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Abstract

Content

Introduction

Hot-dip galvanizing is one of the main ways of corrosion protection of steel structures. Normal service life of galvanized products is 50–70. Zinc coatings are widely used in applications requiring reliability and durability of steel structures.

By hot dip galvanization is meant applying a zinc coating on the surface of metal products, especially steel, ductile and gray cast iron, by immersing the product in molten zinc.

Like any surface treatment, hot dip galvanizing requires proper surface preparation to during the dive details in the galvanizing bath of iron and zinc reaction was homogeneous. Preparations include acid degreasing, pickling, rinsing, fluxing, drying. It was after such a thorough preparation of the surface adhesion (adhesive strength) of the zinc coating to the base metal is very high, and therefore, galvanized metal will be protected for years from the effects of corrosion.

1. Topic relevance

Hot dip galvanizing process, besides the fact that protects against corrosion, improve some of its characteristics; among them, the most obvious: aesthetic appearance and greater resistance to wear and tear, which increases its endurance limit in relation to chemical and mechanical stress. Zinc coating thickness of 100 Life m is an average of 25–50  years, without the cost of funds for maintenance of examination and protective properties of the coating.

However, when coating low quality correction is necessary to cover the defect, sometimes with the complete removal of the latter, with a re-training and re-galvanizing surface, resulting in cost overruns of zinc and other materials, drastically reduces performance and efficiency [1].

This master's work is devoted to an actual scientific problem of increasing the adhesive strength of protective coatings of steel, applied by hot-dip galvanizing. This will improve the corrosion resistance of steel structures and extend their service life.

2. Goals and tasks of the research

The aim is to identify the problems encountered in the implementation of hot-dip galvanizing process

To achieve this goal the following main objectives:

  1. Analysis and Research of species Break.
  2. Examine the process of protective coating by hot dip galvanizing.
  3. Select the factors affecting the adhesion of protective coatings.
  4. Identify the factors that lead to the violation of the integrity of the zinc coating.

The object of this study is low-alloy structural steel, is hot-dip galvanized.

The work will be recommendations for protective coatings of steel structures with high adhesion properties by hot dip galvanizing.

3. Analysis of the influence of the silicon content on the quality of coatings

Hot-dip galvanizing — the oldest and simplest method of applying zinc on the surface of the product. When hot-dip galvanizing protected metal is immersed in a bath of molten zinc whose temperature is about 450 oC. Hot-dip galvanizing is used for coating mainly on large products of simple shapes. To cover thread, narrow holes and other complex elements of hot-dip galvanizing is rarely used. Hot dip galvanized coating applied to the sheets of corrugated iron, pipes, machine parts, beams, angles, wire.

Hot-dip galvanizing method appeared and was first used in France, (1742), chemist Paul Jacques Maluen dipped in molten zinc steel products. As a result, they formed a layer of zinc. In the same year Maluen made a report in the French Royal Academy, which described in detail the process for producing the zinc coating on the steel parts.

Patented method was French chemist Sorel S. (1836). He suggested previously cleaned surface of the steel product in the first 9 % solution of sulfuric acid (H2SO4), and then a fluxing ammonium chloride (NH4Cl). Only after these operations, to use hot-dip galvanizing. In 1847, the first hot-dip galvanizing plant opened in Solingen (Germany). Four years later, he began operating one plant in Austria (at the enterprise Winiwarter & gersheim ).

In the second half of the XX century method of hot-dip galvanizing was recognized as an international standard in the field of corrosion protection.

Numerous studies have confirmed the high performance properties of hot dip galvanized steel. It is widely used both in the industrial and agricultural sectors. Zinc coatings are used in all operating environments, in addition to the strong alkaline and acidic.

Nowadays in Europe it acts more than a thousand plants in which hot-dip galvanizing method used. And each year about 400 thousand. Tonnes of zinc coated steel is consumed in the hot way.

The term hot-dip galvanizing to understand the application of the zinc coating on the surface of metal products, primarily of steel, ductile and gray cast iron. hot-dip galvanizing method is based on submerged metal structures in a bath of molten zinc [2].

In the Donetsk region has one of the largest enterprises specialized in manufacturing metal structures supports high-voltage lines, the open switching centers of substations, guardrails, barrier-type, etc. — PJSC Donetsk Power Transmission Tower Plant, which is carried out applying a protective coating by hot dip galvanizing.

