Several Local Corrosion Characteristics of Titanium Materials

 1.1 Crevice corrosion The crevice corrosion resistance of titanium is particularly strong, and crevice corrosion occurs only in a few chemical media. The crevice corrosion of titanium is closely related to temperature, chloride concentration, pH value and the size of the crevice. According to relevant information, crevice corrosion is easy to occur when the temperature of wet chlorine gas is above 85 ℃. For example, in some factories, a packed tower is used to directly cool the wet chlorine gas before the cooler to reduce the temperature of the wet chlorine gas to 65-70 ℃, and then enter the titanium cooler to improve the resistance to crevice corrosion, and the effect is also very significant. Practice has proved that reducing the temperature is one of the effective methods to prevent crevice corrosion, and crevice corrosion of Grade 5 Ti-6Al-4V Titanium Wire has also occurred in high temperature sodium chloride solution. In short, for the parts and components that are prone to crevice corrosion, such as the sealing surface, the expansion joint between the tube sheet and the tube, the plate heat exchanger, the contact part between the tray and the tower body, and the fasteners in the tower, titanium such as Ti-0.2Pd should be used. Alloys, gaps and stagnation areas should be avoided in the design. For example, the fasteners in the tower should be connected with bolts as much as possible. It is better to use expansion joint and sealing welding structure for tube sheet and pipe than simple expansion joint. For flange sealing surface, asbestos gasket should not be used, but PTFE film-coated asbestos gasket should be used.

1.2 High temperature corrosion

The high temperature corrosion resistance of Grade 5 Ti-6Al-4V Titanium Sheet depends on the characteristics of the medium and the performance of its own surface oxide film. In air or oxidizing atmosphere, titanium can be used as a structural material up to 426 °C, but at about 250 °C, titanium begins to absorb hydrogen significantly. In a complete hydrogen atmosphere, when the temperature rises above 316 °C, titanium Hydrogen absorption becomes brittle. Therefore, without extensive testing, titanium should not be used in chemical equipment with a temperature higher than 330 °C. Considering hydrogen absorption and mechanical properties, the operating temperature of all-titanium pressure vessels should not exceed 250 °C. The upper limit of the use temperature of the tube is about 316°C.

1.3 Stress corrosion

Except for a few kinds of media, industrial pure titanium has excellent stress corrosion resistance, and the phenomenon of titanium equipment damage caused by stress corrosion is still rare. Industrial passive titanium is only used in fuming nitric acid, some methanol solutions or some hydrochloric acid solutions, high temperature hypochlorite, molten salt with a temperature of 300-450 ° C or NaCl atmosphere, carbon disulfide, n-hexane and dry chlorine and other media. Stress corrosion occurs. The tendency of stress corrosion cracking of titanium in nitric acid increases with the increase of NO2 content and the decrease of water content. The stress corrosion tendency of titanium reaches its maximum in anhydrous nitric acid containing 20% ​​free NO2. When the concentrated nitric acid contains more than 6.0% NO2 and less than 0.7% H2O, the stress corrosion cracking of industrial pure titanium occurs even at room temperature. Serious stress corrosion and explosion have occurred in my country when titanium equipment is used in 98% concentrated nitric acid. Industrial pure titanium is sensitive to stress corrosion cracking in 10% hydrochloric acid solution, and titanium produces stress corrosion in 0.4% hydrochloric acid plus methanol solution.

To sum up, titanium has strong corrosion resistance in acid and alkali. It can form oxide film in acid and alkali, but it is also conditional. I hope it can help you when using our materials.

Common specifications and main applications of titanium forgings such as titanium cakes and titanium rings

Titanium forgings have high strength and low density, good mechanical properties, good toughness, and corrosion resistance. In addition, the process performance of titanium forgings is poor, cutting is difficult, and it is very easy to absorb impurities such as hydrogen, oxygen, nitrogen, and carbon during hot processing. There is also poor wear resistance and a complex production process. The industrial production of titanium started in 1948. The need for the development of the aviation industry makes the titanium industry develop at an average annual growth rate of about 8%. At present, the annual output of titanium forgings in the world has reached more than 40,000 tons, and there are nearly 30 types of titanium forgings. The most widely used titanium forgings are Ti-6Al-4V (TC4), Ti-5Al-2.5Sn (TA7), and industrial pure titanium (TA1, TA2 and TA3).

