The generation and prevention of Hydrogen embrittlement in Surface treatment

Hydrogen embrittlement is a brittle phenomenon caused by metal or alloy absorption of Hydrogen and stress. Hydrogen embrittlement mainly occurs in the high strength low alloy steel, stainless steel and elastic parts caused by Hydrogen embrittlement is the main reason of the Surface treatment process of Hydrogen into metals, the lattice arrangement chaos, produce distortion, cause internal stress increase, make the metal or coated produces brittleness, cause fracture of fabrication, the intensity is reduced or cracking and peeling of coating.<br>
1. Causes Hydrogen embrittlement<br>
(1) the influence of the substrate material is not all of the metal Surface treatment process produces Hydrogen embrittlement and failure reports from all kinds of statistical results and research data, and a lot of production practice, the Hydrogen embrittlement mainly produce high strength low alloy steel in black metal (050 mpa tensile strength of not less than 1), stainless steel and elastic part, the reason many scholars have different opinions about at present.<br>
(2) there are a variety of methods for oil removal, except oil, but there are only two kinds of Hydrogen embrittlement, namely, cathode discharge and acidic solution. Cathode to remove oil, send out most of the Hydrogen atoms in the form of Hydrogen gas into the air, few Hydrogen will penetrate into the metal, make the metal produces Hydrogen embrittlement, and the heavier oil, to produce Hydrogen, the higher the risk of embrittlement. The reason is: on the one hand, oil pollution is heavy, except oil time is long, precipitate Hydrogen more, the possibility that produces Hydrogen embrittlement is big; On the other hand, heavy oil can make it difficult to escape the hydrogen produced on the surface of the workpiece, resulting in an increased chance of hydrogen percolation. Acid deoil has the function of oil removal and acid washing. The acid - washed Hydrogen seeps into the metal and causes the workpiece to produce Hydrogen embrittlement.<br>
(3) the influence of the pickling process in the process of pickling, with metal dissolution, the nascent hydrogen on material and the surface concentration, the infiltration of intergranular metal inside, especially on iron and nickel metal, the diffusion trend is bigger.<br>
(4) or chemical electroplating process, the effect of electroplating process, as the cathode of workpieces in addition to occur outside the reduction reaction of metal ions, also can produce the reduction reaction of H +, the higher the cathode overpotential, the more prone to the reduction of H +, hydrogen atoms would enter the coating or iron substrate. The hydrogen absorption of different metals is different, and The hydrogen absorption degree of chromium coating is greater, accounting for 0.45% of The quality of coating. The second is iron metal, accounting for 0.1% of The quality of coating.<br>
2. Preventive measures<br>
(1) the appropriate heating and mechanical processing For easy to produce Hydrogen embrittlement of the material, the Surface treatment before heated, shot peening or vibration, such as processing, to escape the Hydrogen on the workpiece, stress is released in full, reducing the generation of Hydrogen embrittlement phenomenon. Heating is Plating pretreatment of the most commonly used in addition to hydrogen. The heating temperature and dehydrogenation time are also different. Generally speaking, the higher the temperature, the longer the time, the better the hydrogen. But the temperature is too high, which is bad for hydrogen. Therefore, the temperature and time of hydrogen removal can be selected according to the tensile strength of the material. The process parameters are shown in table 1, and Rm in the table is tensile strength.<br>
(2) using the appropriate oil removal process for black metal, especially high strength low alloy steel, elastic, and long-term mechanical parts, high overload, high tensile strength and martensitic stainless steel parts by cathode shall be strictly forbidden to remove oil, such as anode electric lift oil available, such as ultrasonic degreasing method.<br>
(3) USES the high strength alloy steel pickling process reasonable, flexible and long-term stress scale removal of the parts, it is forbidden to use strong acid, can use sandblasting, shotblast, liquid sand blasting or mechanical means such as finishing. In particular, it cannot be washed with sulfuric acid. Because of the viscosity of sulphuric acid and slow pickling rate, the hydrogen produced is difficult to get rid of the workpiece quickly, and the use of high temperature is responsible for the increase of hydrogen absorption. When acid washing must be carried out, dilute hydrochloric acid is applied, and corrosion inhibitor and surfactant are added to reduce the reaction of acid and matrix metal to reduce the generation of hydrogen. On the other hand, the hydrogen generated is easy to get rid of the metal surface and reduce the chance to infiltrate the matrix.<br>
(4) select the appropriate Surface treatmentprocess, such as high strength steel and elastic and long-term stress parts, as far as possible, with the application of coating or chemical treatmentprocess to avoid hydrogen production. The parts that must be electroplated can choose the electroplating layer with less hydrogen absorption. It is found that in the anode coating with steel components, cadmium is more likely to absorb hydrogen than zinc. In the polar coating, nickel is more likely to absorb hydrogen than tin. In addition, still should choose the process with high current efficiency, such as galvanizedprocess, cyanide galvanized! For 60% ~ 85%, ammonium salt galvanized! For 94%~ 98%, galvanized zinc acid! Is 70% ~ 85%. In accordance with ISO2081, steel parts for tensile strength Rm greater than 1 240MPa are not allowed to be galvanized and cadmium plated. It is found that high temperature phosphating has a great influence on the hydrogen embrittlement of high strength, high stress or spring, and the probability of brittle fracture increases with the increase of phosphating time and temperature.<br>
(5) the Surface treatment shall be treated with hydrogen removal: the steel parts of Rm over 1 050MPa must be treated with hydrogen removal within 4h after plating, according to the tensile strength of the material and the type of plating coating, the suitable dehydrogenation process is selected.<br>
Conclusion 3.<br>
In the process of Surface treatment, hydrogen embrittlement is inevitable. The occurrence of hydrogen embrittlement can lead to the distortion of the inner lattice of the metal, resulting in the brittleness of the metal and coating, which can lead to the decrease of the strength, fracture and even failure of the parts. The stress of the material is eliminated by heating, blasting or vibration. Using appropriate oil removal, pickling and Surface treatmentprocess to minimize the generation of hydrogen in the process, the hydrogen absorption may be reduced to the minimum; After treatment, hydrogen removal is performed on the parts, so that hydrogen can escape from the metal. The above measures can effectively prevent hydrogen embrittlement.