Electropolished 316L Stainless Steel Research Articles

Environmentally-assisted cracking of electropolished 316L stainless steel in molten FLiNaK salt

Environmentally-assisted cracking is a well-known and extensively studied phenomenon in light water reactor environments but remains relatively unexplored in proposed molten salt reactor systems where the mechanisms at play are expected to be quite different. Here, slow strain rate testing has been performed on 316L stainless steel tensile specimens during simultaneous exposure to a molten LiF-KF-NaF eutectic salt environment at 600°C with a strain rate between 1E-5 to 1E-7 s-1. Sample ductility and ultimate tensile strength were both observed to decrease for slower strain rates/longer salt exposures. Post-mortem characterization of fractured specimens revealed Fe deposits and Cr- and O-rich precipitates decorating surface grain boundaries and crack surfaces, suggesting a cracking mechanism driven by embrittlement of the crack tip due to oxide formation. Unlike previous studies, intergranular Cr corrosion into the bulk was not observed, and distinct differences in surface morphology between electropolished and non-electropolished sample surfaces are reported. Trace metal salt chemistry measurements showed accumulation of 316L alloying elements over the course of the tests. It appears likely that the surface oxide layer induced by electropolishing, in addition to oxygen and moisture impurities in the salt, led to the formation of LiCrO2 which had a net passivating effect that prevented significant bulk corrosion, though some interplay between mechanical and environmental stressors is still observed.

Electropolishing of 316L Stainless Steel Using Sulfuric Acid-Free Electrolyte

Abstract The study is to investigate the electropolishing characteristics of 316L stainless steel in a sulfuric acid-free electrolyte of phosphoric acid and glycerol and to explore the possibility of using this eco-friendly electrolyte instead of the widely used sulfuric acid-based electrolyte. The influences of process parameters on polishing effects and the corrosion resistance of electropolished samples are investigated. The experimental results show that the electropolishing temperature and acid concentration are directly related to the mass transport mechanism in the limiting current plateau region. The grain boundaries of workpiece were electrochemically dissolved faster than the grain themselves at the beginning of the electropolishing process because they are more reactive than grains. Moreover, the conventional sulfuric—phosphoric acid electrolyte was also used to electropolish the 316L stainless steel, and the electropolished surfaces were compared with the sulfuric acid-free electrolyte proposed in this study. When the sulfuric acid-free electrolyte was used to electropolish the 316L stainless steel, the X-ray photoelectron spectroscopy (XPS) analysis shows that atomic Cr/Fe ratio of 316L stainless steel was increased from 0.802 to 1.909 after electropolishing process in the sulfuric acid-free electrolyte of phosphoric acid and glycerol. The corrosion resistance of the electropolished 316L stainless steel is studied using electrochemical analysis, and the results are verified experimentally.

Comparison of electropolished 316L steel samples manufactured by SLM and traditional technology

PurposeThis paper aims to present the way of modifying surfaces of 316L stainless steel elements that were manufactured in the selected laser melting (SLM) technology and then subjected to mechanical and electrolytic processing (electropolishing [EP]). The surface of the as-generated and commercial produced parts was modified by grinding and EP, and the results were compared. The authors also present an example of the application of EP for the final processing of a sample technological model – an initial prototype of a 316L steel implant manufactured in the SLM technology.Design/methodology/approachThe analyzed properties included surface topography, roughness, resistance to corrosion, microhardness and the chemical composition of the surface before and after EP. The roughness described with the Ra, Rt and Rz was determined before and after EP of samples manufactured from 316L steel with use of traditional methods and additive technologies.FindingsEP provides us with the opportunity to process elements with a complex structure, which would not be possible with use of other methods (such as milling or grinding). Depending on the expected final surface of elements after the SLM process, it is possible to reduce the surface roughness with the use of EP (for t = 20 min, Ra = 3.53 ± 0.37 µm and for t = 40 min, Ra = 3.23 ± 0.22 µm) or mechanical processing and EP (for t = 4 min, Ra = 0.13 ± 0.02 µm). The application of the EP method to elements made from 316L steel, in a bath consisting of sulfuric acid (VI), H2SO4 (35 Vol.%), phosphoric acid (V), H3PO4 (60.5 Vol.%) and triethanolamine 99 per cent (4.5 Vol.%), allows us to improve the surface smoothness and to obtain a value of the Ra parameter ranging from 0.11 to 0.15 µm. The application of a current density of 20 A/dm2 and a bath temperature of 55ºC results in an adequate smoothing of the surface (Ra < 0.16 µm) for both cold rolled and SLM elements after grinding. The application of EP, to both cold rolled elements and those after SLM, considerably improves the resistance to corrosion. The results of potentiodynamic corrosion resistance tests (jkor, EKA and Vp) of the 316L stainless steel samples demonstrate that the values of Vp for elements subjected to EP (commercial material: 1.3·10-4 mm/year, SLM material: 3.5·10-4 mm/year) are lower than for samples that were only ground (commercial material: 4.0·10-4 mm/year, SLM material: 9.6·10-4 mm/year). The microhardness was found to be significantly higher in elements manufactured using SLM technology than in those cold rolled and ground. The ground 316L steel samples were characterized by a microhardness of 318 HV (cold rolled) and 411 HV (SLM material), whereas the microhardness of samples subjected to EP was 230 HV (commercial material) and 375 HV (SLM material).Originality/valueThe 316L samples were built by SLM method. The surface of the SLM samples was modified by EP. Surface morphological changes after EP were studied using optical methods. Potentiodynamic tests enabled to notice changes in the corrosion resistance of 316L. Microhardness results after electropolished 316L stainless steel were shown. The chemical composition of 316L surface samples was presented. The smoothening of the surface amounted to Ra = 0.16 µm.