F9 Titanium Forgings are mainly used to make aircraft engine compressor parts, followed by structural parts of rockets, missiles and high-speed aircraft. In the mid-1960s, titanium and its alloys have been used in general industry for making electrodes in the electrolysis industry, condensers in power stations, heaters in petroleum refining and seawater desalination, and environmental pollution control devices. Titanium and titanium alloys have become a corrosion-resistant structural material. In addition, it is also used in the production of hydrogen storage materials and shape memory alloys.

Standard: National Standard GB/T 16598-1996

American Standard: ASTM B381

Material: TA0, TA1, TA2, TA3, TC4

Delivery state: Annealed state (M) Hot working state (R) Cold working state (Y) (annealing, ultrasonic flaw detection)

Packing: carton or wooden box packing

Surface treatment: car light, chamfering

Surface quality: The surface roughness Ra value of the two end faces should not be greater than 3.2lμm (to meet the requirements of ultrasonic inspection), and the surface roughness Ra of the inner and outer sides should not be greater than 12.5μm (when ultrasonic flaw detection is required for the outer circumferential surface) Ra should not be greater than 3.2μm), the chamfer radius is 5 ~ 15mm. There should be no visible defects such as cracks, folds, and heavy skins on the surface of the product. Local defects on the surface can be removed by grinding, and the cleaning depth should not exceed its dimensional tolerance, and its minimum allowable size should be guaranteed. The ratio of cleaning depth to width should be no more than 1:6 on both ends, and no more than 1:10 on the inner and outer sides. Grinding on the outer side should follow the axial direction.

Surface finish: clean, dust-free, better acid-resistant service life.

Testing: mechanical properties, chemical composition testing, ultrasonic testing.

What is the difference between beta titanium and pure titanium

1. Different metals: pure titanium is a titanium metal material with a titanium purity of more than 99%. Beta titanium refers to a titanium material that is cooled to complete beta particles with a delay in the critical point state of titanium, not a titanium metal material.

2. Different abilities: pure titanium has a high melting point, light material, strong corrosion resistance, and firm electroplating layer. In the glasses industry, pure titanium frames often refer to frames made of a titanium material. Beta titanium has better strength, fatigue resistance, and environmental corrosion resistance than titanium pure powder, and has good shape plasticity. It can be made into wires and sheets, and it can be used to make glasses.

3. Different materials: β-titanium is a material that is relatively larger than pure titanium, but it is lighter than ordinary metals, so its purity is relatively low, about 75%, and in the process of use It has higher strength than pure titanium, and strong environmental corrosion resistance. The proportion of pure titanium reaches 89%, but the material is relatively soft.

Development status of domestic titanium strips

Titanium and titanium alloys are processed into products such as plates, strips, foils, tubes, rods, wires, forgings, castings, and composite materials. Titanium plates and strips are the most important product forms, but the processing of titanium plates and strips The process performance is poor, and the cutting process is difficult. In hot processing, it is very easy to absorb impurities such as hydrogen, oxygen, carbon, and nitrogen. At the same time, the wear resistance of the titanium plate and strip is poor, and the production process is complicated. Before 2007, the industrial production of titanium foil sheet in China was in a blank state. Titanium strips and coils were completely dependent on imports. The procurement costs were high, the cycle was long and the quality was unstable, which seriously affected the development of the downstream application fields of titanium strips and welded pipes. In recent years, many domestic enterprises represented by Hunan Xiangtou Jintian Technology Group have successively rolled hot-rolled titanium strips and cold-rolled titanium strips, the production technology of hot-rolled and cold-rolled pure titanium strips, and the heat treatment of strips. Technology, rolling process control technology, and other aspects have made great progress. With the acceleration of the construction of domestic nuclear power and PTA projects and the demand for localization, the demand for thin-walled titanium welded pipes, chemicals, and plates to replace titanium plates is becoming more and more strong. The total amount of processed materials is 59.2%, of which the output of titanium coils accounts for more than half. For applications in the field of nuclear power condensers and heat exchangers, the use of titanium coils has the advantages of high-quality stability, suitable for batch continuous production, short production cycle, and cost reduction compared with the use of block single sheets. The product has become the preferred material for welded pipes and plate heat exchangers for nuclear power.

