“Batch hot -dip galvanizing has been the most commonly used method of protecting steel products from corros ion. The reasons are simple– hot-dip galvanizing (HDG) process has been around for over 200 years, it is well known and understood, it can be well controlled,and it is relatively cheap. What is most important for many industries, however, is excellent corrosion protection that HDG coating provides for steel structures exposed to harsh environments . Bridge, highway, electrical utility, industrial and marine construction projects have all realized the benefits of hot dip galvanizing. This type of coating is relatively maintenance-free and can commonly prevent corrosion of steel structures for 25 to 75 years in most atmospheric environments (industrial, urban, marine, and rural) with millions of successful projects as a proof of this claim. ng process consists of a number of steps that involve cleaning of the steel parts that produce schemically clean metallic surface and remove s surface oxides and then immersing them into a molten zinc bath. During this stage of the process ,zinc metallurgically bonds to the steel, creating a series of highly abrasion -resistant zinc -iron intermetallic layers, commonly topped by a layer of impact -resistant pure zin c. After the parts are withdrawn from the galvanizing bath, excess zinc is removed by draining, vibrating or – for small items – centrifuging. The galvanized item s are then air -cooled or quenched in liquid. Hot -dip galvanized coatings are known to provide two types of corrosion protection for steel: barrier and galvanic (cathodic ) . First , almost like paint, protects steel by means of a semi -impermeable barrier to environment elements that cause corrosion. In addition, presence of zinc in HDG coatings activates cathodic shielding mechanism. Zinc is more electro -negative (i.e. more reactive) than steel, thus when the two are in contact in presence of electrolyte (moisture) , it stops normal corrosion of steel by donating its electrons to prevent steel from losing its electrons. In other words, zinc “sacrifices” itself in order to protect steel from corroding. However, because the rate of corrosion of zinc is at least 10 times slower than that of steel, a thin coating of zinc can protect steel for a long time”
“Although, as mentioned above, hot-dipped galvanized coating provides excellent core protection to steel structures, it becomes less impressive when used on fasteners. The main drawback is the thickness of the HDG coating. Typical coating thickness on bolts can range from 45 to 90 μ m (1.8 to 3.5 mils ), which can make standard bolt a nd nut tolerances difficult to maintain for correct assembly. If bolts are galvanized, then the nuts should be over-sized to accommodate the 90 to 180 μ m (3.6 to 7.0 mils ) increase in bolt diameter after galvanizing. If this is not done, then assembly of the fastener system will either become impossible or will result in zinc coating being scraped off the thread surface. Either scenario is unacceptable. In addition, depending on thread pitch of a fastener, HDG process can often result in non -uniform coating thickness on the threads with thicker coat in the “valleys” and thinner coat in the “peaks” of the thread. This can also result in the coating being removed during fastener assembly. ”