A study of zinc phosphate coatings on 12·5Cr–21·0Ni stainless steel

Abstract

12·5Cr–21·0Ni stainless steel was chemically treated with zinc phosphate in order to find the most suitable phosphate solution and its operating parameters. The phosphate coatings were tested for their corrosion protection of stainless steel using three methods: the salt spray test, the humidity cabinet test and the brine immersion test. The phosphate coatings were also mechanically tested using a tensile test for determining their mechanical properties. Results clearly show that phosphate coatings with a uniform appearance and full coverage can give high corrosion protection to 12·5Cr–21·0Ni stainless steel by forming a physical barrier against the corrosive environment. The 12·5Cr–21·0Ni stainless steel after coating with zinc phosphate still retains reliable mechanical properties, thereby providing valuable applications in the engineering field.     

Corrosion and protection characteristics of phosphate coatings: effect of yttrium oxide as additive

Abstract

The corrosion and protection characteristics of phosphate coatings formed in a phosphating solution containing mainly ZnO, H₃PO₄ and NaF, using Y₂O₃ as an additive, were investigated through SEM, polarisation curves and EIS diagrams. The results show that the corrosion protection of phosphate coatings has been improved when Y₂O₃ is added to the phosphating solution, making the free corrosion potential shift to the positive direction and causing the corrosion current to decrease. The protection ability of phosphate coatings depends mainly on their barrier performance. The phosphate coatings formed in the phosphating solution with 10 and 20 mg L^–1 Y₂O₃ have finer crystal structures and smaller porosity; therefore, they exhibit better corrosion resistance and adhesion properties than those without Y₂O₃ and with 40 mg L^–1 Y₂O₃.     

Formation of Phosphate Coatings on Steel and Corrosion Performance of Phosphated Specimens in Alkaline Solutions

Crystalline and amorphous phosphate coatings were formed on steel samples with the intention of using them for corrosion resistance enhancement of steel reinforcement in concrete. The physical properties of the phosphate coatings were studied using the following methods: the weights of phosphate coating and those of dissolved metal were calculated from gravimetric measurements, surface morphology studies were carried out with an AFM and the phase composition of coatings was analysed by X-ray diffraction. An indicator of film porosity was considered as the maximum difference between potential values of growth completeness of phosphate film and those of metal dissolution. The corrosion behaviour of phosphated steel specimens was studied in 1 × 10^?5 and 0.1 M NaOH solutions (pH=8.5, 13) without/with 0.1 M Cl^? ions. The phosphate coating protection abilities were related to its composition, thickness and porosity. All the crystalline phosphate coatings studied protect steel from corrosion in alkaline solutions in a wide range of pH even when Cl^? ions are present. The best protective properties were exhibited by medium weight crystalline phosphate coatings. Amorphous phosphate coating is not chemically stable in strongly alkaline solutions because of the hydrolysis of iron phosphate present in it.     

Influence of static magnetic fields on the protective ability of phosphate coatings on aluminium alloys

Abstract

Phosphate coatings on aluminium are used to improve the corrosion resistance of aluminium before the latter is painted. Phosphate coatings deposited in phosphate solution treated with a magnetic field prior to deposition can take on new properties. A magnetic field can either improve or deteriorate the properties of the phosphate coating formed, depending on the composition of aluminium.     

Characterization of calcium-modified zinc phosphate conversion coatings and their influences on corrosion resistance of AZ31 alloy

Two kinds of phosphate conversion coatings, including zinc phosphate coating and zinc-calcium phosphate coating, were prepared on the surface of AZ31 alloy in phosphate baths. The morphologies of these coatings were observed using scanning electron microscopy. Their chemical compositions and structures were characterized using energy-dispersive X-ray spectrum, X-ray photoelectron spectroscopy and X-ray diffraction. The corrosion resistance of the coatings was evaluated by potentiodynamic polarization technique. The results show that the flowerlike Zn-Ca phosphate conversion coatings are mainly composed of hopeite (Zn₃(PO₄)₂·4H₂O). They have a quite different morphology from the dry-riverbed-like Zn phosphate coatings that consist of MgO, MgF₂, Zn or ZnO and hopeite. Both of the zinc and zinc-calcium phosphate coatings can remarkably reduce the corrosion current density of the substrates. The Zn-Ca coating exhibits better corrosion resistance than the Zn coating. Introduction of calcium into the phosphate baths leads to the full crystallinity of the Zn-Ca coating.     

