Scanning auger microprobe study of hot-dipped regular-spangle galvanized steel: Part II. surface composition of chromated sheet

Zinc coatings produced on a hot-dipped-coating pilot line from a zinc bath containing small amounts of aluminum, antimony, and lead were treated with a commercial chromating solution and then examined with a scanning Auger microprobe. The results show that the chromating solution attacked the thin aluminum oxide precipitates in the zinc surface. The invisible conversion-coating film that was formed on the zinc matrix consisted mainly of oxygen, zinc, and chromium in order of decreasing atomic concentration. Surprisingly, the thickness and composition of the film was the same for treatment in 8 and 15 grams-per-liter chromate solutions. The film that was formed on precipitates of lead in the surface of the zinc coating was much thicker and richer in chromium than the film formed on the surrounding zinc. Examination of chromated surfaces exposed to water-saturated air at 100 °F and to normal atmospheres showed that the surfaces corroded rapidly on a microscale. After only a few hours exposure the surface had numerous mounds composed of equal atom concentrations of zinc and oxygen. In each instance corrosion sites were associated with a lead precipitate.     

钕铁硼永磁材料防护镀层

比较系统地研究了钕铁硼永磁材料的单层镍、双层镍、暗镍－铜－亮镍、镍铁－亮镍镀层的厚度对孔隙率及耐蚀性的影响。实验发现,在相同厚度的镀层中,暗镍－铜－亮镍镀层体系的孔隙率低,对基体防护作用强,装饰效果好。该镀层体系的试样已通过１２０ｈ的中性盐雾试验。     

Spangle formation in galvanized sheet steel coatings

Very large grains, termed “spangles,” are produced on galvanized sheet steel coatings when lead is added to the zinc bath. The spangles have been attributed to melt undercooling prior to solidification. The present results indicate this is not the case, undercooling being less than 1 °C. The spangle diameter is shown to be dependent on the alloy addition to the bath, large spangles being obtained with Bi and Sb as well as Pb. The spangle size is related to the surface tension of the alloying addition, the size decreasing as the melt vapor surface tension of the alloying element increases. It is proposed that spangles form dendritically from a nucleus in the melt. Alloy additions with low interfacial energies and very limited solid solubility are highly concentrated ahead of the dendrite tip. This decreases the tip radius and increases the dendrite velocity, producing large grains. The basal plane orientation of the samples varies between 17 and 80 deg with respect to the steel sheet surface, which is inconsistent with basal plane dendritic growth in Zn along (1010) directions. It is proposed that solute additions to the melt and growth in a thin liquid layer can modify the dendrite growth direction, accounting for the spangle orientation. On leave from Obafemi Awolowo University, lie Ife, Oyo State, Nigeria     

热浸镀铝锌硅镀层云纹缺陷的分析及抑制

 对采用薄板坯连铸连轧工艺生产的热浸镀铝锌硅产品镀层横向云纹缺陷和纵向云纹缺陷进行了表面和截面结构的分析。结果表明,产生的云纹缺陷与气刀工作压力、气刀工作距离、气刀喷吹角度、锌液温度和钢板入锌锅温度有关;气刀工作压力过低及锌液温度过高时,易形成纵向云纹缺陷。气刀工作压力过大,气刀间距相应变大,形成镀层横向云纹缺陷;在一定温度下,调整各项参数将钢带抖动控制在3mm以下时,可以获得均匀的镀层厚度、表面锌花尺寸和色泽。     

Conditions of interaction of materials in gas thermal spraying of metallized oxide powders on AK-4 alloy

Conclusions The analysis made from the viewpoints of thermodynamics and the quasichemical theory of the formation of bonds in gas thermal spraying indicates the unquestionable advantages of aluminum oxide composites and the significant role of surface oxide films in the creation of reliable thermal protection coatings on aluminum alloys. It is most desirable to obtain ZrO₂ base coatings and a change in the nature of the oxide film on the surface of AK-4 alloy by preliminary formation of a NiO sublayer on it. The use for spraying of metallized oxide powders substantially increases the energy of contact interaction of the materials on the base. The components of the metallized powders interacts with the formation of types Zr-Al and Zr-Al-Ni oxidized phases. The depth of mutual diffusion of the elements of the coating and the base is 0.5–3 um. The adhesion strength of the optimum composition thermal protection coating with the surface of AK-4 alloy reaches 20–25 MPa.Translated from Poroshkovaya Metallurgiya, No. 8(284), pp. 48–52, August, 1986.     

Surface Modification of AL9 Alloy by Electric-Spark Alloying with Materials of the AlN – Ti(Zr)B2 – Ti(Zr)Si2System

Using x-ray phase, electron-probe microanalysis, and scanning electron microscopy, we have established that heterophase coatings based on an aluminum matrix with a quasi-layered finely dispersed structure reinforced with titanium (zirconium) diboride and borosilicide, and aluminum nitride (alloyed with oxygen) are formed by electric-spark alloying of AL9 alloy with A1N – Ti(Zr)B₂ – Ti(Zr)Si₂ composite ceramics. The wear resistance of the A1N – ZrB2-based coating approached that of the ceramic itself with the sliding speed increased to V ≥ l m/sec. The formation of secondary structures of oxide solid solutions on the coating surface during tribooxidation was investigated.     

