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What’s the Difference between Hot-dip and Cold Galvanizing?
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What’s the Difference between Hot-dip and Cold Galvanizing?

Views: 152     Author: Site Editor     Publish Time: 2018-08-13      Origin: www.fuchun-casting.com

The destructive effects of corrosion on metals have been known for centuries. For almost as long as metals have been a construction material, humans have constantly been seeking ways to improve its longevity in corrosive environments. One of the most widely used techniques for protecting metals (mainly steel) is galvanizing.


There are two main methods of galvanizing : hot-dip galvanizing and cold-dip galvanizing.

Galvanizing, also known as hot-dip galvanized, it is the ingot melted at high temperatures, a number of supplementary material in place, then dipped galvanized metal structure slot, the metal component on a layer of zinc coating.The advantages of hot-dip galvanizing corrosion of his ability, adhesion and hardness of zinc coating is better.

“Cold plated” or “plating”, namely, the zinc salt solution by electrolysis, to the plating on the coating, generally do not have heating, small amount of zinc, the wet environment is very easy to fall off.

metal surface treatment types  

The difference between hot-dip galvanizing and cold-dip galvanizing:

  • In hot-dip galvanizing, the raw material surface is a layer of intermetallic compounds, followed by zinc. However, the cold-dip galvanizing surface is zinc, there is no intermediate layer.

  • The cold-dip galvanizing layer is thinner and the hot-dip galvanized layer thicker.

  • Cold-dip galvanizing can not be produced in large quantities, and the output is low. Hot-dip galvanizing can be produced in large quantities with high output.

  • Surface state and corrosion resistance of cold-dip galvanizing are better than that of hot dipping galvanizing.

  • The anti-corrosion principle of cold galvanizing is the same as that of hot galvanizing. It's just that the two processes are different. 

  • After cold galvanizing, the surface is smooth. The appearance is bright. But because the zinc layer is adhering, it will fall off after a long time.

  • Hot-dip galvanizing------ part is less beautiful than cold galvanizing. However, the zinc layer has penetration, and the service time is longer than that of cold galvanizing.

  • Chemical industry is generally galvanized more, suitable for small parts; hot-dip galvanized sheet is generally used for power equipment and components, suitable for large parts and equipment.



  • What is 'multiple certification'?

    This is where a batch of steel meets more than one specification or grade. It is a way of allowing melting shops to produce stainless steel more efficiently by restricting the number of different types of steel. The chemical composition and mechanical properties of the steel can meet more than one grade within the same standard or across a number of standards. This also allows stockholders to minimise stock levels.

    For example, it is common for 1.4401 and 1.4404 (316 and 316L) to be dual certified - that is the carbon content is less than 0.030%. Steel certified to both European and US standards is also common.

  • What surface finishes are available on stainless steels?

    There are many different types of surface finish on stainless steel. Some of these originate from the mill but many are applied later during processing, for example polished, brushed, blasted, etched and coloured finishes.

    The importance of surface finish in determining the corrosion resistance of the stainless steel surface cannot be overemphasised. A rough surface finish can effectively lower the corrosion resistance to that of a lower grade of stainless steel.

  • Can I use stainless steel at high temperatures?

    Various types of stainless steel are used across the whole temperature range from ambient to 1100 deg C. The choice of grade depends on several factors:

    1. Maximum temperature of operation
    2. Time at temperature, cyclic nature of process
    3. Type of atmosphere, oxidising , reducing, sulphidising, carburising.
    4. Strength requirement

    In the European standards, a distinction is made between stainless steels and heat-resisting steels. However, this distinction is often blurred and it is useful to consider them as one range of steels.

    Increasing amounts of Chromium and silicon impart greater oxidation resistance. Increasing amounts of Nickel impart greater carburisation resistance.

  • Can I use stainless steel at low temperatures?

    Austenitic stainless steels are extensively used for service down to as low as liquid helium temperature (-269 deg C). This is largely due to the lack of a clearly defined transition from ductile to brittle fracture in impact toughness testing.

    Toughness is measured by impacting a small sample with a swinging hammer. The distance which the hammer swings after impact is a measure of the toughness. The shorter the distance, the tougher the steel as the energy of the hammer is absorbed by the sample. Toughness is measured in Joules (J). Minimum values of toughness are specified for different applications. A value of 40 J is regarded as reasonable for most service conditions.

    Steels with ferritic or martensitic structures show a sudden change from ductile (safe) to brittle (unsafe) fracture over a small temperature difference. Even the best of these steels show this behaviour at temperatures higher than -100 deg C and in many cases only just below zero.

    In contrast austenitic steels only show a gradual fall in the impact toughness value and are still well above 100 J at -196 deg C.

    Another factor in affecting the choice of steel at low temperature is the ability to resist transformation from austenite to martensite. 

  • Is stainless steel non-magnetic?

    It is commonly stated that “stainless steel is non-magnetic”. This is not strictly true and the real situation is rather more complicated. The degree of magnetic response or magnetic permeability is derived from the microstructure of the steel. A totally non-magnetic material has a relative magnetic permeability of 1. Austenitic structures are totally non-magnetic and so a 100% austenitic stainless steel would have a permeability of 1. In practice this is not achieved. There is always a small amount of ferrite and/or martensite in the steel and so permeability values are always above 1. Typical values for standard austenitic stainless steels can be in the order of 1.05 – 1.1. 

    It is possible for the magnetic permeability of austenitic steels to be changed during processing. For example, cold work and welding are liable to increase the amount of martensite and ferrite respectively in the steel. A familiar example is in a stainless steel sink where the flat drainer has little magnetic response whereas the pressed bowl has a higher response due to the formation of martensite particularly in the corners.

    In practical terms, austenitic stainless steels are used for “non-magnetic” applications, for example magnetic resonance imaging (MRI). In these cases, it is often necessary to agree a maximum magnetic permeability between customer and supplier. It can be as low as 1.004.

    Martensitic, ferritic, duplex and precipitation hardening steels are magnetic.

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