1. Causes and Types of Steel Corrosion
Corrosion of steel is strongly influenced by the environment, and corrosion under moist atmospheres and other humid gases is the most common corrosion phenomenon. In addition, since steel is the most commonly used metal in the manufacture of industrial equipment, the corrosive environment of industrial electrolytes and gases is even more hostile. Typically, the form of the corrosion process of metals is not static due to the influence of different factors. External factors include temperature, pressure, pH, medium composition, etc.; internal factors include the chemical composition of the metal material, the structural state of the metal surface, and the crystalline form of the metal. Different influencing factors will lead to different corrosion, according to the corrosion reaction process can be divided into chemical corrosion and electrochemical corrosion. Chemical corrosion is a corrosion phenomenon resulting from the chemical reaction between the metal and the environmental medium. Electrochemical corrosion is the metal in the conductive solution medium electrochemical reaction and the corrosion phenomenon. According to the form of corrosion, can be further divided into uniform total corrosion and uneven total corrosion, as well as pitting corrosion, intergranular corrosion, galvanic coupling corrosion, crevice corrosion and other localized corrosion. The protection of steel can be carried out by applying coatings and plating, applying electrochemical protection, and adding corrosion inhibitors.
2. Electroplating Zn-Ni Alloy
Electroplating corrosion-resistant Zn-based metal plating on the surface of steel parts and passivation treatment is one of the effective means of corrosion prevention. Electroplating Zn-Ni (Ni mass fraction of 13%) alloy coating is a typical representative of protective coatings, with excellent corrosion resistance, its corrosion resistance is 6 times that of galvanized layer, which can be used for high protective parts or cadmium plating, and hydrogen embrittlement is small, formability is good, the plating and the substrate are firmly bonded, and it is an ideal protective coating for steel materials. Currently used Zn-Ni alloy plating solution: sulfate, weak acid chloride and alkaline zincate plating solution. Among them, acidic plating solution plating Zn-Ni alloy cathodic current efficiency is high (≥95%), fast deposition speed, hydrogen embrittlement is small, easy to get more nickel content (11% ~ 15%) of the coating. Alkaline zincate bath has high dispersing ability and is suitable for complex workpiece plating, but the cathodic current efficiency is low (50% ~ 80%), and the nickel content of the resulting plated layer is generally low (6% ~ 9%). Take high nickel content acid chloride plating solution as an example, its plating solution composition and process conditions are.
zinc chloride |
70~ 80 g- L-1 |
Nickel chloride |
100~ 120 g- L-1 |
ammonium chloride |
30~40 g- L-1 |
dicalcium phosphate |
190~220 g- L-1 |
boric acid |
20~30 g- L-1 |
sodium acetate (chemistry) |
20~35 g- L-1 |
Additive 721-3 |
1~2mL- L-1 |
pH |
4-5~ 5 |
temp |
25~ 40 °C |
Cathode current density |
1~ 4A- dm-2 |
anodic |
Zinc and nickel sub-control |
Since there are more elements in plating alloy plating than in single plating, the composition of the plating solution is relatively complex. The composition of the plating solution has a greater influence on the quality of the plated layer, and the process conditions also have a greater influence on the quality of the plated layer. Temperature has a greater effect on the alloy composition, on the one hand, it affects the cathodic polarization, on the other hand, it affects the ion concentration in the cathodic diffusion layer. pH has a greater effect on the nickel content of the plating layer, and the nickel content of the plating layer decreases with the increase of pH value. The current density increases, the cathodic potential decreases, and the content of metals with lower potential in the alloy increases. Alloy plating in the anode in addition to conductive and maintain a uniform distribution of cathodic power lines, but also supplement the metal ions, maintain the stability of the plating solution plays an important role, for the Zn ¡ú Ni alloy plating, zinc, nickel sub-hanging, sub-control of the anode method is now commonly used.
3. Passivation of Zn-Ni Alloys
Zn-Ni alloy coating and galvanized layer, the same as the steel substrate are anodic coating, has good corrosion resistance, after passivation treatment can be further - to improve corrosion resistance. For example, Zn-Ni alloy coating after complexate passivation treatment to get different color passivation film, its corrosion resistance than the zinc coating color passivation film to be more than 5 times higher. However, passivation on Zn-Ni alloy is more difficult than passivation on galvanized layer, especially when the nickel content of the plating layer increases, passivation is more difficult, when the nickel content is more than 16%, it is difficult to passivate the Zn-Ni plating layer. passivation of Zn+Ni alloy plating layer is divided into colored, black and white passivation, etc.. Taking color passivation as an example, the main component is chromic anhydride or chromate, and its components and process conditions are as follows.
chrome anhydride |
3~ 15 g- L-1 |
accelerator |
5~20 g- L-1- |
pH value |
0-8~ 1-8 |
temp |
30~70°C |
Immersion time |
10~50s |
4. Corrosion Inhibitors
Corrosion inhibitor is the appropriate concentration and form of the environment (medium), can prevent or slow down the corrosion of materials, chemicals or complexes, corrosion inhibitors, also known as corrosion inhibitors, is - a commonly used anti-corrosion measures. Usually in the corrosive environment by adding a small amount of corrosion inhibitor (a few percent to a few thousandths), its physicochemical effect on the metal surface, can significantly reduce the corrosion rate of metal materials. More types of corrosion inhibitors, according to the chemical composition can be divided into inorganic corrosion inhibitors and organic, corrosion inhibitors; according to the electrochemical mechanism is divided into cathodic corrosion inhibitors, anodic corrosion inhibitors and hybrid corrosion inhibitors; according to the classification of the metal surface layer structure can be divided into oxidized film-type corrosion inhibitors, precipitation film-type corrosion inhibitors, and adsorption-type corrosion inhibitors. Oxide film-type corrosion inhibitors have a passivation effect, so that the metal surface oxidation occurs to form a dense oxide layer, or repair the original incomplete passivation film, thereby inhibiting the anodic dissolution of the metal. Precipitation film-type corrosion inhibitors and metal surface anodic dissolution of metal ions to generate insoluble compounds, in the form of precipitation covered in the anode surface, or the formation of dense and complete oxide film. Adsorption-type corrosion inhibitors mainly achieve the purpose of corrosion inhibition through two adsorption methods.
①Adsorption occurs on the metal surface, covering part of the metal surface and reducing the area where corrosion occurs.
②Adsorption occurs on the active sites on the metal surface, reducing the reactivity of the active sites and reducing the corrosion rate. Submerged surface currently more mature metal corrosion detection methods are: resistance method, linear polarization method, potentiometric method and ultrasonic thickness measurement method. Due to the uncertainty of the location where local corrosion occurs and the long test cycle, the linear polarization method is often used to test the corrosion rate of the metal in the metal corrosion resistance test, and the Tafel curve is the classic corrosion test method.
