The use of Al-Al2O3 cold spray coatings to improve the surface properties of magnesium alloys

Pure Al and 6061 aluminium alloy based Al₂O₃ particle-reinforced composite coatings were produced on AZ91E substrates using cold spray. The strength of the coating/substrate interface in tension was found to be stronger than the coating itself. The coatings have corrosion resistance similar to that of bulk pure aluminium in both salt spray and electrochemical tests. The wear resistance of the coatings is significantly better than that of the AZ91 Mg substrate, but the significant result is that the wear rate of the coatings is several decades lower than that of various bulk Al alloys tested for comparison. The effect of post-spray heat treatment, the volume fraction of Al₂O₃ within the coating and of the type of Al powder used in the coatings on the corrosion and wear resistance was also discussed.     

Microstructure and corrosion properties of plasma-sprayed NiCr-Cr3C2 coatings comparison with different post treatment

The effects of laser and plasma arc remelting on the microstructure and properties of plasma-sprayed NiCr-Cr₃C₂ coatings on steel substrates have been investigated. The microstructure of the coatings has been analyzed by X-ray diffraction (XRD) and scanning electron microscope (SEM). It is found that the Cr₃C₂, δ-(Cr,Ni), Cr₇C₃ and Cr₂₃C₆ phases were obtained for both coatings, before and after remelting treatment. The laser remelting was operated in a continuous way with 800 W power in different scan speed, while the plasma arc remelting was operated with a plasma cladding machine under different scan currents. However, the denser microstructure of both remelted coatings can be obtained, especially for the plasma arc remelted coating. The Vickers microhardness measurement showed certain enhancement values for both remelted coatings. The corrosion behavior was evaluated through salt spray corrosion (SSC) method. Energy-dispersive spectroscopy (EDS) showed that the chloride was produced during SSC process. The higher corrosion resistance for plasma arc remelted coating may be due to the more compact microstructure, less porosity rate and tensile residual stress. Compared with laser remelting method, plasma arc remelting is a cheap, convenient and effective remelting method.     

Corrosion behaviour of MoSx-based coatings deposited onto high speed steel by magnetron sputtering

MoS_x-based films were deposited using magnetron sputtering from a pure MoS₂ target. Alloying was accomplished by “co-deposition” from separate targets onto substrates having a two-fold rotation. An additional experiment had also a Cr⁺-ion etch for surface preparation, followed by a Cr adhesion layer, made using a Cr target mounted on a cathodic arc evaporation source. MoS_x and Al- and Ti-alloyed MoS_x coatings have been deposited onto high speed steel (HSS) and glass substrates for corrosion investigations.The coatings were characterised by scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, hardness and adhesion measurements. The corrosion behaviour of the samples was electrochemically measured by open-circuit-potential (OCP) measurements and by potentiodynamic corrosion tests in 0.8 M NaCl solution (pH 7). Additionally the MoS_x-based coatings on HSS have been exposed to salt spray tests. The corrosion investigations revealed that the addition of Al and Ti to MoS_x shifts the open-circuit-potential of about 80 to 110 mV to lower values, i.e. the alloying elements make the MoS_x coating a little bit less noble. In agreement with the OCP measurements, the corrosion potential E_corr in potentiodynamic corrosion tests was the highest for non-alloyed MoS_x coatings on HSS substrates. After the potentiodynamic corrosion tests, a strong corrosive attack could be observed for all coated samples. In salt spray tests the lowest number of corrosion pits was found for the MoS_x-Al (Cr⁺) coating on HSS.     

Study of structure and corrosion resistance of Fe-based amorphous coatings prepared by HVAF and HVOF

Fe-based amorphous coatings with a composition of Fe_49.7Cr₁₈Mn_1.9Mo_7.4W_1.6B_15.2C_3.8Si_2.4 have been prepared on a mild steel substrate by High velocity air fuel (HVAF) and High velocity oxygen fuel (HVOF) processes. The microstructure and corrosion resistance in 3.5 wt.% NaCl solution of the coatings prepared by the two processes were comparatively studied. It was found that the two coatings exhibit dense structure with the porosity of 0.4% and compact bonding with the substrate. However, HVOF coating contains higher oxygen content than HVAF coating, resulting from the formation of significant oxide contours between the partially melted particles in HVOF process. Electrochemical polarization tests and electrochemical impedance spectroscopy (EIS) analysis indicate that the HVAF coating has better corrosion resistance than the HVOF coating. The preferential corrosion along the oxide contours thus providing efficient diffusion channels for electrolyte accounts for the poor corrosion resistance in HVOF coating. The present results demonstrate that HVAF with less cost can be a promising spray process to fabricate the Fe-based amorphous coating for industrial applications.     

