The behavior of steel during hot-dip galvanizing

Many years a large number of tests and experiments were conducted to establish  does hot-dip galvanizing influence (and to what degree?) on the mechanical properties of steels with low carbon, and steel with limited content of alloying elements.
The following results refer to steel galvanized at normal temperatures (450-460 C) and steel galvanized at elevated temperatures (560 C).

Stretch and impact resistance
The resistance of steel stretching, strength and elongation is presumably stay unchanged after the hot-dip galvanizing as fro parts containing welds and without them.
A high strength of steel (by cool and heat treatment) may decrease during the hot dip galvanizing. The value of decrease depends on treatment’s degree and type of exposure which was subjected to steel processing. The resistance to impact of steels after hot-dip galvanizing partially reduced in as compared to resistance of artificially aged samples. However, it is not enough to prevent that use the steel was designed for.

Plasticity
The degree of plasticity is not associated with a hot-dip galvanizing, but the excessive deformation (e.g., bending to sharp edges) can cause cracks in the zinc coating.

Joints’ exertion

A hot-dip galvanizing causes a great rigidity in welded structures it reduces the effect of stress on the joints by 50/60%.
As a consequence, the whole structure becomes more robust. For example, the hot-dip galvanizing treatment that is widespread in the automobile industry.

Fatigue resistance

The fatigue resistance of the hot dip galvanizing affects differently on different types of steels.
Boiling steel and steel finished by aluminum show a modest decrease in fatigue resistance. And the steel finished by silicon this fatigue can be significantly reduced.
The reason lies in the different composition of the iron-zinc layer.

In the presence of mechanical stress there can be cracks in the layer. They can cause fractures in the steel surface. However during the fatigue data computation in laboratory experiments the hot-dip galvanized material is compared with the “new” (not treated steel). Although you shouldn’t forget that any "untreated" placed outside structure will be immediately attacked by corrosion.
In this case, the deep point corrosion that appears 5-7 times more and fatigue resistance decreases rapidly.
The same can be said about the painted surfaces.
In the place where the damage in the layer of paint the rust begins to attack in the damaged area. On the contrary, fatigue resistance of steel after HDG significantly doesn’t change during the exposure. It happens because the zinc coating remains on the metal surface and with a normal condition there is no corrosion the zinc coating.
Decrease in the resistance to steels’ fatigue after hot dip galvanizing is limited (compare the same phenomenon caused by corrosion). You should also remember that the abrasive machining and welding affect on the fatigue resistance, but galvanizing has a positive effect for both of them.
Steel is sensitive to aging and during the intensive treatment in the cold it can embrittle with time (aging embrittlement). This aging process is accelerated by the temperature during hot dip galvanizing. From that side, a cold treatment can make certain steels too brittle and unsuitable for the desired application and the same in the simple natural aging.
Therefore, when the material is need to expose  to hot-dip galvanizing it is very important to know whether different parts exposed to cold treatment  or not.
Boiling steel is particularly susceptible to embrittlement.  But this effect is less pronounced for steels deoxidized with silicon.
Steel deoxidized with aluminum is also subjected to the negative effects of cold treatment and therefore to the process of hot dip galvanizing.
If the cold treatment of steel can not be avoided, the steel should be free from internal strain. For this you should sustain the steel about 30 minutes at a temperature of 600-650 C and expose it to normalization before hot-dip galvanized.
However, today steel with a tendency to aging is quite rare.

Increase of fragility by hydrogen

In normal mild steel (not alloyed or low alloyed) there are no detected increase of fragility caused by hydrogen absorption.
Hydrogen is absorbed possibly during the etching can be effectively removed by immersion in a zinc bath with a high temperature (about 450-460 C).

Some hardened steel or steel with a high strength can become brittle due to hydrogen diffusion in steel. It would be better to expose these materials to the test before a galvanizing of large batch.

Intercrystalline fracture

In some cases - where the steel has a strong tension - there sometimes can be detected an intercrystalline cracking caused by zinc penetration into the steel in the gap between the grains (LME or liquid metal embrittlement).
The risk of intercrystalline fracture and fracture due to zinc penetration  is small when consider a normal hot-dip galvanizing of mild steel for profiles, while the materials which had a thermal shock (e.g., due to the hot bending or with welding) may be found susceptible to this.
It is also appropriate to test the last when it comes to galvanizee large parties.
The risk of failure can be minimized if the steel annealed at temperatures above 600 C.