Introduction
A stainless steel panel used as an insulating panel for various outdoor applications required surface analysis to determine the integrity of the oxide layer.
Figure 1 shows the possible sources of free ion particles and other contaminants on a panel where Area 1 is the iron-steel layer, Area 2 is the interlayer of chromium oxide and iron and Area 3 is the surface area.
Figure 1 – Surface contamination of panel
Investigation
The standard practice for cleaning, descaling and passivation of stainless steels according to ASTM standard A380-06 was conducted (Table 1).
Table 1: ASTM A380-06 tests
| No. | Test | Method | Reason |
| 7.2 | Gross Inspection | Ensure surface free of contaminants before testing | |
| 7.2.1 | Stereo Microscope Inspection | Visual inspection under light microscope | Ensure surface free of atmospheric deposits and welding flux |
| 7.2.2 | Wipe test | Surface wiped and product sent for scanning electron microscopy | Determine nature of contaminants |
| 7.2.4 | Water-breaktest | Pattern of wetting is observed after continuous water flow. | Detect the presence of hydrophobic contaminants. |
| 7.2.5 | Tests for free iron: Gross indications | ||
| 7.2.5.1 Water-wetting and drying test | Distilled water sprayed on surface of every 8 hours for 24 hours. Signs of corrosion noted. | Perform accelerated corrosion to determine the degrees of corrosion on as-received steel. No rust or rusting patterns should be observed | |
| 7.2.5.2 High humidity test | Panel exposed to 95-100% humidity at 38-46°C for 24-26 hours. | Test the humidity level to which steel was exposed. Should be no corrosion. | |
| 7.2.5.3 Copper sulphate test | Solution of copper sulphate applied for 6 minutes. Sample rinsed and dried to prevent loss of copper deposits. | Determine the type of stainless steel. To check for corrosion by testing for metallic iron oxide. | |
| 7.3.1 | Solvent ring test | High purity solvent placed on microscopic slide, residual pattern after evaporation is observed. Solvent placed on clean panel, drop of resulting mixture is placed on new microscopic slide for comparison. | Determine if contaminants/foreign material has been dissolved by the steel. To test for the presence of adherent transparent films. |
| 7.3.2 | Black light inspection | Panel is inspected under ultraviolet lamp for trace contaminants. | Detect the presence of certain oil films not present under white light |
| 7.3.3 | Atomiser test | Water sprayed on surface and the pattern of wetting is observed. | Test for the presence of hydrophobic films. |
| 7.3.4 | Ferroxyl test for free iron | Solution of potassium ferricyanide and nitric acid spread on clean panel. | Detect iron contamination. |
Results
7.2 and 7.2.1 The as received panels show evidence of pits, suggesting pitting corrosion.
7.2.2 Contaminants from the surface have an oily appearance. The sulphur and chlorides present are generally found in oil and grease or residual adhesive film.
7.2.4 Draining water layer broke into a discontinuous film suggesting the presence of hydrophobic contaminants from oil, grease or residual adhesive film.
7.2.5.1 After 24 hours no signs of corrosion, suggesting absence of free iron on the surface.
7.2.5.2 Minor surface contamination of free iron.
7.2.5.3 Small pits on the surface of the panel experience a colour change from reddish brown to black due to the formation of copper oxide. This confirms the presence of free iron.
7.3.1 Solvent ring test shows confirms the presence of foreign material that has been dissolved by the high purity solvent.
7.3.2 Black light test shows contamination from oil and grease lubricants (Figure 2).
7.3.3 Wetting pattern suggests the presence of hydrophobic contaminants since the spray droplets do not wet the surface but remain as fine droplets.
7.3.4 Presence of blue stain suggests the presence of iron on the surface.
Figure 2: Black light test
- Hydrophobic contaminants are present on the surface of the stainless steel panels. These could come from oil and grease or residue of the adhesive on the surface of the panels.
- There are very few free iron particles on the surface of the stainless steel panels. This contamination is probably in the passive layer since the reaction occurred only when copper sulphate and the ferroxyl prepared solutions were applied.
- No rust stains were observed when water was applied which confirms the absence of free iron particles in AREA 3.