The Study of the Thin Films Formed by Macromolecular Compounds on the Materials Surface Used in Implantology

The corrosion inhibition of 304L stainless steel in physiological serum (PS) in the presence of poly (vinyl) alcohol (PVA) is discussed according to electrochemical measurements. The layer composition formed on stainless steel surface was estimated using Scanning Electron Microscopy (SEM) and X-ray Photoelectron Spectroscopy (XPS) techniques. Electrochemical measurements indicated that PVA presence in PS solution decreases the corrosion current and increases the polarization resistance. XPS analysis showed that the surface layer consisting of PVA contains small amounts of other elements, such as: Na, Cl. The surface characterization of electro polished 316L stainless steel by using poly (O-2-hydroxyethyl) starch (PETA) in physiological serum (PS) is discussed according to electrochemical measurements, Scanning Electron Microscopy and Mossbauer Spectroscopy. The results obtained through polarization curves indicate that PETA reduces anodic dissolution and also decreases the corrosion current (icorr). Scanning Electron Microscopy (SEM) showed no evidence of corrosion pits on the surfaces of electro polished 316L stainless steel, but the formation of a thick film on the steel surface was observed. The Mossbauer spectrometry proves uniformity, compactness and stability of the passive films. More patents detailed the methods for producing a corrosion- inhibiting coating on the implants made of biocorrodable materials. Keywords: Stainless steel, thin film, polymers, XPS, SEM, Mossbauer spectroscopy, cellular micropatterning, biocompatibility, Electrochemical Measurements, XPS surface analysis, polymer coatings, Tafel Polarization, poly ethylenimine, metallic surface by PVA

Plasma polymerized allylamine films deposited on 316L stainless steel for cardiovascular stent coatings

Coronary stents are metallic (316L stainless steel) medical devices used during balloon angioplasty to scaffold diseased arteries and prevent their reblockage. To reduce the restenosis rate, bare metal stent coating is a promising solution. The coating can protect the metallic surface of the stent from corrosion attack caused by the biological environment. In addition, according to Food and Drug Administration (FDA) the coating properties must be guaranteed even after stent expansion. The aim of this study was to develop a dry process to coat the metallic surface from the biological environment by depositing an ultra-thin, stable, cohesive and adhesive plasma polymerized allylamine (CH2=CH―CH2―NH2) coating with high selectivity towards primary amine groups. Plasma polymerized allylamine (PPAA) coatings were deposited on electropolished 316L stainless steel (316L SS) samples using a low pressure plasma reactor (70kHz). XPS (X-Ray Photoemission Spectroscopy) and FTIR-ATR (Fourier Transform Infrared-Attenuated Total Reflectance) spectroscopy measurements were used to investigate the chemical composition of the coatings. A chemical derivatization technique was employed in order to quantify the amine retention rate of the deposited films. Morphology of the films was evaluated by FE-SEM (Field Effect-Scanning Electron Microscopy) imaging. Furthermore, special attention was devoted to study the stability of the coating and its adhesion properties after plastic deformation up to 25%. The effect of the power discharge and treatment time on these properties was also investigated. Our results showed that coatings present the required adhesion and cohesion properties to be stable upon deionised (D.I.) water immersion and to resist to a stent expansion.