4. The technology of anticorrosive coating

The technology of anticorrosive coating consists of several stages. After washing, the product is etched in an aqueous hydrochloric acid solution with an initial concentration of 150 g dm 3, to remove rust and scale. Further processing is carried out at a temperature of 60 oС flux concentrated solution(400–600 g/dm 3), for the final disposal of the remaining oxides. In the basic operation of hot-dip galvanizing steel product is completely immersed in a bath of molten zinc. The bath contains at least 98 % pure zinc and a temperature of about 449 oС. The molten zinc reacts with the iron in the steel to form a layer of iron-zinc alloy. Coupling with zinc layer surface of the product is achieved by a diffusion process between zinc and iron. When removing the product from the hot-dip galvanizing bath, over the iron-zinc alloy lies a thin layer of molten zinc, which solidifies after cooling products. The dwell time in the product melt should be as small as possible to prevent the growth of brittle iron-zinc alloy layer and increase the thickness of the zinc coating.

Hot zinc coating shall comply with GOST 9.307–89 [3] and technical requirements [4]. Control the appearance of the produce at 100 % products. The surface of the zinc coating must be smooth, the coating must be continuous. Colour coating — from the silver–shiny to matt dark gray. The coating thickness should not be less than 40 microns and not more than 200 microns. The thickness of the zinc coating is measured by a magnetic method, as well as metallographic and gravimetric methods.

To control the strength of the coating adhesion following methods are used:

Hot-dip galvanizing is suitable for the protection of low-carbon steel (carbon content of not more than 0,25 %) with the silicon content of up to 0,37 % and at a relatively low content of alloying elements[2].

In Donetsk Power Transmission Tower Plant working with major brands 09G2S structural steels, S355, C345. These steels are classified as low-alloy construction for welded structures with a carbon content of 0.09–0,2 %, 1,3–1,7 % manganese, 0,5–0,8 % silicon.

When operating with said marks steels noted that galvanizing under the same conditions, the resulting adhesive strength varies. There was a violation of the quality of coatings, they become loose, reduces shine, and in some cases it has been observed peeling.

The most likely cause of this process is the fact that the steel structures containing 0,03–silicon; 0,12 % above and 0,2 % soft layer may be formed instead of the iron-zinc alloy layer is dense. It increases with the duration of the dive. This rapid reaction leads to the fact that the zinc coating is formed thick due to an external zinc layer which is not due to an increase of the alloy layer during the cooling of the product.

During the galvanizing of steel structures with critical silicon content can often see gray coating as steel structures very quickly react with zinc and therefore it is very difficult to avoid the penetration of iron-zinc alloy layer during cooling.

Conclusion

Modern continuous casting technology of steel does not allow to predict with sufficient accuracy the residual content of alloying elements in the final product and its uniform distribution throughout the sample. Although each batch of steel products accompanied by a certificate in which the chemical composition of steel, with galvanizing quality problems can occur. Therefore, in the future it is necessary to carry out determination of the content of the alloying element in a silicon steel samples at an appropriate sampling.

The advantage of the method of hot-dip galvanizing: high corrosion resistance of the coated article; the relative cheapness of hot-dip galvanizing; ease of implementation; ease of maintenance facilities; high performance; the ability of the zinc coating to protect the base metal even after mechanical damage or disrupt continuity of the coating; high conductivity coatings; good thermal conductivity; coating prevents hydrogen embrittlement of the base metal.

Disadvantages of the method of hot-dip galvanizing: the limited dimensions of a product designed for hot-dip galvanizing (depending on bath size); difficulties in welding and further processing of galvanized products; relative uneven surface; the inability to obtain a very thin coating; a relatively high consumption of zinc.

Using the method of galvanizing a coating thickness from a few microns to a millimeter.

In writing this essay master's work is not yet complete. Final completion: May 2017 years. The full text of work and materials on the topic can be obtained from the author after that date.

References

  1. Алексеев А. А. Организация эффективного технологического процесса горячего цинкования на судостроительных предприятиях путем внедрения ресурсосберегающих технологий: дис. на соиск. учен. степ. канд. техн. наук: 05.08.04: защищена 24.05.07 / Алексеев Александр Алексеевич. — Санкт-Петербург, 2007. — 127 с.
  2. Рекомендации пользователям услуг цинкования ООО Агрисовгаз
  3. ГОСТ 9.307–89. Единая система защиты от коррозии и старения. Покрытия цинковые горячие. Общие требования и методы контроля.
  4. Технические требования, предъявляемые к металлоконструкциям, подвергающимся антикоррозийному покрытию методом горячего цинкования.