A comparison between the corrosion resistances of some HVOF-sprayed metal alloy coatings

This study compares the corrosion resistance of one Co-based alloy coating, namely Co-28Mo-17Cr-3Si (similar to Tribaloy-800), four Ni-based alloy coatings, namely Ni-17Cr-4Fe-4Si-3.5B-1C (Diamalloy-2001), Ni-20Cr-10W-9Mo-4Cu-1C-1B-1Fe (Diamalloy-4006), Ni-22Cr-9Mo-4Nb-5Fe (similar to Inconel-625), Ni-32Mo-16Cr-3Si-2Co (similar to Tribaloy-700), and a (WC-12Co)-33Ni-9Cr-3.5Fe-2Si-2B-0.5C cermet-Ni alloy blend coating. They were produced by liquid-fuelled HVOF spraying onto AISI1040 steel plates. Electrolytic hard chrome (EHC) plating was characterised as a reference material, to verify whether some HVOF coatings are suitable as an EHC replacement. The microstructure of the coatings was examined by SEM and XRD. Electrochemical polarization tests and free corrosion tests were performed in 0.1 M HCl aqueous solution; the corrodkote test (ASTM B380-97R02) was also performed, to rank coatings qualitatively.The lowest corrosion current densities (I_corr) were recorded for EHC and Tribaloy-700. The latter coating contained few secondary phases and little porosity; the damage was mainly due to corrosion activation along lamellae boundaries. Diamalloy-2001 exhibited the highest I_corr and was significantly damaged after the polarization test, as its multi-phase microstructure had triggered severe galvanic corrosion. During free corrosion in 0.1 M HCl, Tribaloy-700 and Diamalloy-4006 retained rather stable polarization resistance (R_p), whereas the R_p of EHC decreased significantly. Tribaloy-700 survived 40 h of corrodkote test with no apparent damage and EHC underwent limited pitting corrosion. All other coatings had visible corrosion. The Inconel-625 coating failed to protect the substrate after 20 h of testing, due to inadequate processing conditions.     

Electrochemical behaviour of cermet coatings with a bond coat on Al7075: Pseudopassivity, localized corrosion and galvanic effect considerations in a saline environment

The behaviour of an HVOF WC-17Co/Ni-5Al coating on Al7075 in aqueous NaCl is investigated. The coating susceptibility to localized corrosion depended on the potential of polarization reversal. A two-stage pseudopassivity was observed for WC-17Co: At low overpotentials, oxidation occurred in the binder leading to surface films composed of anhydrous Co- and W-oxides. At high overpotentials, oxidation extended to the carbide phase leading to the formation of hydrated WO₃ films. Ni-5Al notably reduced the galvanic effect between WC-17Co and Al7075, whereas it hindered corrosion propagation into the substrate. The coating showed a high corrosion resistance during salt spraying for 49 days.     

The polarization characteristics of Pb-free Sn-8.5Zn-XAg-0.1Al-0.05Ga alloy in 3.5% NaCl solution

The polarization characteristics of Pb-free Sn-8.5Zn-XAg-0.1Al-0.05Ga alloy with varying Ag content were investigated in 3.5% NaCl solution. The value of Ag content i.e X varied from 0.1 wt% to 2 wt%. An increase in the Ag content up to 2 wt% resulted in a progressive increase in the corrosion current density and shifted the corrosion potential (E_corr) towards more active values. These changes were also reflected in the corrosion rate of the solder alloy. However the linear polarization resistance values decreased with increase in the weight content of silver. Increasing the Ag content from 0.1 to 2 wt% did not show a significant improvement in the passivation behaviour of the solder alloy as reflected in the passivation current density (i_p) and critical current density (i_cc) values. XPS and SIMS depth profile results revealed that the oxides of Zn and Sn were present in high concentrations on the outer surface of the solder alloy along with very low concentration of Al₂O₃.     

