Welds cleaning

Aluminium alloys Sensitive to contaminants requires very clean welding attacked very rapidly in concentrations near 100% or with excess acetic anhydride... 

Power ultrasound with its origin in engineering and processing, which includes cleaning, welding and materials processing at the industrial level. 

Surface preparation is most important for plastic parts that will be bonded with adhesives. Solvent and heat welding do not generally require chemical alteration of the surface however, they do require cleaning. Welding procedures are discussed in another section of this chapter. 

High selective performance of the sensor also enable to detect surface cracks in ferromagnetic and non-magnetic welded connections under rust or dirt layer without prior cleaning. 

Cleaning, the removal of unwanted matter, is the beginning of the treatment cycle for metal. The unwanted matter may be carbon smut, welding flux, ink, oxidation products, oil, fingerprints, or other material. Cleaners may be classified as solvent-based or aqueous. Within the aqueous class there are many subclasses, the most important of which are the alkaline cleaners. There are also a variety of ways to apply cleaners. As of the mid-1990s, solvent-based cleaner usage is declining. 

Stainless steel develops a passive protective layer (<5-nm thick) of chromium oxide [1118-57-3] which must be maintained or permitted to rebuild after it is removed by product flow or cleaning. The passive layer may be removed by electric current flow across the surface as a result of dissinulat metals being in contact. The creation of an electrolytic cell with subsequent current flow and corrosion has to be avoided in constmction. Corrosion may occur in welds, between dissimilar materials, at points under stress, and in places where the passive layer is removed it may be caused by food material, residues, cleaning solutions, and bmshes on material surfaces (see CORROSION AND CORROSION CONTROL). 

Thermoplastics. There are five elastomeric membranes that are thermoplastic. Two materials, chlorinated polyethylene (CPE) and polyisobutylene (PIB), are relatively obscure. Thermoplastic materials can be either heat-fused or solvent-welded. In contrast to Hypalon and uncured EPDM, this abiHty to fuse the membranes together remains throughout the life of the material. However, cleaning of the membrane surface after exposure to weather is required. Correct cleaning procedures for specific membranes are available from the individual manufacturer. 

Techniques for handling sodium in commercial-scale appHcations have improved (5,23,98,101,102). Contamination by sodium oxide is kept at a minimum by completely welded constmction and inert gas-pressured transfers. Residual oxide is removed by cold traps or micrometallic filters. Special mechanical pumps or leak-free electromagnetic pumps and meters work well with clean Hquid sodium. Corrosion of stainless or carbon steel equipment is minimi2ed by keeping the oxide content low. The 8-h TWA PEL and ceiling TLV for sodium or sodium oxide or hydroxide smoke exposure is 2 mg/m. There is no defined AID for pure sodium, as even the smallest quantity ingested could potentially cause fatal injury. 

Plate and frame coolers using HasteUoy C-276 plates have been used successfuUy. Anodically protected plate coolers are available as weU as plate coolers with plates welded together to minimize gasketing. Another promising development is the introduction of plate coolers made of HasteUoy D205 (105). This aUoy has considerably better corrosion resistance to concentrated sulfuric acid at higher temperatures than does C-276. Because of the close clearance between plates, cooling water for plate coolers must be relatively clean. 

Welding (qv) of titanium requires a protected atmosphere of iaert gas. Furthermore, parts and filler wire are cleaned with acetone (trichloroethylene is not recommended). The pieces to be welded are clamped, not tacked, unless tacks are shielded with iaert gas. A test sample should be welded. Coated electrodes are excluded and higher purity metal (lower oxygen content) is preferred as filler. Titanium caimot be fusion-welded to other metals because of formation of brittle intermetallic phases ia the weld 2oae. 

The choice of metals for vacuum walls is largely based on the ease of fabrication of the metal, machining, cleaning (26), welding, etc. Aluminum alloys are the material of choice for out-gassing at room temperature. 

Stress Corrosion Crocking. Stress corrosion cracking occurs from the combined action of corrosion and stress. The corrosion may be initiated by improper chemical cleaning, high dissolved oxygen levels, pH excursions in the boiler water, the presence of free hydroxide, and high levels of chlorides. Stresses are either residual in the metal or caused by thermal excursions. Rapid startup or shutdown can cause or further aggravate stresses. Tube failures occur near stressed areas such as welds, supports, or cold worked areas. 

The use of carbon dioxide gas for shielded arc welding with semiautomatic microwire welding equipment has led to welding speeds up to 10 times those obtainable within conventional equipment. No cleaning or wire bmshing of the welds is required (45) (see Welding). 

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