Protection Of Fasteners is the process of destroying metals during their chemical, electrochemical, or biochemical interaction with the environment.
The corrosion process is accompanied by oxidation of the metal and its transformation into various chemical compounds corrosion protection of fasteners (oxides, hydroxides, carbonates, etc.).
Ferrous metals - carbon steel, cast irons - are most intensively corroded. In contrast, many non-ferrous metals, alloy, and stainless steels are very stable in atmospheric conditions and aggressive corrosion protection fasteners environments.
Increasing demands on the level of durability and corrosion resistance of machines and building structures lead to the appearance of more advanced methods and technologies for protecting the surface from the damaging effects of the environment and aggressive operating conditions. Almost all machines, mechanisms, and building structures incorporate fasteners with the same requirements for corrosion resistance and durability.
The use of fasteners made of non-ferrous metals, stainless and alloy steels, and alloys fully satisfy these requirements. However, it is not always economically feasible and technically possible to use these materials. In this regard, protective metallic and non-metallic coatings have acquired great importance in manufacturing products and equipment. Such coatings that protect carbon steel products from corrosion reduce the consumption of expensive metals and alloys and reduce the cost of the final product.
Types of anti-Corrosion Protection Of Fasteners
By connecting the protective layer with the base, adhesive and diffusion coatings are distinguished.
The formation of the adhesive coating is carried out due to the particles' mechanical adhesion of the corrosion protection of the fasteners coated layer with a rough surface of the base. These coatings' corrosion resistance is determined mainly by the adhesion strength (adhesion) of the protective layer to the bottom.
When a diffusion coating is formed, the base material and the coating material undergo diffusion and chemical interaction processes. As a result of this process, a complex layer is formed on the product's surface, associated with the diffusion zone base, consisting of atoms of the coating material and the base material.
According to the type of anticorrosion protection, coatings realizing barrier protection and layers learning electrochemical protection are distinguished.
With barrier protection, the coating creates a barrier for the environment to access the base material. Very sensitive to mechanical damage.
When electrochemical protection creates a pair of dissimilar metals with different electrode potentials in a given operating environment, to provide reliable protection against corrosion of parts, a coating is used that will serve together with the product metal as the anode. So, for steel products, anode coatings are zinc, cadmium, aluminum. If moisture penetrates the coating's pores or places damage and the corrosion process begins, the layer will not undergo dissolution.
Corrosion protection of fasteners
Its nuances and features, which we will consider below. Electrochemical series of metal stresses Historically, in world practice, it has turned out that zinc-based coatings are most widely used due to the optimal cost / protective ratio. Zinc has a more negative (0.2-0.3 mV) stationary electrode potential than iron (steel). When exposed to aggressive media (in the form of electrolytes), it slowly dissolves due to electrochemical reactions with the constant renewal of passive protective films, protecting the fastener's primary material.
Thus, the zinc coating plays the role of not only a barrier but also electrochemical protection. Today in the world, there are several technologies for the production and application of zinc coatings.
Today, the most common technology for applying a zinc coating to fasteners is galvanic galvanizing.
Electroplating coatings are applied to a steel product's surface by the deposition of metals during electrolysis of aqueous solutions of the corresponding salts (electrolytes). The process of plating is quite simple. It lies in the fact that the protected products with a prepared surface and a metal plate, which must be applied as a protective coating, are immersed in a solution of this metal's salts.
The coated product serves as one electrode, and the plate of the deposited metal serves as the other; the direct current is passed between them.
In this case, the product to be coated is the cathode, and the deposited metal plate is the anode. During the coating process, the anode dissolves. From the solution, the metal is deposited on the cathode (the protected product), forming a galvanic coating with a thickness of 5 to 25 microns. A soluble anode is sometimes replaced by an insoluble one (for example, lead), in which case it is necessary to maintain a given concentration of electrolyte.
The resulting coating is adhesive. The adhesion of galvanic coatings is provided by molecular forces acting between zinc and base metal molecules. Most often in industry and construction, they use layers of steel products with zinc, cadmium, tin, chromium, nickel, and lead. The figure shows the microstructure of a galvanic zinc coating on a screw with a diameter of 4 mm. You can see the adhesive area (dark strip) between the substrate and the plating and porosity.