Corrosion behaviour of thermal sprayed nitinol coatings

NiTi alloy is here investigated as an alternative coating to stainless steel since it is considered to possess good corrosion properties. Three different thermal spray techniques (high velocity oxy-fuel -HVOF-, vacuum plasma spray -VPS- and atmospheric plasma spray quenching -APS+Q-) have been used for building the coatings, and electrochemical tests have been carried out for corrosion evaluation. Open-circuit tests have revealed that the VPS-coating shows fairly good corrosion resistance, both in the as-sprayed and polished forms. The HVOF coatings however, showed a strong dependence on the surface conditions and APS+Q is dominated by electrolyte penetration through coating cracks, thus exhibiting a higher i_corr.     

Corrosion assessment of nitric acid grade austenitic stainless steels

The corrosion resistance of three indigenous nitric acid grade (NAG) type 304L stainless steel (SS), designated as 304L1, 304L2 and 304L3 and two commercial NAG SS designated as Uranus-16 similar to 304L composition and Uranus-65 similar to type 310L SS were carried out in nitric acid media. Electrochemical measurements and surface film analysis were performed to evaluate the corrosion resistance and passive film property in 6 N and 11.5 N HNO₃ media. The results in 6 N HNO₃ show that the indigenous NAG 304L SS and Uranus-65 alloy exhibited similar and higher corrosion resistance with lower passive current density compared to Uranus-16 alloy. In higher concentration of 11.5 N HNO₃, transpassive potential of all the NAG SS shows a similar range, except for Uranus-16 alloy. Optical micrographs of all the NAG SS revealed changes in microstructure after polarization in 6 N and 11.5 N HNO₃ with corrosion attacks at the grain boundaries. Frequency response of the AC impedance of all the NAG SS showed a single semicircle arc. Higher polarization resistance (R_P) and lower capacitance value (CPE-T) revealing higher film stability for indigenous NAG type 304L SS and Uranus-65 alloy. Uranus-16 alloy exhibited the lowest R_P value in both the nitric acid concentration. Auger electron spectroscopy (AES) study in 6 N and 11.5 N HNO₃ revealed that the passive films were mainly composed of Cr₂O₃ and Fe₂O₃ for all the alloys. The corrosion resistance of different NAG SS to HNO₃ corrosion and its relation to compositional variations of the NAG alloys are discussed in this paper.     

Wear,erosion and corrosion resistance of HVOF-sprayed WC and Cr3C2 based coatings for electrolytic hard chrome replacement

Thermally sprayed carbide-based coatings are nowadays extensively considered as an alternative to electrolytic hard chrome (EHC) coatings to reduce the environmental impact and the overall cost associated with EHC process. In this investigation, high-velocity oxy-fuel (HVOF) spray process was employed to prepare coatings using the traditional carbide powders namely the WC-10Co4Cr, the Cr₃C₂-25NiCr and a new type of mixed carbide powder WC-40Cr₃C₂-25NiCr. The Powder deposition rate, basic mechanical properties, abrasive wear, slurry erosion and corrosion resistance of the three coatings were then compared with the EHC coating. The results show that WC-10Co4Cr coating exhibited the highest hardness, abrasive wear and slurry erosion resistance followed by WC-40Cr₃C₂-25NiCr, EHC, and Cr₃C₂-25NiCr coating. The deposition efficiency of the powders as per hierarchy was found to be WC-40Cr₃C₂-25NiCr > WC-10Co4Cr > Cr₃C₂-25NiCr and all the HVOF sprayed coatings exhibited higher corrosion resistance than EHC coating. The highest powder deposition efficiency coupled with low density, acceptable tribo-corrosion performance, as well as low post processing cost makes the HVOF sprayed WC-40Cr₃C₂-25NiCr coating a potential candidate to replace the EHC coating.     