Clamping and processing technology of titanium alloy parts

Titanium alloy parts have the characteristics of low density and good corrosion resistance, so they have become ideal structural materials for aerospace engineering; however, there are many factors that affect its machinability at the same time; this is because of the metallurgical properties of 6al4v titanium plate and material properties, which can have serious effects on the cutting action and the material itself.

The clamping principle of titanium alloy parts is the key technology for Grade 2 Pure Titanium Plate manufacturers to process titanium alloys, as follows:

(1) The clamping force in the roughing stage should be large to prevent the parts from loosening during the machining with large cutting force; the clamping force in the finishing stage should be small to prevent the deformation of the clamping.

(2) The clamping force acts on the rigid place, and the force is applied at as many points as possible.

(3) Appropriate auxiliary devices should be added for thin-walled structural parts with poor rigidity to increase the rigidity of the entire processing system.

Antioxidant and flame retardant coating for titanium alloys

At room temperature, a dense oxide film can be formed on the surface of titanium alloy, so it has good room temperature corrosion resistance. Titanium alloy parts used in some aerospace vehicles need to be used at medium or even high temperatures, and the oxide film formed under this condition is porous TiO2, which cannot effectively resist the inward diffusion of oxygen atoms. On the other hand, the ignition point of titanium alloys is lower than the melting point. When the titanium alloy parts of the aero-engine moving at high speed are displaced due to some reasons (such as deformation, fracture, etc.), the relative movement between the parts (such as rotating the stator) high-speed friction, and heat may ignite the Gr5 Ti-6Al-4V Titanium Sheet and cause a titanium fire accident, which seriously endangers the aviation industry. Safe use of spacecraft. Therefore, the development of titanium alloy anti-oxidation coatings and flame retardant coatings has been actively carried out at home and abroad. It is a reliable method to change the oxidation and temperature rise mechanism of titanium alloy surfaces with two types of coatings.

How to deal with the surface defect reaction layer of titanium plate and titanium rod

The surface reaction layer of Gr5 Ti-6Al-4V Titanium Plate and medical titanium rod is the main factor affecting the physical and chemical properties of titanium workpiece. Before processing, the surface pollution layer and defect layer must be completely removed. Physical and mechanical polishing of titanium plate and titanium rod surface polishing process:

1. Sandblasting:

The sandblasting treatment of titanium wire castings is generally better to use white corundum, and the sandblasting pressure is smaller than that of non-precious metals, generally controlled below 0.45MPa. Because, when the injection pressure is too large, the sand particles impact the titanium surface to generate intense sparks, and the temperature rise can react with the titanium surface, resulting in secondary pollution and affecting the surface quality. The time is 15-30 seconds, and only the sticky sand, surface sintered layer, and part of the oxide layer on the surface of the casting can be removed. The rest of the surface reaction layer structure should be quickly removed by chemical pickling.

2. Pickling:

Pickling can quickly and completely remove the surface reaction layer without contamination of the surface with other elements. Both HF-HCL series and HF-HNO3 series pickling liquids can be used for titanium pickling, but HF-HCL series pickling liquids have a large amount of hydrogen absorption, while HF-HNO3 series pickling liquids have a small hydrogen absorption capacity, which can control HNO3 The concentration of HF can reduce the absorption of hydrogen, and the surface can be brightly treated. Generally, the concentration of HF is about 3%-5%, and the concentration of HNO3 is about 15%-30%.