Stainless Steel Post Weld Finishing/Post Fabrication Clean up -A Technical Review by Metal Science Technologies August 2010©

The excellent corrosion resistance of stainless steel can only be achieved if proper cleaning and finishing operations are carried out after any fabrication process as there will have been damage to the surface condition. The corrosion resistance of stainless steel arises from a chromium oxide film that forms a protective layer naturally on the surface of the steel. The chromium oxide film is extremely thin, a few angstroms, but strongly adherent and chemically stable which is commonly referred to as ‘passive’. The film is formed when chromium reacts with oxygen. The film may be described in

chemical terms as a Chromium iii oxide (Cr2O3 ) Maximum corrosion resistance only occurs when the film develops as an unbroken film across the entire structure. Any breaks in the film which will occur with fabrication means that the anodic base metal is in contact with the corrosive environment. As such an electrochemical pathway opens which will result in corrosion of the base metal. Welding leaves a weld scale (oxide scale /heat tint) which prevents the formation of the chromium oxide film. Surface contamination with iron particles, grease and dirt can also occur during fabrication such as cutting, grinding and general handling.

The high temperature of welding also results in severe chromium depletion not only at the weld surface but through out the heat affected area. Therefore removing surface material such as oxide scale is only half the job as the weld area is still chromium depleted relative to the parent stainless steel.

Critically when the weld is depleted of chromium its corrosive resistance is severely reduced. The weld is therefore vulnerable to pitting and crevice corrosion and what is often referred to as tea staining.

Research has shown that the best way to post-weld finish stainless steel is electro-polishing, which is an electrochemical process that increases density of the chromium throughout the weld surface and therefore the ability to create the chromium oxide film. Electropolishing provides the most dense and durable passive film that it is possible to achieve. Electropolishing has two benefits beyond removing weld scale. One is that it selectively dissolves the microscopic high points of the stainless and creates a microscopically smother surface on which contaminates can not bind or hide. Two is that it selectively removes iron at a higher rate than chromium so increases the chromium density. The following scanning electron microscopy Figure 1 (SEM) undertaken by Electromaterials Science Intelligent Polymer Research Institute

1 provides images of the morphology of the surface

before (A) and after post weld finishing with a direct current portable electropolisher (B),pickling paste (C) and an alternating current weld cleaning machine (D) Specimens were examined in the JEOL7500F SEM at 25kV accelerating voltage and 8mm.Representative images were taken along the weld line, in the heat-affected zone, away from the heat affected zone on the treated side and untreated side. Only the weld line is shown here. with a representative image (TIFF) taken in the area at 10,000x

Figure 1© A B

1 Intelligent Polymer Research Institute University of Wollongong NSW 2522 Australia 17/2/2010

C D

Welds cleaned with a direct current electropolisher have significantly better corrosion resistance than either the pickling paste C and electrochemical cleaning using alternating current D due to reduced micro scale and removal of weak points of the microstructure on the material surface and at grain boundaries. The etching effect of pickling paste predominately due to the aggression of hydrofluoric acid can be seen in C while probable contaminates and grains can be seen in D which create the dull effect which is seen by the naked eye Research was recently undertaken on the effects of welding and post weld finishing with electrochemical cleaning by Associate Professor Peter Innis at the University of Wollongong. 2Electrochemical polarisation experiments were used to evaluate the effect of several post weld

treatments. The experiments where conducted on TIG welds completed with 15 bar of standard Argon shielding gas and post weld treated with pickling paste, portable electropolisher with a direct current and an electrochemical weld cleaning machine with an alternating current on the heat affected zone and the welded area. Electrochemical cells Figure 2 were made on the welded and HAZs by sealing silicone tubing (4.2 mm ID) onto the surface with silicone polymer, while a Pt counter electrode and Ag|AgCl (3.5 M) reference completed the cell. Aerated, unbuffered 0.1 M NaCl electrolyte (pH 6.5) was used. The corrosion potential (Ecorr), corrosion current (Icorr), the pitting current at the vertex potential (I+1.5V) and repassivation potential (Erepass) were calculated from the anodic polarisation data and were used for quantitative comparison of samples.

Figure 2

The welded areas displayed high pitting currents and thus a propensity for pitting. Polishing treatments were shown to appreciably lower the pitting current of the welded area and an improved spontaneous repassivation. Anodic polarisation of the welded samples showed that the welded areas exhibited the largest pitting current and the most negative repassivation potentials compared to the HAZ and stainless steel areas. The pitting currents on the welded areas that had undergone polishing were significantly lower compared to their unpolished counterparts The results suggest that polishing treatments using electrochemical cleaning encourage formation a more passive surface oxide on the weld. All polishing treatments improve spontaneous passive oxide film formation compared to unpolished welds, as evidenced by the more positive Er pass. It is likely that this is due to removal of Fe and Fe-based oxides that have formed from the high temperature welding. This was

2 Electrochemical Polarisation Experiments Used To Evaluate the Effect of Polishing

Treatments II. Heat affected zone and welded area. 31/3/2010

consistent with the cathodic polarisation data, where extra cathodic current is also observed on the unpolished welded regions, suggesting reduced chromium oxides on the surface. Energy Dispersive X-ray Spectroscopy (EDS) which allows information on elemental composition to be obtained from the surface of the welded samples when they are in the scanning electron microscope. Tests