Statistical Process Control

Abstract

In order to improve the corrosion resistance provided by a micro-arc oxidation (MAO) coating on AZ31 magnesium alloy, a polypropylene film was prepared on its surface. Scanning electron microscopy, energy dispersive X-ray analysis and Fourier transform infrared spectroscopy were used to characterise the surfaces of the coatings. The corrosion protective performance of the coatings was evaluated by potentiodynamic polarisation curves, electrochemical impedance spectroscopy and immersion testing. The results show that the microdefects of the MAO coating can be filled by PP and the corrosion resistance of the AZ31 magnesium alloy is improved greatly.     

Effect of Heat Treatment on Mechanical Properties and Corrosion Performance of Cold-Sprayed Tantalum Coatings

The cold-spray technique is of significant interest to deposit refractory metals with relatively high melting point for a variety of demanding applications. In the present study, mechanical properties of cold-sprayed tantalum coatings heat treated at different temperatures were investigated using microtensile testing, scratch testing, and nanoindentation. The corrosion performance of heat-treated coatings was also evaluated in 1 M KOH solution, and potentiodynamic polarization as well as impedance spectroscopy studies were carried out. Assessment of structure–property correlations was attempted based on microstructure, porosity, and intersplat bonding state, together with mechanical and corrosion properties of the heat-treated cold-sprayed tantalum coatings. Coatings annealed at 1500 °C, which is very close to the recrystallization temperature of tantalum, were found to perform almost as bulk tantalum, with exciting implications for various applications.     

Corrosion protection properties of new UV curable waterborne urethane acrylic coatings

Abstract

Looking to the development of sustainable and low impact chemical technologies, water based paints offer significant advantage especially for in field applications, where the problem of solvent emission is really stringent. Some water based coatings are represented by alkyd and epoxy paints, capable of cross-linking either by auto-oxidation or with polyamines. Polyurethane aqueous dispersions are finding wide acceptance due to their better durability, adhesion and mechanical properties. The research presented here describes the design, application and characterisation of well defined model structures of waterborne UV curing coatings. The waterborne resins used in this work are segmented anionomeric polyurethanes functionalised with acrylic end groups: they were prepared from polytetramethylene glycol, isophorone diisocyanate, dimethylol propionic acid, and hydroxyethylacrylate. The preliminary results concerning the corrosion protection properties of the new UV curable waterborne coatings applied onto metallic substrates are very promising and were obtained by using electrochemical techniques. Water uptake is the critical process determining the corrosion performance. In particular the necessity to formulate the coating starting from higher molecular weight polytetramethylene glycol for good barrier properties is evident from the electrochemical impedance spectroscopy data.     

Enhancement of Functional Ceramic Coating Performance by Gas Tunnel Type Plasma Spraying

A high-precision plasma system has been pursued for advanced thermal processing. The gas tunnel type plasma jet device developed by the author exhibits high energy density and also high efficiency. Among its various applications is the plasma spraying of ceramics such as Al₂O₃ and ZrO₂. The performance of these ceramic coatings is superior to conventional ones. Properties such as the mechanical and chemical properties of the zirconia coatings were reported in previous studies. In this study, the enhancement of the performance of functional ceramic coatings by the gas tunnel type plasma spraying method was carried out using different powders. Results show that the alumina/zirconia composite system exhibited improvements of mechanical properties and corrosion resistance. The alumina/zirconia composite coating has the potential for use as a high functionally graded thermal barrier coating. Another application of the gas tunnel type plasma is for surface modification of metals. As an example, TiN films were formed in 5 s and, thick TiN coatings were easily obtained by gas tunnel type plasma reactive spraying.     

碳钢超声磷化及在模拟干热岩地热水中的耐蚀性能

采用环境友好的硫酸羟胺为主要促进剂,避免使用强氧化性物质,并以超声处理来改进磷化工艺。通过动电位极化曲线方法研究了超声、促进剂、pH值和磷化时间对磷化膜耐蚀性能的影响。结果表明,超声处理能够细化磷化颗粒,提高其耐蚀性能。pH值和磷化时间显著影响磷化膜的耐蚀性,需保持在较优的范围。采用电化学阻抗谱研究了优化工艺得到的磷化膜在模拟干热岩地热水中的耐蚀性,发现其电荷转移电阻由碳钢的1.44×10³ Ωcm²增加到3.39×10⁶ Ωcm²。     

Effect of nano-Fe2O3 particles on the corrosion behavior of alkyd based waterborne coatings

A nano-composite coating was formed by adding nano-Fe₂O₃ to a specially developed alkyd based waterborne coating system. The interaction of nano-particles with the polymer functionalities was confirmed using FTIR. Curing behavior of the coating was determined by DSC. The uniformity of coating and dispersion of nano-particles in the polymer matrix was investigated using SEM and AFM. The performance of the coating was investigated in terms of their corrosion resistance, UV-resistance, mechanical properties and optical properties. Composite coatings so formed showed enhanced, corrosion resistance, better mechanical properties such as abrasion and scratch hardness and improved UV blocking properties.     