Inhibitory effect of some amino acids on corrosion of Pb-Ca-Sn alloy in sulfuric acid solution

The present article describes the inhibition effect of amino acids cysteine (Cys), methionine (Met) and alanine (Ala), towards the corrosion of lead-alloy (Pb-Ca-Sn) in H₂SO₄ solution by potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), weight loss measurement and scanning electron microscopy (SEM) methods. The influence of inhibitor concentration, temperature and time on inhibitory behavior of the amino acids was investigated. The corrosion data including corrosion current density (I_corr), corrosion potential (E_corr) and charge transfer resistance (R_ct) were determined from Tafel plots and EIS. Recording impedance spectra showed that the charge transfer resistance is increased by increasing adsorption time. The SEM micrographs revealed that the corroded surface area is decreased in the presence of amino acids. Meanwhile, the inhibition efficiency (IE) was found to be depending on the type of amino acid and its concentration. The IE for 0.1 M Cys in 0.5 M H₂SO₄ is greater than 96%. Adsorption isotherms were fitted by Langmuir isotherm.     

Corrosion behaviour of AIP NiCoCrAlYSiB coating in salt spray tests

NiCoCrAlYSiB coatings were deposited by arc ion plating (AIP) and annealed/pre-oxidised under various conditions. The corrosion behaviour of as-deposited and annealed/pre-oxidised coatings was studied by salt spray testing in a neutral mist of 5 wt% NaCl at 35 °C for 200 h. The results showed that the as-deposited NiCoCrAlYSiB coating behaved poorly while the annealed and pre-oxidised ones performed much better in salt spray tests. The dense microstructure in annealed coatings and formation of α-Al₂O₃ scales on the surface during pre-oxidation improved the corrosion resistance in salt spray test. The corrosion process was investigated from the aspects of corrosion products, and its electrochemical mechanism was proposed as well.     

Effect of surface finishing of a Zr-based bulk metallic glass on its corrosion behaviour

Zr-based metallic glasses passivate spontaneously, but exhibit also a certain pitting susceptibility. On the example of the Zr₅₉Ti₃Cu₂₀Al₁₀Ni₈ alloy studied in 0.01 M Na₂SO₄ + x M NaCl (x = 0-0.1) electrolytes it is demonstrated that the surface finishing state and the pre-exposure conditions can significantly influence the free corrosion and anodic polarisation behaviour. Mechanical fine-polishing procedures can lead to extremely smooth topographies but also to Cu enrichment at the surface. This yields a pronounced Cu dissolution at low anodic polarisation prior to stable passivity and increases the pitting initiation susceptibility as compared to mechanically ground surface states.     

Poly(N-methylaniline) coatings on stainless steel by electropolymerization

Poly(N-methylaniline) (PNMA) coatings have been electropolymerized on 304 stainless steel alloy by potentiodynamic, galvanostatic and potentiostatic synthesis techniques from aqueous solutions of 0.1 M N-methylaniline (NMA) and 0.3 M oxalic acid. Characterization of PNMA coatings was carried out by cyclic voltammetry, UV-Vis and FTIR spectroscopy techniques. Corrosion behavior of PNMA coated stainless steel electrodes was investigated using linear anodic potentiodynamic polarization, Tafel test, chronoamperometry and electrochemical impedance spectroscopy (EIS) techniques in 0.5 M aqueous HCl solutions. Corrosion test results showed that PNMA coatings possessed protection to uncoated stainless steel against corrosion.     

Galvanic corrosion between the constituent phases in duplex stainless steel

The exclusive single-phase with the exact chemical composition of the constituent phase in 2205 duplex stainless steel (DSS) could be prepared using selective dissolution method. The respective electrochemical behavior of each constituent phase could then be measured. The experimental results showed that the two distinct peaks in the active-to-passive transition region of the polarization curve determined in 2 M H₂SO₄ + 0.5 M HCl mixed solution were actually corresponded to the respective anodic peaks of the single austenite and ferrite phases. A polarity reversion was found between austenite and ferrite phases in mixed H₂SO₄ + HCl solution and HNO₃ solution. Galvanic current measurements also revealed that austenite was anode in HNO₃ solution, but became cathode when exposed in 2 M H₂SO₄ + 0.5 M HCl mixed solution.     

