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AGA CW Handbook A4_32130-UK
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13 3.5.2
nitrogen
monoxide
, no nitrogen monoxide is formed out of the oxygen and nitrogen in the surrounding air. a spray arc or hot metal starts the following reaction: n 2 + o 2 -> 2no the htp 8h value for nitrogen monoxide in finland is 25 ppm (htp values, 2009). air entering the arc space causes the generation of nitrogen mono- xide (no). the more air enters the arc space, the higher the generati- on of nitrogen monoxide. 3.5.3 nitrogen dioxide
, no 2 part of the nitrogen monoxide formed in the vicinity of the arc can change into nitrogen dioxide (no 2 ) at lower temperatures as follows: 2no + o 2 -> 2no 2 the htp 8h value for nitrogen dioxide in finland is 3 ppm no 2 (htp values, 2009). most nitrogen dioxide (no 2 ) is formed during mma welding, then mig/mag welding and the least during tig welding. if there is ozone in the vicinity, the nitrogen monoxide will primarily react with it forming nitrogen dioxide and oxygen (no + o 3 -> no 2 + o 2 ). this reaction is utilised by the mison ® shielding gases to reduce the ozone concentration in the combustion gases. the generation of small amounts of nitrogen dioxide as a result of ozone elimination is acceptable, as ozone is considered to be clearly more problematic than nitrogen dioxide in welding. 3.5.4 ozone (o 3 ) and nitrogen dioxide (no 2 ) the nitrogen monoxide (no) added to mison ® shielding gases re- moves ozone (o 3 ) but increases the amount of nitrogen dioxide (no 2 ). the combined effect of impurities of similar type and classified as harmful occurring simultaneously in the smoke can be evaluated as follows: c 1 /htp 1 + c 2 /htp 2 +...+ c n /htp n < 1 in the formula, c is the measured concentration of the substance in ques- tion, and htp is the concentration of the substance classified as harmful. in the breathing zone, the total must be less than 1. because the htp value of ozone is clearly lower than that of nitrogen dioxide, it is benefi- cial to reduce the amount of ozone generated in the smoke. the htp 8h value for carbon monoxide in finland is 30 ppm co. (htp values, 2009) correspondingly, the htp 8h value for carbon dioxide is 5000 ppm co2 (htp values, 2009). carbon monoxide is an odourless and colourless gas which prevents oxygen from binding with blood. carbon monoxide poisoning causes fatigue, headaches, chest pains, trou- ble concentrating and, finally, unconsciousness. the higher the carbon dioxide content of a shielding gas, the more carbon monoxide is formed. in normal conditions during mag welding, the generation of carbon mon- oxide is not, however, an especially serious problem. however, danger- ously high carbon monoxide concentrations may occur in closed, poorly ventilated spaces. 3.5.5 carbon monoxide, co carbon monoxide is mainly formed as carbon dioxide (co 2 ) in the shielding gas is broken up as follows: 2co 2 -> 2co + o 2 3.6 other impurities other impurities generated during welding originate from the coating of the metal, solvents used in the cleaning of the metal surface or an unclean sheet metal surface as they come into contact with heat and uv radiation. cleaning the surfaces to be welded in the vicinity of the arc is an effec- tive way of limiting the generation of such impurities. solvents containing chlorinated hydrocarbons, for example ethylene trichloride, can form toxic compounds in the air due to the effect of the arc. such cleaning agents should not be used in the cleaning of welded workpieces. the work environment
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