The selection principle of titanium
Because titanium (titanium and titanium alloy) has good mechanical and physical properties, its density is small and its strength is high. The ratio of tensile strength σb to density ρ σb/p=200, which is almost the highest among all metal materials .
At the same time, it has very good corrosion resistance. Titanium has excellent chemical stability in a strong corrosive environment and strong self-passivation ability in the electrolyte (water), which makes the application and promotion of titanium faster than other metals.
Generally speaking, industrial pure titanium has better corrosion resistance than α-phase titanium alloy and β-phase or α+β-phase titanium alloy, and has a wider application range, although its strength is not as high as β-phase titanium alloy or α+β-phase titanium alloy. , But it has good plasticity and is easy to process and shape. Therefore, industrial pure titanium is the most commonly used material in titanium containers.
1) Deformed titanium materials should be supplied in annealed condition (M), and titanium castings should be supplied in as-cast condition.
2) TA3 in the deformed industrial pure titanium TAO, TA1, TA2, TA3, due to its poor cold deformation ability, is generally not suitable for parts such as cylinders, heads and blisters, and can only be used without cold deformation or cold deformation smaller parts.
3) TA9 titanium palladium alloy (Ti -0. 2Pa) and TA10 titanium nickel molybdenum alloy (Ti -0.8Ni -0.3Mo) are mainly used in high temperature and wet chlorine-containing media and may have crevice corrosion (especially TA9 is more resistant to crevice corrosion) It is especially suitable for parts such as tube sheet and titanium flange.
4) If there are galvanic couples, the following measures can usually be taken:
- ① Compound a metal (usually a metal corroded by galvanic) with an insulating material;
- ② Add a completely isolated insulating material between the two metals to avoid the formation of corrosion batteries;
- ③ Keep the distance between different metals or change the position between them to avoid cathodic pollution;
- ④ Avoid the formation of large cathode and small anode between the two metals to corrode the battery;
- ⑤ Use cathodic protection.
5) If there is crevice corrosion, the following measures can usually be taken:
- ① Adopt reasonable structural design, try to avoid or eliminate the gap retention area and fouling phenomenon, improve the fluid flow in the equipment, and avoid the formation of dead zones. When internal bolt connections, use welding connections as much as possible, and spot welding overlaps as much as possible. Lap welding or butt welding.
- ② Use palladium coating, oxidation or anodization on the surface where crevice corrosion may occur.
- ③ Putting putty mixed with NiO or nickel powder or MoO3 powder in the gap can sometimes avoid crevice corrosion.
- ④ Use titanium materials that are more resistant to crevice corrosion, such as titanium palladium alloy (TA9) or titanium nickel molybdenum alloy (TA10). These titanium materials are especially suitable for flanges with crevice corrosion on the flange sealing surface.
6) If there is hydrogen embrittlement rupture, the following measures can usually be taken:
- ① Use titanium with less hydrogen content.
- ② Prevent hydrogen absorption during processing and manufacturing, that is, avoid embedding iron particles on the surface of titanium during processing and manufacturing processes such as cutting, stamping, coiling, welding, etc.; hot processing and heat treatment heating must be carried out in a micro-oxidizing atmosphere heating furnace; For some titanium equipment with complex structure, it is difficult to realize the welding joint protected by inert gas on the back, so it is necessary to prevent pollution and hydrogen absorption during welding.
- ③ Choose an appropriate environment for use: When used in an environment of dry hydrogen and wet hydrogen at a temperature of 71~316℃, if it contains a certain amount of oxygen and moisture, it can prevent hydrogen absorption. Titanium in oxidizing medium, neutral medium, weakly reducing medium or reducing acid containing oxidizing agent usually does not absorb hydrogen by titanium, or absorb hydrogen very slowly; but when the surface of titanium is contaminated by iron, the surface is defective, When corrosion occurs locally or abnormal working conditions occur, titanium hydrogen absorption embrittlement may occur. Titanium is prone to hydrogen absorption embrittlement in environments where general corrosion or local corrosion occurs.
- ④ Surface treatment, such as high temperature oxidation, anodization, etc., can improve the resistance to hydrogen absorption.
- ⑤ Using corrosion-resistant alloys to improve the corrosion resistance of titanium and prevent hydrogen embrittlement of titanium.
7) Titanium is strictly prohibited to be used in liquid chlorine and dry chlorine applications.
8) Titanium is strictly prohibited to be used in fuming nitric acid with water content less than 2% or free nitrogen dioxide greater than 6%.
9) Titanium materials should be avoided to be used in stress corrosion environments. Any medium with stress corrosion tendency, even if it is slightly corrosive to titanium, still has the risk of stress corrosion cracking, it cannot be used.
Source: China Titanium Flanges Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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