Methods of Zinc Coating Application on Fasteners

 

 

 

1. Hot Galvanizing of Fasteners

This is the highest quality method ofzinc coating application on steel. A coating is applied by brief dipping of pre-degreased, etched or mechanically treated ferrous fasteners into a bath with molten zinc (~450-520°С). Before dipping into the molten zinc, fasteners pass through a flux layer on the bath surface consisting of a mixture of zinc chloride and ammonium chloride. Fluxing may alsobe  performed in a separate bath by dipping into a concentrated solution of zinc chloride with subsequent drying.

In terms of anticorrosion properties, hot dip galvanized fasteners are surpassed only by stainless steel fasteners. The advantage of this coating is its ability to make a double anticorrosion barrier – directly as a covering and due to cathodic reduction of steel even in the instance of zinc layer damage.

During the assembly of metal structures, the weakest part is undoubtedly their butt joints made with application of fasteners. The main loads exposed to temperature changes and external exposure bear precisely on joints. Therefore, the very strict requirements to this type of fasteners are completely justified. Nowadays fasteners with no coating are still widely used for these purposes.

Recently, designers of facilities applying metal structures have begun to recommend the usage of high-strength fasteners galvanized by the thermal-diffusion method. This is undoubtedly better than uncoated fasteners; however, due to the imperfection of the technology its quality does not completely satisfy some customer requirements. Moreover, fasteners with thermal-diffusion coating cannot be used without the additional varnish-and-paint surface coating since the surface does not have a homogeneous zinc oxide film providing the main corrosion protection, as is the case with hot dip galvanized fasteners. But the main thing is that this type of coating is exposed to intergranular corrosion which leads to destruction of the product under certain conditions.

All the problems mentioned above are completely eliminated when hot dipped galvanized fasteners are used. Firstly, hot dip galvanized fasteners do not need any additional treatment. The durability of these fasteners without additional protection (painting, etc.) is 50-120 years, depending on the environment. 

The coating produced has a non-homogeneous composition. At the zinc-steel border the coating is represented by a layer of intermetallic compounds of zinc and iron (FeZn7 and FeZn3). The upper layer of the coating consists of pure zinc.

The thickness and quality of the produced coating depends on the melt temperature, the duration of dipping, the rate of extraction from the bath and subsequent operations for elimination of excessive molten zinc.

The coating layer applied on fasteners is usually 20-70 ηm. Application of a thicker coating leads to a change in physical and technical properties, primarily in joints (bolt-nut), such as tightening coefficient, rupture of the bolt-nut couple, etc.

Continuous hot galvanizing is widely used in the manufacture of sheet products, pipes and wires on high-speed automated lines. Development of the hot galvanizing technique and technology has allowed manufacture of thin-sheet cold-rolled galvanized steel for the automotive industry. In this process, excessive zinc is blown away from the sheet surface by air-operated knives, and a light gauge coating is produced (8-10 ηm), which simplifies subsequent forming, welding and painting of autoparts.

In modern cars, 60-90% of  car body panels are manufactured mainly from hot dip galvanized steel.

2. Metal Spraying

Metal coating is performed by spraying of molten metal on a coated surface from special gas-flame or electric arc guns. Zinc in the form of wires is put into a spraying gun, and melted and pulverized on products. Molten zinc drops harden on the surface in the form of numerous small flakes, forming the coating.

The coating structure has the form of separate plate layers. One of the important conditions providing rigid adherence of the coating and the base is sufficient unevenness of the coated surface, which is achieved by sand blasting or etching.

As compared to hot galvanizing, metal spraying does not require application of energy-consuming and cumbersome equipment, e.g. baths. Zinc sputtering may be applied not only in workshop conditions but also in field conditions during almost all seasons.

This method is applied for protection of large metal structures and for local metal coating; in this instance it is possible to provide selective adjustment of the coated zinc amount and to apply thicker coatings of ~250 ηm and over. The drawbacks of the method include large (up to 35%) losses of zinc in the course of spraying.

