Metals surface contaminants

Critical factors. In general, porosity is caused by the entrapment of gas during the welding process or during solidification of the weld metal. Surface contamination may provide a gas source during the welding operation. 

In addition to the electrochemical techniques, many surface analytical techniques are constantly in use, such as ellipsome-try for the surface thin oxide thickness, multiple reflection infrared spectroscopy (MIR), and X-ray photoelectron spectroscopy (XPS) for surface layer composition, total reflection X-ray fluorescence spectroscopy (TXRFS) for the metal surface contaminants, and naturally atomic force microscopy (AFM) for the surface roughness profile. 

Chaudry and Bhide [57] demonstrated a process for cleaning metallic surfaces contaminated by a spent-fuel radioactive wastewater stream. Using an aluminum cathode and tapwater as the electrolyte, a current density of 15 pA/cm on a contaminated pipe wrench made of mild steel (rendered anodic) reduced the radioactivity from 26 pGy/h to 0.26-0.39 pGy/h in 15 min. Similar results were obtained when studies were performed on contaminated aluminium plates. 

From Tables 7.7 and 7.8, it can be forecast that epoxy adhesives will wet clean aluminum or copper surfaces. However, epoxy resin will not wet a substrate having a critical surface tension significantly less than 47 dynes/cm. Epoxies will not, for example, wet either a metal surface contaminated with silicone oil or a clean polyethylene substrate. 

Blister. A large bubble sometimes present as a fault in ceramic ware. In glass-ware, if near the surface it is a skin BLISTER and if on the inside surface of blown glass-ware it is a pipe blister (q.v.). The common causes of blisters in vitreous enamelware are flaws in the base-metal, surface contamination of the base-metal, and too high a moisture content in the atmosphere of the enamelling furnace. 

Solvent dip methods are generally expensive to run and do not usually, unless a number of dip tanks are used, completely remove oils and greases from the metal surfaces. Contaminants are easily carried from tank to tank and it is difficult to ascertain whether the metal surface is completely cleaned after its passage through the tank series. This method would not normally be used for anything other than small scale operations. Fast drying solvents such as methylene chloride and acetone evaporate so quickly that they lower the temperature of the metal surface and water condenses. 

Glass bead and walnut shell blasting are very effective in removing metallic surface contamination without damaging the surface. It is sometimes necessary to resort to blasting with clean sand to restore heavily contaminated surfaces such as tank bottoms, but care must be taken to be certain the sand is truly clean, is not recycled, and does not... 

Atmospheric corrosion results from a metal s ambient-temperature reaction, with the earth s atmosphere as the corrosive environment. Atmospheric corrosion is electrochemical in nature, but differs from corrosion in aqueous solutions in that the electrochemical reactions occur under very thin layers of electrolyte on the metal surface. This influences the amount of oxygen present on the metal surface, since diffusion of oxygen from the atmosphere/electrolyte solution interface to the solution/metal interface is rapid. Atmospheric corrosion rates of metals are strongly influenced by moisture, temperature and presence of contaminants (e.g., NaCl, SO2,. ..). Hence, significantly different resistances to atmospheric corrosion are observed depending on the geographical location, whether mral, urban or marine. 

Metal Preparation. Preparation of the metal surfaces to be bonded usually is required because most metals contain surface imperfections or contaminants that undesirably affect bond properties. The cladding faces usually are surface ground, using an abrasive machine, and then are degreased with a solvent to ensure consistent bond strength (26). In general, a surface finish that is >3.8 fim deep is needed to produce consistent, high quaUty bonds. 

Boiler Deposits. Deposition is a principal problem in the operation of steam generating equipment. The accumulation of material on boiler surfaces can cause overheating and/or corrosion. Both of these conditions frequentiy result in unscheduled downtime. Common feed-water contaminants that can form boiler deposits include calcium, magnesium, iron, copper, aluminum, siUca, and (to a lesser extent) silt and oil. Most deposits can be classified as one of two types scale that crystallized directiy onto tube surfaces or sludge deposits that precipitated elsewhere and were transported to the metal surface by the flowing water. 

Surface preparation, always important in obtaining optimal coatings performance, is critical for marine coatings (see Metal surface treatments). Surface preparation usually comprises about half of the total coating costs, and if inadequate may be responsible for early coating failure. Proper surface preparation includes cleaning to remove contaminants and roughening the surface to faciUtate adhesion. 

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