Hydrogen embrittlement and plating

1 basic definition of Hydrogen embrittlement
Hydrogen embrittlement (HE), also known as Hydrogen induced cracking or Hydrogen damage, is a phenomenon of material plasticity, cracking or damage caused by Hydrogen in metal materials. Hydrogen embrittlement is a Hydrogen dissolved in steel, which is polymerized into Hydrogen molecules, resulting in stress concentration, exceeding the strength limit of steel and forming tiny cracks inside the steel.
Hydrogen embrittlement can only be guarded against. When Hydrogen embrittlement is created, it can't be eliminated. In the smelting process of materials and parts manufacturing and assembly process (such as electroplating, welding) into the inside of the steel in the trace hydrogen (10 to the negative power 6 magnitude) in internal residual or additional stress under the action of material embrittlement cracking even. In the case of not cracking can be dealt with through dehydrogenation (such as heated to 200 ℃ or more hours, can reduce the hydrogen) resume the performance of the steel. Therefore, internal Hydrogen embrittlement is reversible.
2 Hydrogen embrittlement phenomenon
Hydrogen embrittlement usually shows delayed fracture under stress. There have been galvanized parts such as car springs, washers, bolts and spring springs, and broken in succession within hours after assembly. The fracture ratio reaches 40-50%. In the process of the use of cadmium in a special product, there have been batch cracks in the process, and the national offensive has been organized and strict dehydrogenation process has been established. In addition, there are some Hydrogen embrittlement is not characterized by delayed fracture phenomenon, for example: electroplating hung with (steel wire, copper wire) due to the many electroplating and pickling Retreat plating, serious seepage Hydrogen, often occurs in the use of ten percent occurred the phenomenon of brittle fracture; The core of the shotguns is broken after multiple Chrome plated. Some quenching parts (internal stresses) are cracked when the acid is washed. These parts are highly hydrogenated, and there is no need for extra stress to crack, and hydrogen is never used to restore the original toughness.
The mechanism of Hydrogen embrittlement
The delay fracture occurs because the hydrogen in the part is concentrated in the concentrated part of the stress concentration, and the metal defects in the stress concentration area are many (the atoms are misaligned, holes, etc.). Hydrogen diffusion to these defects, hydrogen atoms into molecular hydrogen, produce a great pressure, the pressure and the residual stress inside the material and material by the applied stress, to form a resultant force, when the resultant force exceeds the yield strength of material, would lead to break in. Since Hydrogen embrittlement is related to the diffusion of Hydrogen atoms, diffusion takes time, and the velocity of diffusion is related to the concentration gradient, temperature and material type. Therefore, Hydrogen embrittlement usually manifests as delayed fracture.
Hydrogen atoms have the smallest atomic radius and are easily diffused in metals such as steel and copper, and the diffusion of hydrogen in cadmium, tin, zinc and their alloys is difficult. Cadmium plating is the most difficult to diffuse. The hydrogen produced during cadmium plating is very difficult to diffuse and hydrogen is difficult to disperse. After a period of time, the hydrogen diffuses into the metal, especially the hydrogen inside the metal's internal defects, which is difficult to diffuse. At normal temperatures, hydrogen is spreading slowly, so it needs to be heated immediately to hydrogen. Temperature, increase the solubility of hydrogen in steel, high temperature will reduce the hardness of the material, so the hydrogen stress before and after the plating to the temperature of the choice, must consider not to reduce the material hardness, shall not be in a certain steel brittleness tempering temperature, does not destroy the performance of the coating itself.
4 reduce and eliminate hydrogen embrittlement measures
4.1 reducing the amount of hydrogen percolating in the metal must be minimized by the acid washing of high-strength/high hardness steel fasteners, as the acid washing can increase hydrogen embrittlement. When rust removal and oxide skin, try to adopt the method of sand blasting, if rockwell hardness is equal to or greater than the HRC 32 fasteners for pickling, must ensure parts when making pickling process in the acid soaking time the longest do not exceed 10 minutes. The acid concentration should be reduced as far as possible, and the time for the parts to be soaked in acid is not more than 10 minutes. When oil removal, use cleaner or chemical oil removal methods, such as solvent degreasing seepage hydrogen quantity is less, if using electrochemical oil removal, anode and cathode before high strength parts are not allowed to use the cathode electric lift oil; During heat treatment, the drip volume of methanol and propane is strictly controlled. In electroplating, alkaline bath or high current efficiency has less hydrogen plating.
4.2 coating with low hydrogen diffusion and low hydrogen solubility. It is generally believed that in Cr, zinc plating, Cd, Ni, Sn, Pb, seeping into the steel easy remnant of hydrogen, and Cu, Mo, Al, Ag, Au, W and other metal coating with low diffusible hydrogen and the hydrogen solubility, less hydrogen permeability. Could satisfy the requirement of product technical conditions in the circumstances, can be used not cause hydrogen permeability of coating, such as mechanical galvanized or chromium-free zinc aluminum coating, not happen hydrogen embrittlement, Corrosion resistant high, good adhesion, than electroplating and environmental protection.
4.3 go to the stress and plating to remove hydrogen embrittlement. If the parts after hardening, welding process, such as the internal residual stress is larger, should carry on the tempering treatment before plating, tempering eliminating stress can actually reduce the amount of parts within the trap, so as to reduce the occurrence of hydrogen embrittlement.
4.4 control coating thickness, because the coating is covered in the fastener surface, the coating will act as a hydrogen diffusion barrier to some extent, which will hinder the diffusion of hydrogen to the external fastener. When the coating thickness exceeds 2.5 mu m, it is very difficult to diffuse the hydrogen from the fastener. Therefore, hardness <32HRC fastener, coating thickness can be required at 12 mu m; High strength bolts with a hardness greater than 32HRC, the thickness of the coating shall be controlled at 8 mu m Max. This requires that the hydrogen embrittlement risk of high strength bolts must be taken into account in the design of the product, and the type and thickness of the plating shall be selected rationally.
Dehydrogenation of electroplating parts
Hydrogen embrittlement can be eliminated by hydrogen treatment (such as heating, etc.) and hydrogen embrittlement can be avoided by using vacuum, low hydrogen atmosphere or inert atmosphere. For ordinary galvanized workpiece dehydrogenation, it is generally in 200 ~ 240 degree temperature, heat 2 ~ 4 hours can remove most hydrogen.
The international standardization organization has formulated special criteria for the removal of stress and post-plating hydrogen treatment. The following list is for reference only.
6 galvanized low hydrogen embrittlement of alternatives For the present some electroplating factory customer the hydrogen embrittlement of the galvanized parts problems facing the confusion and distress, use Tak tinZNSPIRIT alkaline any Zinc nickel alloy plating process will be a very perfect solution!
Note: this article is transferred from the Tak tin technology public number, scanning the qr code below, and focusing on more industry information.