Welding Contamination

In corrosive envirorunents, corrosion will be initiated by surface imperfections in stainless steel plate. This corrosion can occur in the presence of media to which stainless steel is normally resistant. Such imperfections can be caused by  

Welding slag from coated electrode arc strikes Welding stop points Heat tint 

Weld splatter produces small particles of metal that adhere to the surface, at which point the protective film is penetrated, forming minute crevices where the film has been weakened the most. If a splatter-prevention paste is applied to either side of the joint to be welded, this problem will be eliminated. Splatter will then easily wash off with the paste during cleanup. 

Whenever coated electrodes are used, there will be some slag around the welded joints. This slag is somewhat difficult to remove, but if it is not done, the small crevices formed will be points of initiation of corrosion. 

Arc strikes and weld stop points are more damaging to stainless steel than embedded iron because they occur in the area where the protective film has already been weakened by the heat of welding. Weld stop points create pinpoint defects in the metal, whereas arc strikes form crevice-like imperfections in or adjacent to the heat-affected zone. 

---

Titanium Argon Good arc stability minimum weld contamination inert gas backing is required to prevent air contamination on back of weld area. 

Iron contamination Organic contamination Welding contamination... 

Welding contamination removal is best accomplished using abrasive discs and flapper wheels. Although grinding has been used, this procedure tends to overheat the surface, thereby reducing its corrosion resistance. Its use should be avoided. 

Electrodes must be dry and free from oil and grease to prevent weld contamination. 

Flux must be clean and free from moisture to prevent weld contamination. 

Lead and its alloys are generally melted, handled, and refined in cast-iron, cast-steel, welded-steel, or spun-steel melting ketdes without fear of contamination by iron (qv). Normal melting procedures require no dux cover for lead. Special reactive metal alloys require special alloying elements, duxes, or covers to prevent dross formation and loss of the alloying elements. 

Nondestmctive testing (qv) can iaclude any test that does not damage the plastic piece beyond its iatended use, such as visual and, ia some cases, mechanical tests. However, the term is normally used to describe x-ray, auclear source, ultrasonics, atomic emission, as well as some optical and infrared techniques for polymers. Nondestmctive testing is used to determine cracks, voids, inclusions, delamination, contamination, lack of cure, anisotropy, residual stresses, and defective bonds or welds in materials. 

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. 

As in the case of many metal—ahoy systems, weld ductihty is not as good as that of the base metal. Satisfactory welds can be made in vanadium ahoys provided the fusion zone and the heat-affected zone (HAZ) are protected from contamination during welding. Satisfactory welds can be made by a variety of weld methods, including electron-beam and tungsten-inert-gas (TIG) methods. It is also likely that satisfactory welds can be made by advanced methods, eg, laser and plasma techniques (see Lasers Plasma technology). 

Metal Preparation. Sheet-steel parts are formed by stamping, bending, and shearing. Many parts require welding (qv), which needs to be carried out in a uniform, smooth manner so that the welded joint can be enameled without defects. Cast-iron parts are formed by the usual cast-iron foundry methods however, additional care is given to prevent contamination of the surface. Surface contamination causes defects in the enamel, particularly bUsters and bubbles. Aluminum metal can be formed in sheets, extmsions, and as castings. 

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. 

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

Resistance and arc welding operations, and plasma and laser cutting produce fumes by expulsion or evaporation of the base material, coating, and electrode wear. Larger particles deposit on the surrounding surfaces, while smaller particles move upward with convective flows. Specific contaminants associated with different welding and cutting operations are listed in AWS. ... 

Furnace-type air heaters are manufactured from cast iron or steel, and cased in brickwork or steel. The cast-iron heaters are sectional, cemented and bolted together. The steel type are welded or riveted. It is essential that the joints are airtight, so that the cold air can pass over the heated surfaces of the furnace and flues without contamination by the flue gases. 

In general, the Aaberg principle is suitable for all open processes which demand an open work area. These processes avoid the use of closed or partially closed systems, for instance a conventional hood with specific walls around the pollution source. In particular, the principle is very suitable for welding (i.e., spraying particle sources) or hot sources of contaminant. Some examples of open... 

Air contaminants in solid or liquid state (aerosols), e.g., wood dust, welding smoke, or oil mist, are all in principle directly visible. The dispersion of those contaminants and the airflow patterns around the source may therefore be studied without any special tools. It is, however, not always possible to see the contaminant if, for example, the concentration in the air is low, the size of the particles is small, or the lighting is poor. The fact that the contaminant can t be seen may stem from the acceptable low level of the concentration but that can of course not be used to conclude that the control is acceptable. That conclusion depends not only on the contaminant s toxicological qualities but on how visible it is iit air. The ability to see the particles directly is also, as said above, a function of their size. Small particles, able to be transported deep into the thinner airways of the lungs, are many times also difficult to see directly. 

This method has been used for a variety of applications to visuali2e the concentration of different air contaminants in the breathing zone and how it relates to factors like design of local ventilation, etc. lypical situations where it has been used are when the contaminant source is close to or handled by the worker, e.g., welding, painting, and woodworking. 

---