Cast iron with good abrasion resistance, low cost, simple operation, high efficiency, good damping, low notch sensitivity, and the advantages of easy to machining, in the automotive, aerospace, shipbuilding and other industries widely used.
But easy to corrosion, cast iron surface rust after difficult to handle, decorative, need through the subsequent processing to improve its surface properties.
Plating can effectively improve the appearance and corrosion resistance of cast iron, has become a hot application and research.
The particularity of iron casting
and plating countermeasures compared with base material, such as rolling, forging steel, cast iron pieces of special processing technology, complicated structure, plating is relatively difficult.
According to the particularity of cast iron parts, use of electroplating countermeasures are: (
The high carbon content on the surface of the cast iron, silicon surface residue of sand or carbon compounds, overpotential of hydrogen in carbon is low, easy to precipitate and produce a large number of bubbles [
, in a given flow is difficult to reach the potential of metal deposition, coating is not easy to grow, so the iron nickel alloy galvanized can only choose the acid system with high current efficiency,
Acid zinc nickel alloy current efficiency is about 90%, alkaline zinc nickel alloy in only about 50%)
General is more complex, the structure of cast iron and thus to adopt decentralized strong coverage and good plating solution.
Cast iron base material structure is loose and porous, there are a lot of sand holes and shrinkage cavity, rough surface, uneven, so the micro electroplating surface area than the macro area is much bigger, lead to the actual current density is much smaller than the apparent current density, so not only should use acidic plating system, initial current should be 1 ~ 2 times larger than that of other steel substrate.
Instant activation by pulsed high current shock surface, can make loose surface is covered by dense coating [
On the surface of the cast iron in the process of high temperature casting a non-conductive layer of oxide film, hinder the growth of zinc nickel alloy coating.
So before plating must be thoroughly remove oxide on the surface of the skin.
Cast iron pieces in general machining process for the: casting ─ ─ blasting machine.
Blasting is an important process step of iron plating, the main use of sandblasting machinery hit the surface, not only can remove scale, and makes forms when casting high carbon layer falls off, cause micro roughness, is beneficial to improve the bonding strength, so the blasting quality directly affect the quality of cast iron plating.
In machining process, on the surface of the cast iron will be a large amount of residual oil, lubricating oil and coolant, as well as from the remains of the mold, sand blasting residual sand and iron filings in the store are influenced by the outside world will be generated in the process of oxidation film and other insoluble contaminants, and itself is the presence of large amounts of graphite cast iron surface, so the iron needed by special pickling and ultrasonic cleaning pretreatment process, to form the wet, good conductivity, easy to clean, uniform plating surface.
Cast iron plating process is cast iron parts adopt more acid zinc plating process and equipment is relatively simple, but the zinc coating anticorrosion ability is poor, cannot satisfy the requirement of the automobile industry such as high corrosion resistance of zinc nickel alloy coating while can greatly improve the corrosion resistance, but due to the acidic nickel alloy galvanized process is more complex, the process control is difficult, has struggled to large-scale popularization and application.
In recent years, with the rapid development of chemical and mechanical manufacturing process, cast iron acid electric zinc nickel alloy technology to enhance the performance, production process control is increasingly mature, become a hot research topic in recent years, and gradually large-scale promotion.
Iron nickel alloy galvanized mainly has two ways: acidic zinc + alkaline nickel alloy, galvanized or acidic nickel alloy galvanized directly.
The former in order to solve the problem of casting can not directly alkaline plating, using acidic zinc base, but in the case of alternating between double coating binding force is not good, easy to fall off, and the technological process is more complex, thickness, nickel and other quality control risk.
The characteristics of the different plating processes are shown in table 1.
Pickling nickel alloy galvanized process 3.
1 hot dip process ultrasonic cleaning wash ─ ─ except oil wash acid wash ─ ─ ─ ─ ─ ultrasonic cleaning washing ─ ─ electric lift oil washing ─ 40% (
Hydrochloric acid activation wash ─ ─ ─ electroplating bath ─ 2% (
Nitrate out light close wash passivation ─ ─ ─ ─ ─ water washing flow flat ─ and dry.
2 treatment before plating.
1 ultrasonic cleaning iron loose structure, high porosity, surface residual dust, sand and other insoluble contaminants through general cleaning method is difficult to eradicate.
Ultrasonic cleaning USES powerful shock waves can penetrate into the surface pore and make a thorough cleaning on the surface of the cast iron clean [
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