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.     

Corrosion Resistance of Copper Coatings Deposited by Cold Spraying

In the article, a study of corrosion resistance of copper and copper-based cermet (Cu+Al₂O₃ and Cu+SiC) coatings deposited onto aluminum alloy substrate using the low-pressure cold spraying method is presented. The samples were subjected to two different corrosion tests at room temperature: (1) Kesternich test and (2) a cyclic salt spray test. The selected tests were allowed to simulate service conditions typical for urban, industrial and marine environment. Examination of corroded samples included analysis changes on the coating surface and in the microstructure. The physicochemical tests were carried out using x-ray diffraction to define corrosion products. Moreover, microhardness and electrical conductivity measurements were conducted to estimate mechanical and physical properties of the coatings after corrosion tests. XRD analysis clearly showed that regardless of corrosion conditions, for all samples cuprite (Cu₂O) was the main product. However, in the case of Cu+Al₂O₃ cermet coating, chlorine- and sulfate-containing phases such as Cu₂Cl(OH)₃ (paracetamite) and Cu₃(SO₄)(OH)₄ (antlerite) were also recorded. This observation gives better understanding of the lowest microstructure changes observed for Cu+Al₂O₃ coating after the corrosion tests. This is also a justification for the lowest decrease in electrical conductivity registered after the corrosion tests for this coating.     

An investigation on oxidation,hot corrosion and mechanical properties of plasma-sprayed conventional and nanostructured YSZ coatings

Conventional and nanostructured YSZ coatings were deposited on IN-738 Ni super alloy by atmospheric plasma spray technique. The oxidation was measured at 1100 °C and hot corrosion resistance of the coating was measured in a mixed salt of V₂O₅ and Na₂SO₄ at 1050 °C using an atmospheric electrical furnace. According to the experimental results, nanostructured coatings showed a better oxidation and hot corrosion resistance than conventional one. The improved oxidation resistance of the nanocoating could be explained by the change of structure to a dense and more packed structure in this coating. The improvement in hot corrosion resistance was not as good as the oxidation but much better than conventional coating. The thermo-mechanical properties of the coating were tested using thermal cycles, nanoindentation, and bond strength tests during which nanostructured YSZ coating again showed a better performance by structural stability.     

A comparative study of tribological behavior of plasma and D-gun sprayed coatings under different wear modes

In recent years, thermal sprayed protective coatings have gained widespread acceptance for a variety of industrial applications. A vast majority of these applications involve the use of thermal sprayed coatings to combat wear. While plasma spraying is the most versatile variant of all the thermal spray processes, the detonation gun (D-gun) coatings have been a novelty until recently because of their proprietary nature. The present study is aimed at comparing the tribological behavior of coatings deposited using the two above techniques by focusing on some popular coating materials that are widely adopted for wear resistant applications, namely, WC-12% Co, A1₂O₃, and Cr₃C₂-MCr. To enable a comprehensive comparison of the above indicated thermal spray techniques as well as coating materials, the deposited coatings were extensively characterized employing microstructural evaluation, microhardness measurements, and XRD analysis for phase constitution. The behavior of these coatings under different wear modes was also evaluated by determining their tribological performance when subjected to solid particle erosion tests, rubber wheel sand abrasion tests, and pin-on-disk sliding wear tests. The results from the above tests are discussed here. It is evident that the D-gun sprayed coatings consistently exhibit denser microstructures and higher hardness values than their plasma sprayed counterparts. The D-gun coatings are also found to unfailingly exhibit superior tribological performance superior to the corresponding plasma sprayed coatings in all wear tests. Among all the coating materials studied, D-gun sprayed WC-12%Co, in general, yields the best performance under different modes of wear, whereas plasma sprayed Al₂O₃ shows least wear resistance to every wear mode.     

