Surface and Coatings Technology, 49 (1991) 83—86
Niobium coatings on 316L stainless steel for improving corrosion resistance J. H. Hsieh, R. Lee, R. A. Erck and G. R. Fenske Materials and Components Technology Division, Argonne National Laboratory. Argonne, IL 60439 (U.S.A.)
Y. Y. Su, M. Marek and R. F. Hochman School of Materials Engineering, Georgia Institute of Technology. Atlanta, GA 30332 (U.S.A.)
Abstract Niobium coatings were deposited onto 316L stainless steel substrates by ion-beam-assisted deposition. The coatings, deposited under 250 eV ion bombardment with [Ar ]/[Nb] ratios ranging from 0.68 to 0.8, were dense and showed no sign of pittingcorrosion in a 3% NaCI solution. Also, based on the result of scratch tests, niobium coatings may act assacrificial anodes and protect substrates.
1. Introduction Niobium coatings can be used in applications where protection from aqueous corrosion or hot-gas erosion Ellis needed. The resistance of these coatings to liquidmetal corrosion and the low-capture cross-section for thermal neutrons make them attractive for reactor applications . Also, niobium coatings applied to SiC  and Al20~ substrates greatly reduce sliding friction and wear, implying that these coatings can be used as a solid lubricant on heat-resistant ceramic materials, Despite the usefulness of niobium coatings, only a few studies have been conducted on their applications, One reason is that they cannot be made by conventional as deposition electroplating or cladding. Althoughmethods physical such vapor (PVD) is a possible method, niobium coatings deposited by PVD at low substrate temperatures (low T/Tm value) usually consist of columnar grains with open boundaries. As a result, coating performance is impaired. In this study, niobium coatings were deposited onto 316L stainless steel (316L SS) substrates by ion-beamassisted deposition (IBAD). In IBAD, the concurrent bombardment of energetic particles during coating growth will change the coating density and microstructure, depending on the particle energy and flux. Therefore, to produce an effective corrosion protection coating, it is desirable to understand the effects of ion bombardment on growing niobium films and the subsequent corrosion behavior. 2. Experimental procedure 2.1. The ion-beam-assisted deposition The deposition of 2.5 pm thick niobium coatings was
performed at room temperature by IBAD. Figure 1 is a schematic diagram of the IBAD system. The electronbeam evaporation of niobium with a deposition rate of 0.4 nm s~ was controlled with a quartz-crystal rate monitor. During growth, the films were bombarded with 250 or 500 eV argon ions generated by 3 cm Kaufman-type ion guns. Various [Ar~]/[NbJ atomic ratios, from 0 (i.e. no ion bombardment) to 0.8, were used. The ion flux was measured with a beam calorimeter that is sensitive to ions and neutrals but insensitive to secondary electrons .The substrate was positioned at 450 to the vapor and ion fluxes, which were vertical and horizontal respectively. Before deposition, the substrates were sputter cleaned 2. for 20 mm with a 250 eV ion beam at 0.25 mA cm 2.2. Corrosion and scratch tests Protection of the niobium coatings was evaluated at room temperature in a 3% NaCI solution open to air. Potentiodynamic anodic polarization was chosen as the test technique. Although this technique may not be capable of distinguishing coatings deposited under different conditions, it can show if the niobium coatings can isolate SS substrates and behave like bulk niobium; thus it permits the determination of optimum deposition conditions. Galvanic scratch tests were performed to investigate whether a coating made of highly anodic material (i.e. niobium) can protect a less anodic substrate (i.e. 3l6L SS). In this test, the Nb—SS couple was immersed in the solution for I h, and the galvanic current was dropped to an unmeasurable amount. Niobium, SS, and both electrodes were scratched sequentially with a ceramic stylus, and galvanic currents were recorded against time.
Elsevier Sequoia, Lausanne
J. H. Hsieh
al. / Niobium coatings on stainless steel
Deposition Rate Montor
Substrate Energetic Ions
Vacuum Pump Fig. I. Schematic diagram of the ion-beam-assisted deposition (IBAD) system.
The corrosion cell was a 500 ml flat-bottomed glass vessel containing a working electrode (specimen) and a platinum counterelectrode. A Luggin probe was used to connect the test cell to a reference electrode where a standard calomel reference electrode (SCE) was located. The test cell was monitored by a corrosion measurement system (EG & G model 351).
-, _____ -
was the anode before it repassivated. When the niobium sample was scratched, the niobium sample became the anode. When both electrodes were scratched,
the niobium sample was the anode before both electrodes repassivated. When both electrodes were in the passive state, the potential of this couple was close to that of SS.