3 as presented in Figure 3 (chart below) showed that after electrochemical post

weld finishing that the composition of 304 BA Stainless Steel which has been TIG welded is returned to 19% Cr on the weld line being greater than the parent (The parent 304 material being minimum 18% chromium) There is no carbon detected on the surface noting that 304 SS specification would have a maximum of .08% carbon throughout the material

Figure 3 After Post Weld Finishing (EDS)

304 TIG Weld SS

Chemical formula

ms% mol% Sigma Net K ratio

Carbon (C)

Not detected

Not detected

Chromium (Cr)

19.11 20.32 .22 147696 0.3650586

Iron (Fe) 72.93 72.19 .31 372067 1.2464804

Nickel (Ni) 7.96 7.49 .52 27102 0.1255519

EDS detects x-rays emitted from the sample during bombardment by an electron beam to

characterize the elemental composition of the analyzed volume. Features or phases as small as 1 µm or less can be analyzed.

Electropolishing can be accomplished by immersion of the item in a large, acid-containing bath. It is

impractical to perform this on-site but it can also be achieved with a portable Electropolishing

machine or weld cleaning machine that runs on a direct current. This method uses an

electrochemical cell on the metal surface, akin to electropolishing. The weldment is connected as

the anode at which metal is dissolved, removing the oxide layer, leaving a clean surface. With a

direct current and specific solution chemistry, electropolishing can occur, which results in a high

surface lustre. Passivation which could be simply called cleaning re-establishes an oxide film on the surface only and is typically carried out by application of nitric acid, which is a highly corrosive and dangerous to exposed skin. Nitric acid is not aggressive enough to remove severe weld burn so nitric passivation is not used on welds as they would remain chromium depleted relative to the parent stainless steel Pickling using pickling paste which is typically a highly toxic chemical cocktail of nitric and hydrofluoric acids is also used for post weld finishing but as the scanning electron microscopy shows it has an etching effect, which can increase the surface roughness profile, resulting in an obvious contamination problem. This can appear to the naked eye as a dull stain. The correct OH&S requirements for workers handling pickling paste are onerous in process, time and cost and while they reduce the health risk they do not remove it. There are a growing number of restrictions on storage and use of pickling paste. The pickling process should generally include a pre-treatment, an after-treatment and a waste treatment, to reach the required result and to meet environmental requirements.

Cleaning with tools such as mechanically grinding, buffing, wire brushing and/or polishing the weld is a surface treatment and leaves scale residues and abrasion and often some residue from cleaning so is typically a poor corrosive resistant finish. It can be used as a preparatory step. While it aids corrosion resistance by reducing surface roughness the smearing effect creates an amorphous, deformed surface layer. Iron-oxides, polishing compounds and other materials become embedded and entrapped in the distorted structure, creating a vulnerability point for surface corrosion. . An investigation by Dr. Ian Ward Technical Manager at Sandvik Materials Technology, Australia

4

has been conducted to assess the performance of mechanically cleaned with 3M scotchbrite pad.

3 SEM & EDS Weld Analysis by Intelligent Polymer Research Institute University of Wollongong

21/12/2009 4 Sandvik Report on weld cleaning methods 15/8/2007 Author; Dr. Ian Ward

pickling paste and electrochemical cleaning to determine their effects on the corrosion resistance of weldment using 316L 2B mill finish MIG welds with argon purge on the root side. This investigation was carried out using a portable electrochemical potentiostat, which allows easy access to virtually any size object with various surface geometries even on-site. This is particularly important in the case of weldments, where analysis of specific areas of the heat affected zone and weld metal are needed as they can produce different results. The instrument runs polarization scans at a rate of 10 mV/s in order to determine the pitting potential. Corrosion tests were conducted on the parent metal, similarly treated parent metal, low temperature HAZ, high temperature HAZ and weld metal, after various forms of cleaning using 1M NaCl solution at 18 degC. The test results are summarized in Figure 4

Figure 4

A follow up test undertaken in May 20105 by Sandvik Materials Technologies and Metal Science

Technologies on MIG welded grade and Figure 6 shows the as-welded surface which retains

surface oxidation is susceptible to corrosion in the high temperature HAZ. The heavy oxidation of

the weld metal makes this susceptible also. Figure 7 show the surface which has been

electrochemical cleaned with a carbon fibre brush has given excellent all round results

Figure 6 as welded 304 MIG Weld ©

5 Weld Cleaning Corrosion Testing Report: Sandvik & for Metal Science Technologies 5/5/2011.Dr Ian Ward and

Dean Klower Document Ref. IW 20100505

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Figure 7 Electrochemical Cleaned with a carbon fibre electrode © The experiments concluded that pickling paste improves the corrosion resistance compared with that of the as-welded material but did not quite match that of the parent metal while the electro cleaning process not only recovered, but can improve the corrosion resistance to a level above that of the original parent metal. Best results are archived with electropolished direct current which gives a significantly better result than mechanical cleaning and pickling paste ©Contact Richard Ray: Metal Science Technologies 10a Jones Lane Teven NSW 2478 0424 030 187 Email: richard@metalscience.com.au DATE: August 2010

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