Improving corrosion properties of high-velocity oxy-fuel sprayed inconel 625 by using a high-power continuous wave neodymium-doped yttrium aluminum garnet laser

Thermal spray processes are widely used to protect materials and components against wear, corrosion and oxidation. Despite the use of the latest developments of thermal spraying, such as high-velocity oxy-fuel (HVOF) and plasma spraying, these coatings may in certain service conditions show inadequate performance,e.g., due to insufficient bond strength and/or mechanical properties and corrosion resistance inferior to those of corresponding bulk materials. The main cause for a low bond strength in thermalsprayed coatings is the low process temperature, which results only in mechanical bonding. Mechanical and corrosion properties typically inferior to wrought materials are caused by the chemical and structural inhomogeneity of the thermal-sprayed coating material. To overcome the drawbacks of sprayed structures and to markedly improve the coating properties, laser remelting of sprayed coatings was studied in the present work. The coating material was nickel-based superalloy Inconel 625, which contains chromium and molybdenum as the main alloying agents. The coating was prepared by HVOF spraying onto mild steel substrates. High-power continuous wave Nd:YAG laser equipped with large beam optics was used to remelt the HVOF sprayed coating using different levels of power and scanning speed. The coatings as-sprayed and after laser remelting were characterized by optical microscopy and scanning electron microscopy (SEM). Laser remelting resulted in homogenization of the sprayed structure. This strongly improved the performance of the laser-remelted coatings in adhesion, wet corrosion, and high-temperature oxidation testing. The properties of the laser-remelted coatings were compared directly with the properties of as-sprayed HVOF coatings and with plasma-transferred arc (PTA) overlay coatings and wrought Inconel 625 alloy.     

Investigation of wear and corrosion resistance of nanocomposite coating formed on AZ31B Mg alloy by plasma electrolytic oxidation

Ceramic-WC coatings were prepared on AZ31 B Mg alloy by plasma electrolytic oxidation (PEO) from a phosphate based bath containing suspended tungsten carbide nanoparticles at various process times. Scanning electron microscope results indicated that increase of coating time and incorporation of tungsten carbide into the ceramic coating during the PEO process led to a decrease in the number and diameter of coating pores. Phase analysis showed that the nanocomposite coating was composed of MgO, Mg₃(PO₄)₂ and WC. Tribological properties and corrosion behaviour of uncoated AZ31 B Mg alloy and ceramic coatings were evaluated using a pin-on-disc tribometer and potentiodynamic polarisation technique in 3.5% NaCl solution, respectively. The wear and electrochemical tests showed that wear and corrosion resistance of ceramic-WC nanocomposite coatings were better than ceramic only ones. In addition, wear and corrosion behaviour of coatings improved with increasing the coating time.     

Influence of aluminum phosphate on the tribocorrosion performance of chemically bonded phosphate ceramic coatings

In this study, chemically bonded phosphate ceramic coatings (CBPCCs) with different contents of aluminum phosphate (AP) are prepared on stainless steel (AISI 304L). Differential scanning calorimetry, X-ray diffraction, contact angle test, and a tribocorrosion experiment are carried out to clarify the role of AP in the tribocorrosion performance of CBPCCs. The results show that, with the increase in the AP content, the enthalpy of curing increases because of the greater formation of the bonding phase AlPO₄. Both in static corrosion and in tribocorrosion, the corrosion current density of CBPCCs achieves the lowest value when the weight ratio of AP to polytetrafluoroethylene is about 0.78. Additionally, the influence mechanism of AP on tribocorrosion is clarified. AlPO₄ from the reaction between AP and Al₂O₃ has excellent mechanical properties and can enhance the wear resistance of CBPCCs by reducing the mechanical wear and the increased wear due to corrosion. The alumina particles wrapped by AlPO₄ can form a dense and smooth surface and change the direction of electrolyte propagation, which leads to the increase in the tribocorrosion resistance of CBPCCs.     

