developed as shown in Figure 16.4. An initial layer of sulfur-free nickel is applied to the steel substrate, followed by an inner layer of a bright nickel containing sulfur, with an outer layer of microcracked chromium. 

Duplex coatings are superior to bright nickel coatings in many atmospheres. Since the chromium deposit is the topmost layer in a multilayer coating, the durability of the chromium deposit controls the corrosion behavior of the duplex nickel during the initial stage of exposure. Duplex nickel coatings provide excellent corrosion resistance in natural atmospheres. 

Consider nickel coating on a steel substrate. A layer of bright nickel is laid on the dull layer of nickel. Over the bright nickel a layer of chromium is laid. The bright nickel (high sulfur content) is more noble than the steel substrate. Such a coating system is called duplex coating. 

---