Plasma Surface Modification of 316L Stainless Steel for Cardiovascular Stent Coating

Coronary stents are metallic (316L stainless steel) devices employed during balloon angioplasty to reopen and prevent the re-obstruction of a diseased narrowed area within a coronary artery. To reduce restenosis rate, bare metal stent coating is a promising solution. The coating can act as an anticorrosive barrier against the aggressive properties of biological environment, improving the long-term safety of the device. The goal of this study is to develop a dry process to isolate metallic surface from the biological environment by depositing a thin plasma polymerized allylamine (CH2=CH-CH2-NH2) film on the metallic surface. Plasma polymerized allylamine films were deposited on flat electropolished 316L stainless steel samples in a low pressure plasma reactor (70 kHz). Chemical composition of the coatings has been analysed as a function of the discharge power and treatment time. Moreover, special attention has been paid on the stability of the coating after immersion during 24 hours in D.I. water. Finally, to mimic stent expansion conditions, a “small punch test” has been used to investigate the adhesive properties of the coating. Our results demonstrate that is possible to deposit a stable, cohesive and adhesive plasma polymerized allylamine thin film which can be used as a coating for cardiovascular stents

Deposition of gold–titanium and gold–nickel coatings on electropolished 316L stainless steel bipolar plates for proton exchange membrane fuel cells

The effects of electropolishing and coating deposition on electrical resistance and chemical stability were studied for the stainless steel bipolar plates in proton exchange membrane fuel cell (PEMFC). A series of 316L stainless steel plates, selected as the substrate for a proton exchange membrane fuel cell (PEMFC) bipolar plate, were electropolished with a solution of H 2SO 4 and H 3PO 4 at temperatures ranging from 70 to 110 °C. The surface regions of the two electropolished stainless steel plates were coated with gold and either a titanium or nickel layer using electron beam evaporation. The electropolished stainless steel plates coated in 2-μm thick gold with a 0.1-μm titanium or nickel interlayer showed remarkably smooth and uniform surface morphologies in AFM and FE-SEM images compared to the surfaces of the plates that were coated after mechanical polishing only. The electrical resistance and water contact angle of the deposited stainless steel bipolar plates are strongly dependent on the surface modification treatments (i.e., mechanical polishing versus electropolishing). ICP-MS and XPS results indicate that after electropolishing, the coating layers show excellent chemical stability after exposure to an H 2SO 4 solution of pH 3. Finally, it was concluded that before coating deposition, the surface modification using electropolishing was very suitable for enhancing the electrical property and chemical stability of the stainless steel bipolar plate.

Study on corrosion resistance of gas cylinder materials in HF, HCl and HBr environments

Corrosion properties of Mn steel, Cr–Mo steel and 316L stainless steel and effects of surface treatments including Cr or Ni coating, nitriding and oxynitriding on the corrosion resistance of steels were examined in halogen acid gas and solutions. Corrosion behaviours were evaluated by the potentiodynamic polarisation tests in 0·001M HF, HCl and HBr solutions and the exposure test in HBr gas with 1·13 ppm H2O for 24 h. The surface morphology of the tested samples was examined by optical microscope and SEM and the composition of the corrosion product was analysed by EDS after exposure of the tested alloys to HBr gas. The corrosion resistance of Mn steel and Cr–Mo steel were very poor but was considerably improved by Cr or Ni coating. Oxynitriding also reduced the corrosion rate while nitriding was not effective. However the corrosion resistance of steels with Cr, Ni plating or oxynitriding was still much inferior to electropolished 316L stainless steel, which was practically not damaged by corrosion tests in aqueous solutions nor in gas.

Evaluation of the Adhesion of Ultra-Thin Teflon-Like Films Deposited by Plasma on 316L Stainless Steel for Long-Term Stable Drug-Eluting Stents