Corrosion characterization of new tin-silver binary alloys in nitric acid solutions

Electrochemical techniques were used to characterize the corrosion behavior of four new binary alloys xSn-Ag (x = 26, 50, 70 and 96.5 wt%) alloys and their individual metal components in nitric acid solutions. The experimental data were collected by using open-circuit potential, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). Over the concentration range studied (0.075-4.5 M), each of the two corrosion parameters (E_corr and i_corr) shows a regular dependency on both the alloy composition and the solution concentration. In general, for all studied samples, especially pure Ag and those with lower Sn contents (26 and 50 wt%), increasing the acid concentration increases i_corr, meanwhile causes a shift of the corresponding E_corr towards more positive values. This is probably due to the increase in the effect of cathodic depolarizer as the nitric acid concentration is increased. EIS results at the free corrosion potential confirmed well this behavior, where at concentrations ?1.5 M the thickness of the surface film increases while its resistance decreases with increasing tin wt%, indicating formation of less protective thicker film. However, at higher concentrations all samples exhibit identical behavior.     

Corrosion behavior of equal-channel-angular-pressed pure magnesium in NaCl aqueous solution

Effect of microstructure change on corrosion behavior of equal-channel-angular-pressed (ECAPed) pure Mg was investigated. The ECAPed sample after 6 passes obtained finer grains (50-100 μm) compared with as-cast one (800-1500 μm). The strain-induced grain refinement with more crystalline defects weakened corrosion resistance of pure Mg, resulting in more and deeper pits after in-situ corrosion, higher mass-loss rate immersed in NaCl solution, larger I_corr values in polarization curves and lower fitted R_t values in EIS plots. However, the enhanced initial OCP values indicate better weather resistance. Furthermore, corrosion improvement can be expected by reducing defects via subsequent annealing.     

Electrochemical corrosion and impedance study of SAE1045 steel under gel-like environment

To present simulative study on corrosion of metal by sewage sludge, three kinds of gel-like systems based on SiO₂ gel and polyacrylamide gel are proposed. Comparative results of electrochemical polarization and impedance studies together with coupon test of SAE1045 steel under these gel-like surroundings and deionized water are investigated. Obvious pitting corrosion characteristic of the steel can be seen in gel-like systems with decreasing I_corr and more negative value of E_corr. It is considered to be caused by electric field concentration effect at defected sites in covering layer on the surface of the steel formed by gel particles.     

Corrosion behavior of Ni-based amorphous alloys and their crystalline counterparts

In this paper, we report the corrosion behavior of Ni₅₉Zr₂₀Ti₁₆Si₂Sn₃ and Ni₅₃Nb₂₀Ti₁₀Zr₈Co₆Cu₃ (at.%) amorphous alloys with glass forming ability (GFA) of ∼3 mm, and their crystalline counterparts in severe corrosive environment of 1 M HCl aqueous solution. From the results of polarization curves and X-ray photoelectron spectroscopy (XPS), it was found that the corrosion behavior in Ni-based amorphous alloys is very sensitive to the compositions and the structural homogeneity is favorable for their corrosion-resistance in 1 M HCl aqueous solution.     

Electrochemical behaviour of Ni-base alloys exposed under oil/gas field environments

The electrochemical behaviour of Ni-base alloys (Inconel 625, Inconel 718, G3 and Incoloy 825) is carried out at 80 °C in CO₂/H₂S corrosion environments using cyclic potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) techniques. The passivity mechanisms are analysed and discussed. In addition, some significant characterisation parameters such as E_corr, I_pass, E_pit, E_pp, ΔE and I_pass in cyclic polarisation curves are analysed and compared to reveal the corrosion resistance of various Ni-base alloys. The equivalent circuit model and ZsimpWin software are utilised to discuss the Nyquist plots of various Ni-base alloys. The diffusion mechanism in EIS measurement is discussed. The result shows that the corrosion resistance of the Ni-base alloys to CO₂ corrosion or CO₂/H₂S corrosion follows the sequence: Inconel 625 > G3 > Inconel 718 > Incoloy 825. H₂S works as a cathodic depolariser with accelerating initiation of the corrosion process.     

