Shankar Sengupta | Head – Energy Engineering Group, Adani Corporate House, Ahmedabad
- Preface
Steel is the most common material used in almost all products i.e., from households to Industrial application. Significant part of the economy is steel sector based; however, steel has one large disadvantage – its high corrosion rate. Hence protection of steel structures and components is of great economic importance. In the Indian subcontinent, the rate of corrosion is quite high considering hot and humid environmental conditions.
Zinc is mostly used as an anti-corrosion agent and being used from very early ages i.e., from 200BC to obtain brass which is an alloy of copper and zinc. It has an inherent natural capacity to protect steel against corrosion.
In 1742, a chemist known as Melouin found that a zinc coating could be applied to iron by dipping it into molten zinc which indeed laid the foundation for galvanizing. In 1780, an Italian physicist, Luigi Galvani, observed that the contact between two dissimilar metals resulted in the flow of an electrical current. He worked in the field of galvanization and in the name of this physicist, the process is named as Galvanization. Subsequently in 1836, a French civil engineer, inventor cum chemist, named as Stanislas Sorel patented a method of galvanization by cleaning steel and then coating it with zinc by dipping it into molten zinc. This was the beginning of modern hot-dip galvanizing.
cross-section of the galvanized steel coating, showing a typical microstructure comprised of three alloy layers and a layer of pure metallic zinc
- Benefits of Galvanization
Hot-dip galvanizing has been in use over 100 years to protect steel corrosion worldwide; however, over the decades, many new material and process have been evolved. Comparing all the use of hot-dipped galvanization remained most trusted based on their superb performance and till date experience as users.
In the HDG process, the base material is getting three level protection.
i. Zinc Layer Barrier Protection which protects steel by insulating direct contact with air and moisture.
ii. Cathodic Protection where in zinc acts as anode and sacrifices itself protecting the steel from corrosion
iii. Zinc Patine where in zinc oxidizes and builds a protective layer over steel
Formation of Zinc patina – which itself acts as a protective layer for the active zinc metal
- Reasons for opting Hot Dipped Galvanization over Pre-Galvanization from Developers perspective
i. Galvanization is the process of applying a protective zinc coating to steel or iron to prevent corrosion. There may be various other ways of corrosion protection; however galvanized coating is one of the most durable forms of corrosion protection. In an ideal condition i.e., without any operation loads on the steel structure i.e., if the galvanized steel is kept, with average coating
ii. Thickness of 85 microns, it will protect the base material for almost 100 years. It means, the galvanization is very durable.
Hot dipped galvanization is a proven and well-established method. HDG members comes with guarantees of service life; however, Pre galvanized steel suppliers do not provide performance guarantees beyond 5 to 10 years. Scientific literature is present in case of HDG stating the year-on-year degradation of the coating in various corrosive environments, whereas no such data is available for Pre-Gal material and the life of the asset is at risk beyond 10 years.
iii. Hot Dipped Galvanization is almost maintenance free. Once it is done, there is no requirement of reapplication or recoating. Hot dipped Galvanization is very much sustainable.
In case of pre galvanized material, after some service life the pre-galvanized material needs replacement and / or maintenance.
iv. Indian solar developers are trying to increase the solar project life to 35-40 years instead of the present standard of 25 years, and as most of the upcoming solar projects in India will be at C3-C4 corrosion zones, it is important to design the BOP systems for 40-50 years as well. Galvanization is techno – economically, cost effective solution when compared with grit blasted equivalent painting systems. PU or alike painting systems must be applied in multiple layers over grit blasted steel surfaces. Usually the painting scheme shall have 275 to 325micron thickness as per manufacturer prescription.
However, this cost effectiveness is achievable in HDG only. Corresponding thickness of galvanization compared to 325micron painting is 60 to 80 microns. These 60 to 80 microns of galvanization thickness provide a minimum 25 to 30 years of life expectancy under worst condition like corrosion conditions greater than C4.
Pre galvanized material does not provide the requisite thickness of galvanization and service life guarantee comparable with HDG and painting systems.
v. Hot Dipped Galvanizing ensures complete coverage over steel surface, irrespective of it’s shape, geometry. As a process point of view, in case of hot dipped galvanization, once the structural member is completely fabricated including welding, punching, drilling, cutting etc. all operation, it is immersed into molten zinc and kept for requisite time duration. This ensures complete zinc coverage all over steel members.
However, in case of pre- galvanized steel members, this is not possible. The steel members which are fabricated using pre- galvanized are highly susceptible to corrosion. In the manufacturing process of the pre-galvanized sheet (or similar products), a bare steel plate of thickness is passed galvanization process. As a result, zinc coating is applied (of 20/30micron) only on the exposed surfaces. While fabricating the structural member to requisite shape and size it is subjected to welding, punching, drilling, cutting i.e., all activities involved in fabrication work exposes and damages the existing pre- galvanized layer., so corrosion starts and life of the steel structure member comprises.
vi. Hot Dipped Galvanizations provide superior abrasion resistance. In hot dip galvanization process, a zinc coating which is formed is firmly bonded metallurgically to the steel surface. Due to its unique metallurgical bond, a galvanized coating is incredibly tough, offering exceptional performance for all kind of abrasion and shocks.
An initial outer layer of galvanization provides reliability by acting as a buffer zone, helping to absorb any type of initial shock, impacts, abrasions etc. to the galvanized coating and metal surface. Also, the underlying zinc-iron alloy is harder than steel itself and will further reduce any potential penetration of the coating or the exposure of bare steel. This means that a galvanized coating is highly shock, wear, and tear resistant which is particularly suited to areas of high frequency industrial application. A Hot Dipped Galvanization coating can also help prevent damage during construction, transport, erection, and other mechanical activities.
This type of protection is not possible in pre-galvanized steel as the galvanized thickness is itself very thin compared to HDG.
vii. Hot Dipped Galvanized is a fast activity. Once the steel members are fabricated, it simply requires just a few minutes of immersion time into molten zinc tank post cleaning of the members. The molten zinc reaches all exposed surfaces and does provide complete protection.
However, such all protection in such a short time is not possible in case of pre galvanized material, as the punched, drilled, cut surfaces remain unprotected or it needs special treatment in special case requirements.
viii. Typically, in Pre-Galvanized material, the claimed 80-micron thickness of galvanization represents 40micron on inner face + 40 micron on outer face of steel; however, in Hot Dipped Galvanization 80 microns it is a coating thickness on each side.
ix. In Renewable Power sector, all the structures which are comprising of thin structural members and cost is worked out on basis of weight of the structural members; in such cases owner or project developer leverage benefit of steel weight from Hot Dipped Galvanized members, this tonnage benefit is not available in pre-gal structures. In Hot Dipped Galvanized process, the bond between steel and protective zinc coating is metallurgical bond. This bond is very strong, in some cases, the strength of this interface layer i.e. zinc + steel is stronger than bare steel, hence it is possible to optimize the base material thickness.
However, this advantage of weight reduction is not possible in pre galvanized or PosMAC like materials.
- Points to Ponder
i. As India is accelerating towards its goal of 300 GW solar capacity by 2030 from 58 GW at present, do we need to change our engineering designs for making a long-term & sustainable solution or should we continue focusing on the designs developed by Western countries, for totally different environmental conditions.
ii. Which all components should be innovated to increase the plant life to 50 years & beyond.
