Electrochemical cleaning versus GreenOne pickling paste
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During the autumn of 2002, a study comparing electrochemical cleaning of welding oxides with conventional cleaning using pickling paste was carried out by the Process Chemistry department at the Outokumpu Research Centre in Avesta. A modern electrochemical apparatus used for cleaning welding oxides and "GreenOne 120", a new, low-acid pickling paste supplied by Avesta Finishing Chemicals, were tested.
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For the purposes of the study, an evaluation of GTA/TIG-welded joints in 3 mm cold rolled material was carried out for the grades ASTM 304 and 316. The pickling paste used was the ”GreenOne 120”, a new product with a low acid content supplied by Avesta Finishing Chemicals in Malmö.
The comparison shows that electrochemical treatment, although an effective method, has a number of negative effects: After electrochemical treatment, the surface showed signs of intergranular corrosion, which results in inferior corrosion resistance in the weld than if pickling had been carried out using pickling paste.
Furthermore, the method attacks the stainless steel surface more dramatically, resulting in higher levels of heavy metals (Fe, Cr, Ni) in the rinsing water after treatment compared to the rinsing water after pickling with paste. However, both types of rinsing water require special handling and treatment before the purified water is discharged to the receiving body of water. According to the study, considerable volumes of electrolyte fluid were used compared to the volume of pickling paste required to clean the welds, which suggests that electrochemical cleaning is a more expensive method than pickling with paste.
1. BACKGROUND
In recent years, the use of electrochemical cleaning apparatus has been growing, particularly for the cleaning and post-weld treatment of TIG weld joints. The apparatus uses a direct current source that is connected to an electrode surrounded by an electrolyte, normally phosphoric acid (H3PO4), but it may also be a combination of phosphoric acid (H3PO4) and sulphuric acid (H2SO4). The electrolyte is then used to soak a piece of material, which is attached to the tip of the electrode. The equipment uses direct current (12V square wave) in the transpassive area and the process produces a certain anodic dissolution and polishing of the material. This type of apparatus is usually used on thin-gauge stainless steel material (ASTM 304 and 316), that is normally TIG-welded. The cost of the apparatus is in the region of US$ 1500 - 2000.
During the autumn of 2002, the Process Chemistry department at the Outokumpu Research Centre in Avesta tested and compared a modern electrochemical apparatus (see picture 1) with "GreenOne 120" (picture 2), a new pickling paste with a low acid content, supplied by AvestaPolarit Welding Chemical Products.
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| 1. Electrochemical cleaning machine |
2. GreenOne pickling paste 120 |
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