Subsequent impregnation of the layer with different protective compounds or application of organic priming and paintwork is used for thickening of metallized zinc coatings and to increase their protective properties.

3. Thermal-Diffusion Galvanizing

This method (the process was previously called “sherardization”) consists in zinc saturation of an iron surface and is accomplished under increased temperatures in a zinc-containing mixture of powders. Coatings may be applied on low- and high-carbon steels as well as on cast iron.

Once heated, zinc diffuses into iron with formation of intermetallic Zn-Fe compounds of different composition in the surface layer; these compounds form the basis of the thermal-diffusion coating.

The process is accomplished in slowly rotating, closed steel drums loaded with several hundred kilograms of fasteners at 300~450°С. Thermochemical processes executed for 2-4 hours result in formation of a rather even coating on small parts.

The chemical composition of steel does not have a noticeable influence on the thickness and structure of the coatings produced, and the limiting step of galvanizing is achieved through input of the powder mixture onto the surface of the fasteners. The thickness of the produced coating is adjusted through the composition and volume of the zinc mixture supplied to the drum, and the temperature and duration of the process. The thermal-diffusion method is used for application of coatings with a thickness of 10-150 ηm.

Finishingsurface treatment is applied for improvement of appearance and corrosion resistance of the coating, either by means of phosphatizing with thickening impregnation or by painting.

Thermal-diffusion galvanizing (with additional surface treatment) is mainly applied in the construction industry as an alternative to hot galvanizing for long-term corrosion protection of fasteners.

4. Zinc-Rich Coatings

Zinc-rich coatings include coatings based on non-organic or organic binding materials with a high content of finely dispersed zinc powder. Due to the high content of zinc powder in dry film (usually not less than 80%), zinc-rich coatings exercise some anodic properties in relation to steel. Moreover, zinc-rich coatings also feature the barrier mechanism of protection which is typical for paint-and-lacquer coatings.

Ethyl silicate compositions are widely used as non-organic binding materials. Organic binding materials include resins used in traditional paintwork materials – urethane, epoxide, acrylic or organic-silicon. Thus, zinc-rich coatings embody the advantages of zinc metal and paint-and-lacquer coatings. Coating thicknesses usually amount to tens of microns. The high resistance properties allow the use of zinc-rich coatings in cases when it is economically unprofitable to apply zinc coatings using traditional methods. Water storage tanks, metal structures and oil & gas equipment used under arduous conditions may serve as examples of steel structures protected from corrosion by means of these coatings. Zinc-rich coatings represent a worthy alternative to hot or thermal-diffusion galvanizing.

Further development of zinc-rich coatings is associated with the so-called “zinc-lamellar coatings” with additional layers which do not contain hexavalent chrome. The system of lamellar zinc coating includes a basic layer consisting of thin aluminum and zinc flakes (lamellas) and, if necessary, one or several additional layers, giving special properties to the coating: friction properties, corrosion and chemical resistance, color, etc.

The zinc-lamellar coating is applied on the pre-treated surface of parts upon their dipping into a finely-dispersed suspension of zinc and aluminum powders (having the form of flakes) in the binding material, or upon suspension spraying on parts with their subsequent heating up to 240°Сfor drying and hardening. The formed basic coating contains over 70% of zinc powder and up to 10% of aluminum powder and also a binding organic material. It consists of numerous layers of aluminum and zinc particles with a thickness less than a micrometer and width of about 10 ηm, which are situated parallel to each other and to the coated surface and are connected by the binding material. The small size of particles makes it possible to apply the zinc-lamellar coatings with a thickness of 4-8 ηm used in the automotive industry. Thicker coatings are used for application on parts and elements of building structures.

The coating has electrically conductive properties, and its more electronegative potential as compared with steel creates electrochemical protection in addition to the barrier protection.

Application of zinc-rich coatings does not lead to hydrogen embrittlement of steels with coatings.

 

5. Mechanical Galvanizing

Mechanical galvanizing is referred to as a “currentless” method for application of metal coatings and is used to provide reliable corrosion protection of fasteners and it is required to prevent hydrogenation which usually accompanies electrochemical galvanizing.

