Many fabricators experience welding problems at different times of the year. Moisture (H20) is a prime source of hydrogen. At arc temperatures, water breaks down releasing hydrogen atoms that cause porosity in weldments. Shielding gas supplies are controlled to very low moisture content (-70°F dewpoint or lower). Likewise, the atmospheric conditions in a fabricating facility need to be controlled to prevent moisture condensation from forming on electrode or base metal.
Aluminum, which is allowed to repeatedly come intocontact with water, will eventually form a hydrated oxide (AIOH) coating. Moisture from condensation present on either the electrode or the base metal can cause two problems during welding:
Porosity caused by hydrogen generated from the breakdown of water or from the breakdown of hydrated oxide (AIOH) present on the metal surfaces.
Entrapment of the actual oxide (AIOH), present on the metal surfaces, in the weldment.
Relative Humidity – The ratio of the quantity of water vapor present in the atmosphere to the quantity which would saturate the air at the existing temperature. Relative humidity is expressed as a percentage number and needs to be monitored in the welding area. Dip tanks, cleaning stations, etc., affect relative humidity.
Dew Point – The temperature at which condensation of water vapor in the air takes place. Moisture will condense on metal surfaces when their temperature is equal to or below the dew point. For each relative humidity percentage, there is a corresponding dew point.
Air Temperature – The temperature of the air in the welding area at any given time.
Metal or Electrode Temperature – The temperature of the electrode or base metal at any given time.
DEW POINT CONDITIONS VERSUS RELATIVE HUMIDITY:
(Tair-Tmetal)º Humidity (Tair – Tmetal)º Humidity
ºC (ºF) % ºC (ºF) %
0 (0 ) 100 12 (21.6) 44
1 (1.8) 93 13 (23.4) 41
2 (3.6) 87 14 (25.2) 38
3 (5.4) 81 15 (27.0) 36
4 (7.2) 75 16 (28.8) 34
5* (9.0*) 70* 18 (32.4) 30
6 (10.8) 66 20 (36.0) 26
7 (12.6) 61 22 (39.6) 23
8 (14.4) 57 24 (43.2) 21
9 (16.2) 53 26 (46.8) 18
10 (18.0) 50 28 (50.4) 16
11 (19.8) 48 30 (54.0) 14
(Tair – Tmetal)º- Temperature of the air minus the temperature of the metal shown in ºC and ºF.
The above chart shows the relative humidity at which detrimental water condensate will form for a number of given differential temperatures.
* Example – If the relative humidity in the weld area is 70%, the base metal and electrode must be no colderthan 9ºF below the air temperature to prevent moisture condensation.
In an aluminum welding shop, the uniformity of air and metal temperatures is important especially when the relative humidity is high. Electrode and base metal should be allowed to stabilize to the weld area temperature. The electrode should not be opened in the weld area for 24 hours after entry from a cooler storage area. The base metal should be cleaned and brushed with a clean stainless steel brush prior to welding. AlcoTec recommends mild alkaline solutions and commercial degreasers that do not evolve toxic fumes during welding. Welders should wipe joint edges with a clean cloth dipped in a volatile petroleum based solvent. All surfaces must be thoroughly dried after cleaning.