Mechanical caracteristics of the connecting elements represent an important item when designing metallic constructions of machine parts. Corrosion could significantly effect this caracteristics in a negative way. The lifetime of each connecting element is easily improved by accurate choice of surface protection.
The most suitable processes to protect bolts from corrosion are:
Brushing – proces that performs a ferrous oxiduloxid coating, enabling a surface protection with own oxide against atmospheric affects. Screws are firstly heated and afterwards, cooled down in a special oil emulsion. With this process is a thin oxid surface/coating up to 1 milimikrons (µm) created. In oiled conditions it performs an impermeable surface film.
Galvanic zinc coating - metalic elements' surface protection, when due to electric energy influence zinc cells from the electrolyte are depositated on metallic piesec/objects surface. Depending on process duration the thichness of this coating can be different (usually protection coat thickness is from 3-10 milimicrons /µm). White zinc coating is used mostly. Beter than zinc plateing is thick zinc plateing with additional laquer finish - thickness is between 12 - 15 µm. It's important to emphasize only bolts grade 4.6, 5.8, 6.8 or 8.8 could be protected by this galvanic zinc coating. By galvanisation process hydrogenis absorbed in the base material surface, in case of improved steels over 1.000 N/mm² that could easily cause fragility. Therefore a special hydrogen expulsion process for bolts grade 10.9 and 12.9 is needed (several hours lasting hold of objects/bolts at 200-400°C temperature between zincing phase).
HDG – hot dip galvanisation – is the best protection against corrosion – bolts are dipped into melted zinc at 450-480°C temperature, resulting with a coating thickness of 0,043-0,1 mm. Due to greater coating thickness incisions between coils on screws and nuts decrease, therefore this process is not recommended for coils under M 12 sizes. In all situations all coils on nuts must be cleaned due thickness of HDG with threaded drills.