Mechanically applied zinc coatings are currently used in industry and are included into specifications of automotive companies.

The galvanizing principle consists in the mechanical interaction of the coated surface, finely-dispersed (2-5 ηm) suspended zinc particles and glass balls in an aqueous environment. The process is accomplished in drums or bells where coated parts, glass balls and an acidic aqueous chemical solution are subsequently loaded. Zinc powder is also added. While the drum is rotating, micron zinc particles are pressed to the metal base of the part by the glass balls. The points of their contact with the base feature high contact pressure and formation of adhesive bonds.

Organic compounds contained in aqueous solutions which may cause the formation of thin adsorption films on the surface of coated metalshave the determining role in application of mechanical coatings. These substances include amines, amides, condensation products with ethylene oxides, quaternary aliphatic ammonium salts, simple and compound aromatic ethers, alcohols, aldehydes and some others.

For improvement of zinc coating adhesion with the base, a thin intermediate layer (less than 1 ηm) of more noble metals (copper and tin) is preliminarily mechanically applied to the product.

Zinc powder is partially dissolved in the acidic solution, and hydrogen is extracted on the surface of particles, which is then removed from the solution in the form of gas. Inhibitors are specially injected into the solution to restrain active interaction of the zinc and acidic solution and to reduce hydrogen extraction, and the under-layer of copper prevents atomic hydrogen diffusion into the steel base. Thus, due to mechanical galvanizing, hydrogenation of the base does not occur, there is no hydrogen embrittlement of high-strength and hardened steels and the dehydrogenation operation is not needed.

Galvanizing is accomplished in automated lines or in single-position bell units operated manually. The produced coatings may be chromatized (passivated), and in terms of corrosion resistance in salt spray they are not surpassed by traditional galvanic coatings.

Mechanically applied zinc coatings with the thickness of 7-12 ηm are used in different branches of the machine-building industry for corrosion protection of parts made of high-strength and hardened and also low-carbon steels. For application in construction, coating thickness may be 25 ηm and over.

6. Electrogalvanizing

Electrogalvanizing is a widely applied method nowadays and is used almost in all industrial branches for corrosion protection of different metal products, such as bolts, nuts, washers, and all kinds of fastening and constructional elements. By means of electro galvanizing, zinc is also applied to cold-rolled thin-sheet steels.

This is the most practical and cost effective method of galvanizing, allowing adjustment of thickness and the properties of the settled zinc layer across a wide range.

Electrolytic zinc coatings are usually not applied without finishing treatment. Finishing treatment means creation of conversion films on the zinc surface – chromate, phosphate and their varieties, and also additional impregnation of conversion films with thickening compounds and/or application of organic polymer films on the conversion films.

Electrolytic Zn-Ni, Zn-Co, Zn-Fe zinc alloys and others with a subsequent finishing treatment have a higher corrosion resistance as compared to coatings based on pure zinc.

The options of finishing treatment for coatings made of zinc and its alloys are becoming more and more diverse, and application of electrogalvanizing is constantly extended.

The thickness of zinc coatings is regulated depending on purpose, conditions and durability, and varies from 3 to 40 ηm. For instance, for low-duty operating conditions coating thickness is 6-9 ηm, for mild and severe operating conditions - 15-21 ηm, for especially severe conditions - 24-40 ηm. Coating thickness may be reduced if additional protection is used and applied above conversion films.

For instance, in the automotive industry the minimum thickness of zinc coating is usually set in the range 6-15 ηm, in some special cases it may be increased up to 20-25 ηm. It is important to note that with the same coating thickness its actual corrosion resistance may differ by several times depending on the coating composition, type of conversion film and additional protection.

None of the above-mentioned galvanizing methods – melt dipping, diffusion, mechanical, electrolytic or application of zinc-rich compounds – is universal. To some extent, they all have advantages and disadvantages and solve different technical tasks related to corrosion protection of products and provision of their surface with the required functional properties.