Self healing ability of silicate conversion coatings on hot dip galvanized steels

The self-healing ability of a coating is an important aspect for evaluating its corrosion resistance. Chromate conversion coatings show excellent anti-corrosion properties because of their compact microstructure and their strong self-healing ability. Silicate conversion coatings, which are potential replacement candidates for chromate conversion coatings, have good corrosion resistance, but their self-healing abilities have not been determined. In this paper, silicate conversion coatings are prepared by immersing hot dip galvanized steel sheets in sodium silicate solutions with SiO₂:Na₂O molar ratio of 1.00 and 3.50. The coatings are scratched with a blunt knife edge and corroded in a neutral salt spray chamber for certain lengths of time. The corrosion products in and near the scratched area are investigated by scanning electron microscopy and energy-dispersive spectroscopy. The self-healing ability of silicate conversion coatings is discussed. The results show that silicate conversion coatings have self-healing abilities under the experimental conditions. Higher SiO₂:Na₂O molar ratios lead to improved self-healing abilities. During the corrosion process, the silicate anions in the coating migrate to the scratched area, where a new conversion coating composed of Zn, O, and Si is formed. This delays corrosion in the scratched area.     

Glass-like CexOy sol–gel coatings for corrosion protection of aluminium and magnesium alloys

This work reports the preparation of glass-like, environmentally-friendly, cerium-based coatings for active corrosion protection of aluminium and magnesium alloys. It describes the production of cerium sol–gel sols from cerium nitrate and their deposition by immersion and automatic spray onto aluminium and magnesium alloys to produce uniform coatings with amorphous (glass-like) structures (Ce_xO_y). The coatings have been characterised by profilometry, scanning electron microscopy (SEM), spectral ellipsometry and UV–visible, in order to analyse the structure and density of the glass-like cerium coatings as well as their redox ratio Ce⁴⁺/Ce³⁺ as a function of pH and sintering temperature. Finally, electrochemical measurements (EIS) and standard corrosion tests (neutral salt spray, filiform corrosion, immersion–emersion test and adhesion on embossing and T-bend test) have been performed to study the corrosion behaviour of the cerium glass-like coatings on aluminium and magnesium alloys. EIS measurements confirm the healing or blocking of the defects by the corrosion inhibiting species. Excellent corrosion protection is provided by cerium glass-like coatings, satisfying the most demanding industrial requirements.     

Tribocorrosion behaviour of HVOF cermet coatings

The main purpose of this work is to analyze the degradation mechanisms induced on industrial HVOF cermet coatings by tribocorrosion. Tribocorrosion of cermet coatings is a subject that has not been widely analyzed in research studies: in fact, while many works dealing with wear or corrosion of HVOF cermet coatings are published, studies relevant to the combined processes (wear and corrosion) are relatively few.The tribocorrosion mechanisms of the cermet coatings were studied in a sodium chloride solution under sliding wear, trying to combine and integrate differently produced mechanical and electrochemical damage phenomena.Electrochemical techniques such as potentiodynamic polarization curves as well as potentiostatic (I vs t) or galvanostatic (E vs t) methods were used in order to stimulate and to interprete tribocorrosion degradation mechanisms.It was shown that coating post grinding, which is a mechanical operation usually performed after the deposition of conventional cermet coatings in order to obtain a desired roughness, could produce structural damages, which can greatly affect the mechano-chemical behaviour of the cermet coatings.Mainly abrasive-adhesive wear mechanisms were observed on the coating surface and sometimes, depending on coatings mechanical properties (fracture toughness), cracks developed during wear causing the coating continuity breaking. In the latter case, the degradation mechanism is no longer governed only by surface tribocorrosion, but undermining corrosion can occur, greatly affecting sample performances and promoting coating detachment.Cr₃C₂-NiCr coatings, under all the selected experimental conditions, showed good barrier properties and substrate corrosion was never observed. Moreover, when chromium was added to the metal matrix of WC-Co based systems, tribocorrosion behaviour was enhanced and the lower tribocorrosion rates were measured.Finally, it was shown that electrochemical techniques can be used to govern the coating corrosion processes and to interpret the main degradation mechanisms, even though they seem not to provide a precise quantitative analysis of tribocorrosion.     

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.     