Metallic intravascular stents are medical devices commonly made of 316L stainless steel or nitinol used to scaffold a biological lumen, most often diseased arteries, after balloon angioplasty. Stenting procedures reduce the risk of restenosis, but do not eliminate it completely. Indeed, restenosis remains the principal cause of clinical complications, leading to up to 30 % of failure after 3 months of implantation. During the last few years, several works have been focused on the development of an appropriate coating able to act as a carrier for specific anti-restenosis drugs. Moreover, this coating would act as an anti-corrosive barrier, thus inhibiting the release of potentially toxic ions. Actually, the main challenges in stent coatings are to synthesize a biocompatible polymer coating resistant to blood flow, wall shear stress and tensile force after the stent deployment which results in a permanent strain of up to 25%. The adhesion and chemical resistance after deployment are critical properties to investigate for the improvement of the long-term reliability of polymer coated stent. The aim of this study was to evaluate the effect of a 25% equivalent plastic deformation on chemical, mechanical and adhesion properties of Teflon-like films deposited on 316L stainless steel. These properties were studied by chemical spectroscopy and atomic force microscopy. Teflon-like films were deposited by pulsed plasma glow discharges on flat electropolished 316L stainless steel. An original method has been developed to induce the deformation, and preliminary results have showed that the 12 nm thick Teflon-like films successfully resist to deformations of up to 25%.

Effects of welding on the passive oxide film of electropolished 316L stainless steel

In semiconductor manufacture the corrosive fluid delivery systems are subjected to the same scrutiny as the chemicals and gases used, to prevent any contribution to contamination, corrosion, or particle generation. Electropolished 316L stainless steel has become the industry standard due to its superior corrosion resistance and the passive oxide layer formed by the electropolishing. However, welding of the tubing often leads to discoloration in the heat-affected zone (HAZ) leading to corrosion. In this study, specimens from various heat lots were welded under identical parameters, with varying concentrations of oxygen leaked into the argon purge gas during the welding. The chemical composition and thickness of the discoloration in the HAZ and a clean reference area were analyzed by Auger electron spectroscopy. The cause of the discoloration was due to iron diffusing through the chromium oxide layer in the HAZ in the sensitizing temperature range and forming an iron oxide layer, the thickness of which depended upon the concentration of oxygen available in the purge gas. The composition of the original steel was found to be only a minor contributor to the extent of the discoloration.

Materials selection for HBr service

This study was undertaken to determine the compatibility of hydrogen bromide (HBr) with common materials of construction used for specialty gas delivery systems. Reactions between reactive gases and materials of construction can result in the formation of particles and volatile metal complexes as well as the creation of corrosion products that can retain water. We found that when moisture is below 1 ppm v (designated as anhydrous), bromine from HBr is not incorporated beyond the native oxide of electropolished 316L stainless steel (EP316L) and no macroscopic degradation of the metal occurs. Also, if adequate purging and evacuation procedures are followed to remove the HBr, this material can be exposed to moist air without diminishing the initial surface quality. However, if adequate precautions are not followed to eliminate water in the presence of HBr, iron-and bromine-rich crystalline deposits form on the surface. Purge and evacuation procedures are inadequate for removal of the reactive species on this surface and corrosion proceeds upon subsequent exposure to air. EP316L exposed to HBr containing 1700 ppm v H 2O appears visually unaltered, but close inspection by SEM reveals the onset of corrosion. Of the materials examined in this study, Nickel-200 and Hastelloy C-22 are more resistant to HBr corrosive environments. In contrast, deleterious reactions occur between anhydrous HBr and elemental iron and the iron-rich surface of oxygen-passivated 316L.

New technique for the measurement of adsorbed moisture concentration on a solid surface

We have established a new measurement method to evaluate the adsorbed moisture concentrations on solid surfaces exposed to the atmosphere having high moisture content ranging from 100 ppm to a few percent. The technique is based on the variation of the electric conductivity of anhydrous hydrogen fluoride. It has been found that the amount of adsorbed moisture on electropolished 316L stainless steel surface in an atmosphere with moisture concentration of 1% and 3% at 25 °C (relative humidity of 32% and 96% respectively) are 2.5× 1016 and 1.25×1017 molecules/cm2, respectively. This represents 25 and 125 molecule layers of water. The adsorbed moisture has been confirmed to reach an equilibrium level within an exposure time of 1 s.

Environmental scanning electron microscopy for dynamic corrosion studies of stainless steel piping used in UHP gas distribution systems.

An ElectroScan ESEM was used for in situ corrosion studies on cold drawn electropolished 316L stainless steel tube surfaces in the as-received and passivated conditions. Corrosion product was removed as it formed and the tube surface was viewed before, during, and after corrosive attack. The corrosion process was followed in situ, and the sample features most susceptible to corrosion (draw lines, inclusions, etc.) were identified. In addition, X-ray photoelectron spectroscopy (XPS) was used to study the changes in surface chemistry after corrosive attack. This information provided clear evidence of relevant corrosion mechanisms and relative corrosion susceptibility.