titanium equipment damage types and protective measures
Types of damages and protective measures of titanium equipments
In addition to considering many advantages of titanium, it is also necessary to recognize the shortcomings of titanium in some aspects, such as low thermal conductivity, high melting point, low coefficient of linear expansion, low modulus of elasticity, poor anti friction performance, large fluidity of molten titanium, difficult drilling and tapping, poor creep resistance, gas protection for welding and large welding residual stress, etc. in the specific design, we should develop strengths and avoid weaknesses to avoid unnecessary corrosion and damage. See Table 4 for damage types and protection design of common titanium equipment.
1. Damage type: Uniform corrosion
Wrong design:
1. Lack of oxidizing corrosion inhibitor in reducing media such as hydrochloric acid and sulfuric acid.
2. General titanium are used in high temperature sulfuric acid, hydrochloric acid and phosphoric acid solutions.
3. Reducing acid or high fluorine ion concentration shall be used for chemical cleaning
Right design:
1. Add corrosion inhibitor and strictly control the concentration of oxidant or metal ion
2. Use Ti-32Mo alloy instead
3. Add appropriate corrosion inhibitor to control the concentration of fluoride ions.
2.Damage type: crevice corrosion
Wrong design:
1. sealing surface, plate heat exchanger, contact part of tower body and tray, and fasteners in the tower are pure titanium
2. The tube and tube sheet of heat exchanger are expanded
3. flange sealing surface adopts rubber and tetrafluoro gasket
4. threaded connection of tower internals
Right design:
1. Use gr7, gr12 or partial Pb coating
2. Use expansion welding instead
3. Use ti/ tetrafluoro spiral gasket, Ti-0.2 metal rhombic gasket and serrated gasket
4. Use wedge fasteners instead
3. Damage Type: Stress Corrosion
Wrong design:
1. used in fuming nitric acid, methanol solution containing hydrochloric acid, high temperature hypochlorite, dry chlorine, carbon disulfide and other media
2. Structural design, local stress concentration exists in the equipment
3. residual stress exists after equipment fabrication
Right design:
1. prohibit the use of titanium
2. Improve the design, try to make smooth transition and avoid stress concentration
3. stress relief annealing to eliminate residual stress of processing and welding
4. Damage Type: Pitting Corrosion
Wrong design:
1. High concentration of reducing acid exists locally in the equipment
2. choose titanium material with low oxygen content
3. titanium weld discoloration and welding defects
4. Iron pollution on the surface after equipment manufacturing and maintenance
Right design:
1. add sodium carbonate for neutralization and oxidant for inhibition
2. use titanium materials with high oxygen content
3. Improve argon protection during welding to improve welding quality
4. pickling and anodizing
5. Damage Type: Erosion Corrosion
Wrong design:
1. equipment flow rate is not controlled when it exceeds 6m/s
2. No baffle is set at the part directly facing the air flow or material impact
Right design:
1. control fluid less than critical flow rate
2. Reasonably set anti impact baffle
6. Damage type: Corrosion under scale
Wrong design:
1. There are dead corners in the structure, and the drain outlet is too small
2. Materials and catalysts are easy to agglomerate and deposit
3. Pure titanium shall be used for parts prone to scaling and pipeline
Right design:
1. improve the design, avoid dead corners and increase the drain outlet
2. regular cleaning or appropriate improvement
3. use gr7 or gr12 instead
7. Damage Type: Hydrogen Embrittlement
Wrong design:
1. hydrogenation equipment, design temperature greater than 315 ℃, moisture is 2%。
2. The temperature of the equipment is greater than 77℃, and the PH of the contact corrosive medium is less than 3 or greater than 12. No measures have been taken during the design.
3. production of atomic hydrogen by chemical process side reaction in the reactor。
4. Iron pollution caused by using steel tools during equipment manufacturing and maintenance。
5. Improper welding process, weld cracking, resulting in leakage of corrosive materials, making titanium cladding and steel shell form a couple
6. the tested parts have hydrogen absorption to a certain extent, but there is no abnormality on the surface, so they are still used。
Right design:
1. do not select titanium material or change the process so that the design temperature is less than 315℃, water content is greater than 2%.
2. add oxidant to prevent over temperature operation of the equipment, and conduct atmospheric thermal oxidation treatment after the equipment is manufactured.
3. avoid using titanium
4. austenitic stainless steel tools or anodizing
5. improve welding process and strengthen inspection
6. Conduct dehydrogenation heat treatment for reuse