Effect of surfactant on the alumina dispersion and corrosion behavior of electroless Ni?P?Al2O3 composite coatings

Electroless Ni? P? Al₂O₃ composite coatings have been synthesized on mild steel shafts using surfactant cetyltrimethyle ammonium bromide (CTAB) as dispersant. The effects of the surfactant on the alumina dispersion, weight fraction in coatings, and corrosion resistance of the composite coatings under salt spray test were investigated. Results showed that alumina dispersion was improved, whereas weight fraction was decreased, with the increasing concentration of the dispersant CTAB. The corrosion resistance of the composite coatings was found to increase with the increase in CTAB concentration up to a certain optimum of 20 mg/L, beyond which a decreasing trend of corrosion resistance was observed under salt spray test. Compared with Ni? P coating, all the Ni? P? Al₂O₃ composite coatings showed improved corrosion‐resistant properties.     

Effect of Ceramic Particles on Properties of Cold-Sprayed Ni-20Cr+Al2O3 Coatings

Cold spraying is a thermal spray process enabling the production of metallic and metal-ceramic coatings with low porosity and low oxygen content, capable of, e.g., resisting corrosion. The aim of this study was to characterize the microstructural and mechanical properties of cold-sprayed Ni-20Cr+Al₂O₃ coatings and to clarify the effect of the hard particles on different coating properties. Accordingly, the research focused on the microstructure, denseness (impermeability), adhesion strength, and hardness of the coatings. Scanning electron microscopy (SEM) analysis and corrosion tests were run to gain information on the through-porosity. Ceramic addition in cold-sprayed Ni-20Cr+Al₂O₃ coatings improved their quality by lowering their porosity. Moreover, hardness was slightly higher than those of cold-sprayed Ni-20Cr coating, indicating a hardening effect by the ceramic particles. The addition of Al₂O₃ also made it possible to use high gas temperatures without nozzle clogging, which affects coating properties, such as coating thickness, denseness, and hardness.     

Characterization of plasma sprayed Fe-17Cr-38Mo-4C amorphous coatings crystallizing at extremely high temperature

A Fe-17Cr-38Mo-4C alloy powder was plasma sprayed by three processes: an 80 kW low-pressure plasma spray (LPPS), a 250 kW high-energy plasma spray (HPS), and a 40 kW conventional plasma spray (APS). The as-sprayed coating obtained by the LPPS process is composed of only amorphous phase. As-sprayed coatings obtained by the HPS and APS processes are a mixture of amorphous and crystalline phases. The three as-sprayed coatings exhibit a high hardness of 1000 to 1100 DPN. The amorphous phase in these coatings crystallizes at a high temperature of about 920 K. A very fine structure composed of hard ϰ-phase and carbides is formed after crystallization. The hardness of the coating obtained by LPPS reaches a maximum of 1450 DPN just after crystallization on tempering and retains a high hardness more than 1300 DPN after tempering at high temperatures of 1173 or 1273 K. The corrosion potential of the amorphous coating is the highest among the three coatings and higher than that of a SUS316L stainless steel coating. The anodic polarization measurements infer that the corrosion resistance of the amorphous coating is superior or comparable to SUS316L stainless steel coating in H₂SO₄ solution.     

Structure and corrosion resistance of ZrO2 ceramic coatings on AZ91D Mg alloys by plasma electrolytic oxidation

The aim of this work is to study the structure and the corrosion resistance of the plasma electrolytic oxidation ZrO₂ ceramic coatings on Mg alloys. The ceramic coatings were prepared on AZ91D Mg alloy in Na₅P₃O₁₀ and K₂ZrF₆ solution by pulsed single-polar plasma electrolytic oxidation (PEO). The phase composition, morphology and element distribution in the coating were investigated by X-ray diffractometry, scanning electron microscopy and energy distribution spectroscopy, respectively. The results show that the coating thickness and surface roughness were increased with the increase of the reaction time. The ceramic coatings were of double-layer structure with the loose and porous outer layer and the compact inner layer. And the coating was composed of P, Zr, Mg and K, of which P and Zr were the main elements in the coating. P in the coating existed in the form of amorphous state, while Zr crystallized in the form of t-ZrO₂ and a little c-ZrO₂ in the coating. Electrochemical impedance spectra (EIS) and the polarizing curve tests of the coatings were measured through CHI604 electrochemical analyzer in 3.5% NaCl solution to evaluate the corrosion resistance. The polarization resistance obtained from the equivalent circuit of the EIS was consistent with the results of the polarizing curves tests.     