After coating for more excellent stability and durability, it is subjected to clarification - activation of the coating surface with nitric acid and passivation (chromatin) - creating an additional passive protective layer (it is also called a conversion coating) on the surface of the protective coating itself. Passivation can be rainbow (yellow), colorless (white or blue), olive and black. For passivation, hexavalent chromium, a carcinogen, and poison is used in most enterprises. The main disadvantages of such coatings are the loss of corrosion resistance when heated above 100 ° C and technology's environmental unacceptability. Since the beginning of 2007, a ban has been issued on using hexavalent chromium in passivation films in the automotive industry.
Electrochemical series of metal stresses
Replacing chromate films with chromite films that do not contain hexavalent chromium solves these problems. Some manufacturers of automotive fasteners already use passivation's with trivalent chrome. In this embodiment, the coatings meet the current standards for corrosion resistance but are estimated by experts as unpromising.
The use of passivating solutions and electrolytes containing acids, cyanides, and other chemically active compounds forces us to organize methods of neutralization and deep cleaning of environmentally hazardous waste in galvanic plants to build expensive treatment facilities, in the end, eliminates the positive qualities of high-performance electroplating processes.
Therefore, such coatings, phosphate, oxide, and others are called conversion coatings. Due to their barrier and, in some cases, electrochemical protective property, they significantly increase the corrosion resistance of galvanic coatings. There is a wide variety of conversion films on zinc coatings: colorless, rainbow, olive, black, which differ not only in appearance but also in corrosion resistance, the composition of working solutions, and treatment regime.
Advantages and disadvantages
Electroplated coatings have several advantages and disadvantages.
- coating of high chemical purity;
- electrochemical or barrier protection;
- good decorative properties;
- the ability to adjust the thickness of the coating with acceptable accuracy; do not require special preparation of threaded parts of products.
- low wear resistance;
- low durability;
- deterioration of the mechanical properties of fasteners made of alloy steels as a result of hydrogenation of the base material;
- lengthy coating process; - high toxicity and complex waste disposal process.
APPLICATION OF MELTED DIP COATING, HOT
The method of coating by immersion in the melt is based on the diffusion interaction of the base metal with the melt of the coating metal, on
Hot-dip galvanizing the boundary between the two phases.
The coating is formed by the liquid phase's contact interaction — the coating metal with the solid phase — the base metal. In this case, at the interface between the two phases, processes of wetting, mutual diffusion, and after removing the coated product from the melt, crystallization occurs. The resulting coatings have a complex structure. The layer adjacent to the steel consists of intermetallic compounds of the coating metal and the base. The surface layer, which determines the coating's durability, is made of the hardened coating metal.
Hot-dip galvanizing technology includes degreasing steel products in alkaline solutions, chemical etching in acid solutions, and fluxing, most often in solutions of zinc and ammonium chlorides, followed by drying, after which the products are placed in a drum and dipped in a bath (usually ceramic) with zinc melt.
The drum rotation provides a flow of zinc relative to the products to fill all pores and microcracks. Then the drum is removed from the bath and untwisted to remove excess zinc by centrifugation. However, excess zinc remains on the internal thread (on the nuts) (Fig. 4), so the internal thread is re-machined. The lack of coating on the inner line does not affect the connection's corrosion resistance if the nut is used with a hot-dip galvanized mating component - a bolt or stud.
Due to the high anodic of zinc concerning iron at temperatures up to 70 ° C, the nut thread sections' electrochemical protection with a damaged coating layer is carried out. In this case, zinc from the bolt's external thread, due to the potential difference between zinc and iron in a naturally moist and acidic environment, is transferred to the sections of the nut's internal line, which remained uncoated when the cable was machined.
Advantages and disadvantages
- corrosion resistance is 5-7 times higher than the resistance of the galvanic coating;
- electrochemical and barrier protection;
- poor decorative properties;
- special preparation of threaded parts of products is required;
- large, in comparison with galvanic coating, the coating thickness of 40-60
- low wear resistance;
- possible deterioration of the mechanical properties of fasteners made of alloy steels due to exposure to high temperatures;
- lengthy coating process;
- high toxicity and complex waste disposal process.
