“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zinc -iron intermetallic layers, commonly topped by a layer of impact -resistant pure zin c. After the parts are withdrawn from the galvanizing bath, excess zinc is removed by draining, vibrating or – for small items – centrifuging.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. ”

In Addition, When It Comes To Hot – Dip Galvanizing Fasteners, There Are A Few More Drawbacks :

High variability in the relationship between torque and induced tension . Because of this, torque cannot be used as a reliable method for gauging the required minimum bolt tensi on. High friction of galvanized surfaces and inconsistent torque -tension relationship result in high rate of bolt failures in torsion.
“Second drawback concerns high -strength bolts. Since the 1970s, ASTM A490 standard prohibited the application of metallic coatings on high – strength structural bolts. At the heart of the prohibition was the attempt to eliminate the risk of hydrogen embrittlement . This phenomenon may occur when at omic hydrogen is absorbed by high -strength steel during the acid cleaning process that takes place prior to galvanizing . This leads to significant decrease in ductility of the bolts and permit brittle cracks to grow at fairly low stress levels. ”

Mechanical Galvanizing

In an attempt to address aforementioned concerns a process of mechanical galvanizing had been developed. Mechanical galvanizing (MG) is similar to hot -dip galvanizing in that it also applies a coating of zinc on top of bare steel. During MG process fasteners are placed in a large rotary barrel along with zinc powder, special promoters, and glass impact beads. The mechanical energy generated from the barrel’s rotation is transmitted to the glass beads that bombard fastener surface with zinc particles. This causes zinc powder to be mechanically welded to the surface of the fasteners. With a proper glass bead size mix all exposed surfa ces can be coated uniformly , and the buoyancy of the glass beads cushions the fasteners in the rotating barrel to minimize thread nicking

“In contrast to HDG fasteners, mechanically galvanized nuts are tapped oversized prior to coating which results in their threads to be coated during the MG process . However , MG plating consists of round particles of zinc loosely bonded together which is believed to be orders of magnitude weaker than the metallurgical bond found in hot -dip galvanized coating. This , in turn, raises concerns about adhesion strength of MG coating. Also, depending on fastener thread pitch and size of glass beads used in MG process ,the resulting coating thicknes s in the thread area can be non -uniform, with “peaks” and “valleys” having thinner than expected coating. These factors might affect long -term corrosion resistance of fasteners “