Corrosion Properties of Cold-Sprayed Tantalum Coatings

Cold spraying enables the production of pure and dense metallic coatings. Denseness (impermeability) plays an important role in the corrosion resistance of coatings, and good corrosion resistance is based on the formation of a protective oxide layer in case of passivating metals and metal alloys. The aim of this study was to investigate the microstructural details, denseness, and corrosion resistance of two cold-sprayed tantalum coatings with a scanning electron microscope and corrosion tests. Polarization measurements were taken to gain information on the corrosion properties of the coatings in 3.5 wt.% NaCl and 40 wt.% H₂SO₄ solutions at room temperature and temperature of 80 °C. Standard and improved tantalum powders were tested with different spraying conditions. The cold-sprayed tantalum coating prepared from improved tantalum powder with advanced cold spray system showed excellent corrosion resistance: in microstructural analysis, it showed a uniformly dense microstructure, and, in addition, performed well in all corrosion tests.     

Hot corrosion behaviour of plasma sprayed YSZ/LaMgAl11O19 composite coatings in molten sulfate–vanadate salt

Hot corrosion studies of thermal barrier coatings (TBCs) with different YSZ/LaMgAl₁₁O₁₉ (LaMA) composite coating top coats were conducted in 50 wt.% Na₂SO₄ + 50 wt.% V₂O₅ molten salt at 950 °C for 60 h. Results indicate that TBCs with composite coating top coats exhibit superior oxidation and hot corrosion resistances to the TBC with the traditional YSZ top coat, especially for which has a LaMA overlay. The presence of LaMA can effectively restrain the destabilization of YSZ at the expense of its own partial degradation. The hot corrosion mechanism of LaMA coating and the composite coatings have been explored.     

Properties of Reactive Plasma Sprayed TiB2-TiC0.3N0.7 Based Composite Coatings with Cr Addition and Laser Surface Treatment

Thick TiB₂-TiC_0.3N_0.7 based composite coatings were deposited by reactive plasma spraying (RPS) successfully in air. The influence to the coating properties (morphology, Vickers microhardness and corrosion resistant property) with Cr addition in the thermal spray powder and TiB₂-TiC_0.3N_0.7 based coatings treated by laser were investigated. The phase composition, structure and properties of composite coatings were studied using XRD, SEM, EDS, Vickers microhardness and electrochemical testers. The results show that the Vickers microhardness values and the density of laser surface treated coatings are improved significantly. The Cr addition in the thermal spray powder can increase the density, improve the wettability of ceramic phases, uniform the phase distribution and enhance the corrosion-resistant property of coatings. However, due to lower microhardness of metal Cr than ceramic phases in coatings, the Vickers microhardness values of plasma sprayed coatings and plasma sprayed coatings with laser surface treatment are a little lower than that of each coating without Cr addition in the thermal spray powder.     

Comparison of electrochemical corrosion behaviour of MgO and ZrO2 coatings on AM50 magnesium alloy formed by plasma electrolytic oxidation

Two types of PEO coatings were produced on AM50 magnesium alloy using pulsed DC plasma electrolytic oxidation process in an alkaline phosphate and acidic fluozirconate electrolytes, respectively. The phase composition and microstructure of these PEO coatings were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The corrosion behaviour of the coated samples was evaluated by open circuit potential (OCP) measurements, potentiodynamic polarization tests, and electrochemical impedance spectroscopy (EIS) in neutral 0.1 M NaCl solution. The results showed that PEO coating prepared from alkaline phosphate electrolyte consisted of only MgO and on the other hand the one formed in acidic fluozirconate solution was mainly composed of ZrO₂, MgF₂. Electrochemical corrosion tests indicated that the phase composition of PEO coating has a significant effect on the deterioration process of coated magnesium alloy in this corrosive environment. The PEO coating that was composed of only MgO suffered from localized corrosion in the 50 h exposure studies, whereas the PEO coating with ZrO₂ compounds showed a much superior stability during the corrosion tests and provided an efficient corrosion protection. The results showed that the preparation of PEO coating with higher chemical stability compounds offers an opportunity to produce layers that could provide better corrosion protection to magnesium alloys.