Effect of surface mechanical attrition treatment (SMAT) on zinc phosphating of steel

The effects of surface mechanical attrition treatment (SMAT) of EN8 steel on the growth of phosphate coatings, morphological features and corrosion resistance of the resultant coatings have been studied. SMAT enabled the formation of a uniform surface profile although the average surface roughness is increased after treatment. SMAT increased the extent of metal dissolution and the rate of growth of phosphate coating. In spite of the similarity in phase composition, the phosphate crystallite size is relatively high for samples treated by SMAT. Compared to the untreated one, a cathodic shift in E_corr with a corresponding decrease in i_corr is observed for SMAT treated EN8 steel after phosphating. For all tested samples, zinc phosphate coatings deposited on EN8 steel after SMAT using 8 mm Ø balls for 30 min offers the highest corrosion resistance. The increase in surface roughness by SMAT is partly compensated by the expected improvement in corrosion resistance.     

The EIS investigation of powder polyester coatings on phosphated low carbon steel: The effect of NaNO2 in the phosphating bath

The effect of different type of iron-phosphate coatings on corrosion stability and adhesion characteristic of top powder polyester coating on steel was investigated. Iron-phosphate coatings were deposited on steel in the novel phosphating bath with or without NaNO₂ as an accelerator. The corrosion stability of the powder polyester coating was evaluated by electrochemical impedance spectroscopy (EIS), adhesion by pull-off and NMP test, while surface morphology of phosphate coatings were investigated by atomic force microscopy (AFM).The adhesion and corrosion stability of powder polyester coatings were improved with pretreatment based on iron-phosphate coating deposited from NaNO₂-free bath.     

Corrosion and Wear Response of Oxide-Reinforced Nickel Composite Coatings

Various grades of fuels are used in automobiles, as a result the engine components are continuously subjected to simultaneous action of corrosion and wear. Ni-SiC composite coating is the most widely investigated and commercialized wear-resistant coating in the automotive industry. However, this coating cannot be used at temperatures above 450 °C due to the tendency of SiC to react with Ni and form brittle silicides. An alternate approach is to use oxide-reinforced coatings. In the present study, zirconia, ZrO₂ and, yttria-stabilized zirconia, YSZ-reinforced Ni composite coatings have been developed by electrodeposition method. It was observed from the microhardness studies that there is no significant difference in the values for Ni-SiC and Ni-ZrO₂ coatings. The corrosion behavior was evaluated using polarization and electrochemical impedance studies. The studies showed that oxide particle-reinforced Ni coatings possessed better corrosion resistance due to their lower corrosion current density, I _corr. Tribo-corrosion studies were carried out to understand the synergistic effect of wear and corrosion on the performance of Ni-based composite coatings in 0.5 M Na₂SO₄. Among various composite coatings, Ni-YSZ exhibited less material loss thereby showing better tribo-corrosion behavior.     

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.     

Air plasma sprayed metallic coatings for sour environments

Abstract

Among protective coatings for components used in petrochemical applications, plasma sprayed alloys containing chromium, nickel, and molybdenum have potential to provide functional surfaces with good resistance to corrosion and wear. They could offer a cost effective solution by limiting the use of expensive bulk materials such as Inconel or Hastelloy superalloys or highly alloyed chromium stainless steels, since a low cost base material could be selected to give the required mechanical strength combined with suitable corrosion resistance from its modified surface. To limit fabrication costs of coating deposition, air plasma spraying techniques can be proposed, but proper optimisation of the plasma processing parameters is required to increase coating performance. In this paper, coatings based on Ni–Cr–B–Si and Mo alloys have been deposited following a design of experiment methodology. Results of characterisation tests for preliminary spraying trials (metallographic, tribological, and corrosion tests) are reported and discussed. Following analysis of numerical results, a coating obtained by the codeposition of two commercial powders (Metco 505 and Metco 16C) was sprayed on carbon steel substrates. Sulphide stress corrosion cracking tests, according to the NACE TM 0177-90 standard, at up to 90% of the yield stress of the coated specimens did not induce any damage for test durations of up to 720 h. Tribological properties have also been evaluated in terms of the slurry abrasivity response number (SAR), friction coefficients, and mass loss after ASTM standard ball on disc tests. Good resistance to erosion damage was obtained with a final SAR number of 37.     

Expert system to choose coatings for flue gas desulphurisation plant

Abstract

The development and use of an expert system to recommend coatings for flue gas desulphurisation plant is described. The system ranks coatings by their material properties and experimental test and plant performance scores when the component to be coated and its working environment are specified. The user interface, the inference engine, the knowledge base, and the process used to implement the expert system are presented with comments on its suitability and application for corrosion consultations.