Repassivation of the damage generated by cavitation on UNS N08031 in a LiBr solution by means of electrochemical techniques and Confocal Laser Scanning Microscopy

The objective of this work is to study the influence of cavitation on the corrosion behaviour of Alloy 31, a highly-alloyed austenitic stainless steel (UNS N08031), in a LiBr heavy brine solution (992 g/L) at 25 °C. The presence of cavitation shifted the OCP value towards the active direction by 708 mV_Ag/AgCl, increased anodic current densities and passivation current density, i_p, and reduced the pitting potential, E_p.Repassivation behaviour of Alloy 31 has been investigated by using potentiostatic tests at different potentials. The current density transient obtained after interrupting cavitation was used to obtain the repassivation index, n, provided by the slope of the log i(t) vs. log t representation. The value of n decreased as the applied potential was increased, reaching values near zero for potentials close to the pitting potential. The damage generated during the potentiostatic tests has been quantified by means of Confocal Laser Scanning Microscopy.     

A newly synthesized glycine derivative to control uniform and pitting corrosion processes of Al induced by SCN anions - Chemical, electrochemical and morphological studies

A newly synthesized glycine derivative (termed GlyD), 2-(4-(dimethylamino)benzylamino)acetic acid hydrochloride, was used to inhibit uniform and pitting corrosion processes of Al in 0.50 M KSCN solutions (pH 6.8) at 25 °C. For uniform corrosion inhibition study, Tafel extrapolation, linear polarization resistance and impedance methods were used, complemented with SEM examinations. An independent method of chemical analysis, namely ICP-AES (inductively coupled plasma atomic emission spectrometry) was also used to test validity of corrosion rate measured by Tafel extrapolation method. GlyD inhibited uniform corrosion, even at low concentrations, reaching a value of inhibition efficiency up to 97% at a concentration of 5 × 10⁻³ M. Results obtained from the different corrosion evaluation techniques were in good agreement. This new synthesized glycine derivative was also used to control pit nucleation and growth on the pitted Al surface based on cyclic polarization, potentiostatic and galvanostatic measurements. The pitting potential (E_pit) and the repassivation potential (E_rp) increased by the addition of GlyD. Thus GlyD suppressed pit nucleation and propagation. Nucleation of pit was found to take place after an incubation time (t_i). The rate of pit nucleation and growth decreased with increase in inhibitor concentration. Morphology of pitting was also studied as a function of the applied anodic potential and solution temperature. Cross-sectional view of pitted surface revealed the formation of large distorted hemispherical and narrow deep pits. GlyD was much better than Gly in controlling uniform and pitting corrosion processes of Al in these solutions.     

Influence of flow on the corrosion inhibition of St52-3 type steel by potassium hydrogen-phosphate

Influence of hydrodynamic conditions on the corrosion of St52-3 type steel rotating disc electrode, RDE, in 3.5% NaCl and its corrosion inhibition using K₂HPO₄ have been studied. Results showed that by rotating the electrode in blank and inhibited solutions, corrosion current density, i_corr, increased, corrosion potential, E_corr, shifted toward more positive values and charge transfer resistance, R_ct, decreased. The inhibition efficiencies increased with electrode rotation rate. This increase was attributed to the enhanced mass transport of inhibitor molecules toward the metal surface and formation of more protective films. Little decrease of efficiencies at higher rotation speeds was probably because of the separation of protective films due to high shear stresses.     

Study of the electrochemical behaviour of nanocrystalline zinc by statistical methods

Pulse electrodeposition was used to synthesize nanocrystalline (NC) zinc coatings from citric acid bath. The electrochemical behaviour of the NC zinc coatings was investigated by using potentiostatic and potentiodynamic polarization methods in 0.5 mol/L NaCl (pH = 12) solution and compared with that of cast zinc. Pitting corrosion behaviour was characterized by pitting potential, induction time and stable pit growth rate which were analyzed according to statistical method. The results showed that nanocrystallization increased the sensitivity of E_pit refer to potential sweep velocity, changed the type of the pit generation from B1 (parallel) to B2 (series), accelerated the pitting initiation process and inhibited the stable pit growth process of NC zinc.