-500 -70C -10
LOG I A/cm
3.1. Corrosion behavior of 316L SS, niobium, and a 316L-SS—Nb couple The anodic polarization behavior of niobium and 316L SS is presented in Fig. 2. In this figure, 316L SS shows a pitting potential E~at 200 mV(SCE), whereas niobium shows no sign of pitting within the range of the scan. The passive current density of SS in this figure seems to be lower than that of niobium. However, under potentiostatic conditions, the passive current density of niobium is much lower than that of SS. Figure 3 shows the result of galvanic scratch tests of 316L SS and niobium. Both materials were in their passive state prior to scratching. When the SS sample was first scratched, the galvanic current reached a peak, then dropped to the initial value. In this case, SS
Fig. 2. Anodic polarization curves for pure niobium and uncoated 3l6L SS in a 3% NaCI solution,
3.2. Corrosion tests The corrosion behavior of the niobium-coated SS substrates after anodic potentiodynamic test scanning
J. H. Hsieh
al. / Niobium coatings on stainless steel
Time After Scratch (second)
Fig. 3. Changes in galvanic current after scratching of niobium, 316L SS, and both (Nb, +; SS, —). (a)
up to 800 mV(SCE) is presented in Table 1. The table also includes the deposition parameters. Samples 3 and 4 show a behavior similar to that of bulk niobium, i.e. no E~was observed. The 3l6L SS substrates were isolated from a 3% NaCI solution by niobium coating. Figure 4 shows the surfaces of the uncoated and niobium-coated 3l6L SS samples after corrosion tests. 3.3. Microstructure of niobium coatings Examination of niobium coatings by scanning electron microscopy (SEM) shows that the microstructure of the coatings changed, depending on the ion-bombardment parameters. Samples 3 and 4 appear to have the finest structure of all. Some cross-sectional micrographs of the niobium coatings are shown in Fig. 5. The microstructure of sample I (without ion bombardment) is columnar with open boundaries. Figure 5(b) shows a (b) typical cone-type columnar structure with open boundaries in the coating deposited under weak ion Fig. micrographs uncoated 316L4.SS,Scanning and (b) electron niobium-coated 3l6L of SSsurfaces (sample of 4) (a) after anodic bombardment. In Fig. 5(d), 500 eV ion bombardment at polarization test. [Ar~]/[Nb] 0.8 produced a somewhat different microstructure but one that still contained much porosity. The microstructure of sample 4, as illustrated in Fig. 5(c), is dense. 4. Discussion =
TABLE 1. Deposition parameters of niobium coatings and corrosion test results (coating thickness 2.5 pm) Sample
Ion energy (eV)
2 3 4 5 6
250 250 250 500 500
0.4 0.68 0.8 0.4 0.8
Corrosion behavior Cl a
Pitting Pitting Similar to Nb Similar to Nb Pitting Pitting
pitting potential (SCE).
As shown in Fig. 5, certain deposition conditions must be used to produce dense coatings. With little or no ion bombardment during deposition, the niobium coating contains coarse columnar grains and open boundaries, because of a self-shadowing effect during deposition. By applying 250 eV ion bombardment during deposition, the coating density is increased as the bombardment rate is increased. It has been suggested that densification of coatings by ion bombardment is a result of collisional events occurring near or at the surface of growing films . The collision of ions of .
sufficient energy with newly deposited atoms drives “knock-on” atoms deeper into the films, filling in porosities. However, the 500 eV bombardment seems,
J. H. H,sieh ef al. / Niobium coatings on stainless steel
sity increased after scratching, and pits were found inside the scratch track. Further study is under way.
5. Conclusions IBAD can be used to produce dense and protective
J Fig. 5. Scanning electron micrographs of some niobium coatings: (a) sample I. (b) sample 2. (c) sample 4. and (d) sample 6.
instead, to have created gaps between the columns. In part, this may have occurred because higher-energy ions produce cascade damage deeper into the film surface . Similar results were found for several coatings produced by IBAD, e.g. silver , chromium , nickel [101, and optical films . The observations of microstructure are consistent with the results of corrosion tests. The niobium coatings with fine microstructure (samples 3 and 4) prevent corrosive solutions from reaching the SS substrates. As a result, samples 3 and 4 show little or no pitting corrosion. Ion bombardment under the deposition conditions for samples 3 and 4 apparently reduces boundary porosity in these coatings. The results of galvanic scratch tests show that the anodic behavior of niobium coatings can be beneficial to SS substrates. As illustrated in Fig. 3, when niobium and SS are scratched simultaneously, niobium becomes the anode, in contrast to the cathode of the SS. Therefore, when niobium-coated SS is scratched, the newly exposed niobium may behave as a sacrificial anode and protect newly exposed SS. In some of our initial tests, a niobium-coated SS sample was scratched at a constant potential of — 100 mV. The current density dropped to the initial value after scratching, and no pitting was found inside the scratch track. When uncoated SS was scratched under the same conditions, the current den-
niobium coatings at room temperature. However, concurrent ion bombardment during deposition can be either advantageous or disadvantageous. Thus, deposition conditions necessary to produce dense coatings should be determined. In this study, the optimum conditions for bombarding ions during deposition were 250 eV and [Ar~]/[Nb] ratios ranging from 0.68 to 0.8. Under these conditions, 316L SS coated with niobium showed corrosion behavior similar to that of bulk niobium, where no pitting was observed. With the use of 500 eV Ar ions, IBAD was found to be less useful than it was with the use of 250 eV ions. An Nb—3l6L-SS galvanic couple repassivated after it was scratched. This suggests that a highly anodic material (niobium) coated on an anodic material (SS) may protect the substrate by sacrificial behavior.
Acknowledgment This work was partly supported by the Tribology Program, Office of Advanced Transportation Materials, U.S. Department of Energy, under contract W-31-109Eng-38.
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