Zinc-lamellar coatings are a type of non-electrolytic coatings using finely dispersed zinc and electrically conductive.
Binder. The name of this coating comes from its filler - dispersed zinc, which is presented in the form of small flakes (lamellas) with a thickness of several tenths of a micron, with a width (length) of 20 ÷ 30 microns. One of the most famous in the world and at the same time presented on the market is the German technology for applying zinc dispersed anti-corrosion systems.
The protective system of zinc-lamellar coatings consists of a base layer and an additional insulating layer, which, along with the corrosion protection of metal products, provides different properties - friction, elastoplastic, thermal, chemically resistant, mechanical, decorative, etc.
The base layer is an electrically conductive matrix of inorganic origin (varnish) with zinc flakes located parallel to it and passivated without using Cr (VI) chromium. The base layer, due to the electrical conductivity and the presence of dispersed zinc, is an anode for the protected surface of the steel part since the electrode potential of zinc is more enormous in magnitude more electronegative than the electrode potential of iron (steel), the cathodic protection process is implemented.
Thus, with external mechanical damage to the coating, zinc particles are first corroded. And only after complete corrosion of zinc does the steel component begin to erode. The thickness of the base layer may be in the range of 5 ÷ 10.
An additional protective layer is a solvent-based organic varnish (highly network polymer) applied to the base layer. This is an additional anti-corrosion protection of the cathode type with electrical insulating properties. This extra layer protects the base layer from the occurrence of “white corrosion” - zinc corrosion, isolating from environmental influences. An additional layer also allows you to paint the parts' surface in a wide range of colors. It can be a carrier of integrated grease, which will enable you to adjust the friction coefficient in threaded joints. The thickness of the finish layer can be in the range of 3 ÷ 5 microns.
Application technology coating technology includes these key steps:
Preparatory - stage-by-stage washing in aqueous detergents solutions, followed by shot peening to activate locksmith the surface layer of the products.
Coating - cyclic immersion of corrosion resistance classifications in a solution followed by centrifugation, thereby removing excess solution on the products.
Heat treatment - preheating to a temperature of 60–80 ° C. Exposure of coated fasteners in a continuous furnace at a temperature of 180–250 ° C.
Locksmith area covered protection of fasteners:
Cooling - forced cooling and unloading
The processes can be repeated several times until the desired coating thickness is obtained.
Advantages and disadvantages
- electrochemical or barrier protection;
- extremely small thickness, usually 4 - 12 microns;
- excludes temperature effects on the material of products, the maximum temperature during application 250 ° C;
- eliminates hydrogen disturbance (hydrogen saturation) of the surface layer of products from high-strength steels;
- Protective Coating
- the attractive appearance of products, there is the possibility of integrating color into the finish layer;
- friction coefficients for threaded parts are set by the requirements of customers; also, they satisfy other properties of bolted joints
- Do not use heavy metals hazardous to health, such as chromium (VI);
- have high resistance to chemicals and organic solvents;
- maintains from 6 to 10 cycles of twisting of a threaded connection on extreme moments;
- maximum constant application temperature up to 200 ˚C;
- suitable for flexible and resilient elements such as spring washers, Grover washers, coil and cup springs, etc. due to elasticity;
- allows you to adjust the coefficient of friction in the threaded connection within µ = 0.09-0.18 due to the solid lubricant integrated into the base or additional layers.
This advantage allows you to have, along with highly effective anti-corrosion protection, a stable coefficient of twisting of fasteners, which is very important for the high-quality installation of critical metal structures and other joints. In our opinion, this coating has no drawbacks.
In our opinion, this coating has no drawbacks.
By evaluating the zinc-lamellar coating's Protection of fasteners resistance using the neutral salt spray method in extraordinary chambers (CCT) per ASTM (American Society for Testing Materials) B117 / DIN 59021 and analyzing the values according to DIN 50961, comparative data were obtained.