镍基合金镀层在北方老工业城市冬季降水中的腐蚀行为

为研究镍基合金镀层在北方老工业城市冬季降水中的腐蚀行为,采用电沉积技术制备了Ni-P、Ni-Fe合金镀层,对其结构与耐蚀性能进行检测.结果表明:不同镍基合金镀层在冬季降水中耐蚀性能不同,其中Ni-Fe合金镀层由于Fe掺杂增加了镍基合金与氧的亲和力,可以快速在Ni-Fe合金镀层表面形成一层连续的氧化膜,对基体起到很好的防护作用,电位最正,自腐蚀电流密度值最小,耐蚀性较好,年腐蚀速率约为20号钢的1/2;而Ni-P合金镀层表面形成氧化膜的速率较缓慢,阻抗值较低,不适合在北方老工业城市的室外装饰与防护中应用.      

Structure and Properties of Composite Electrospark,Laser, and Magnetron Deposited AlN ― TiB2 Coatings

A comparative investigation of the coatings produced by electrospark alloying, laser gas-powder deposition, and magnetron sputtering using composite AlN ― TiB₂ material was carried out. It is shown that the presence of oxygen in the working atmosphere is responsible for the oxidation of aluminum nitride, and also the appearance of refractory compounds of the Ti ― Al ― O system in the coatings. During electrospark and laser alloying globular regions form on the surface by a mechanism of liquid-phase sintering, due to the mass transport of a metallic component from the substrate. The composite layer of a magnetron coating sinters by a mechanism of solid-phase interaction. The dispersion-strengthened structure of electrospark coatings is highly wear resistant, while the continuity, high dispersion, and heterogeneity of the structure of magnetron coatings permits their recommendation for corrosion protection.     

Comparison in the Oxidation and Corrosion Behavior of Aluminum and Alumina-Reinforced Ni/Ni-Co Alloy Coatings

In this study, a comparison in the oxidation and corrosion behavior of Ni/Ni-Co aluminum and alumina-reinforced electrodeposited composites has been made. The developed coatings were characterized for the morphology, structure, microhardness, oxidation, and corrosion resistance. It was found that the incorporation of Al particles in NiCo matrix is higher (9 wt pct) compared to Ni matrix (1 wt pct). In the case of aluminum oxide particles, about 5 and 7 wt pct had been obtained in Ni and NiCo matrices respectively. The difference in the surface morphology was observed with respect to metallic (Al) and inert ceramic (Al₂O₃) particle incorporation. X-ray diffraction studies showed the presence of predominant Ni (200) reflection in the coatings. Also, peaks corresponding to Al and Al₂O₃ particles were present. The Ni/NiCo-Al coatings exhibited higher microhardness values at 1273 K (1000 °C) compared to alumina-reinforced coatings, indicating better thermal stability of the former coatings. The NiAl coating showed one and two orders of magnitude improved oxidation resistance compared to NiCoAl and Ni/NiCo-Al₂O₃ coatings, respectively. It was observed that the Ni-Al composite coating exhibited poor corrosion resistance in 3.5 pct NaCl solution compared to the other coatings studied.     

Effects of zinc coatings on the stress corrosion cracking and hydrogen embrittlement of low-alloy steel

The effects of electroplated and hot-dip zinc coatings on the fracture of low-alloy steel AISI 4140 bars tempered to hardnesses in the range Rc 33 to 49 were studied. Either electroplated or hot-dip zinc coatings decrease resistance to stress corrosion cracking,i.e., they reduceK _sc, the threshold stress intensity for stress corrosion cracking in 3.5 wt pct NaCl solution. AboveK _scelectroplated-zinc coatings do not appear to affect the crack-growth rate, although the incubation period prior to the onset of crack growth is reduced. Hot-dip zinc coatings increase stress corrosion crack growth rates slightly because of the additive effect of internal dissolved hydrogen. Hot-dip zinc coatings reduce the critical stress intensity for fracture in the absence of a corrosive environment because of embrittlement by internal hydrogen which is released from traps during hot-dip coating and confined by the inter metallic coatings which form on the steel surface in the hot dip bath. A simple fracture mechanics analysis indicates that either increasing diameter or the presence of a zinc coating lowers the critical hardness at which the stress corrosion cracking of structural bolts can occur.     

Deformation and damage mechanisms of zinc coatings on hot-dip galvanized steel sheets: Part I. Deformation modes

Zinc-based coatings are widely used for protection against corrosion of steel-sheet products in the automotive industry. The objective of the present article is to investigate the deformation modes at work in three different microstructures of a thin (8 μm) zinc coating on an interstitial-free steel substrate under tension, plane-strain tension, and expansion loading. Damage mechanisms are addressed in a companion article. The plastic slip and twinning activity in the zinc grains of an untempered cold-rolled coating (labeled NSK), a tempered cold-rolled coating (labeled SK), and a recrystallized coating are compared with the response of the corresponding bulk low-alloyed zinc material. The in-plane grain size in the NSK and SK coatings ranges from 300 to 600 μm, vs about 30 μm in the recrystallized coating and bulk material. The coatings exhibit a strong crystallographic texture, with the c-axis generally normal to the sheet plane. Basal slip is shown to be the main deformation mechanism in bulk zinc and the recrystallized coating, whereas pyramidal π₂ slip and mechanical twinning are found to be major modes in the NSK and SK coatings. These results, obtained from an extensive, quantitative slip-line analysis combined with electron backscattered diffraction (EBSD) measurements, are explained by the constraining effect of the substrate. This effect is successfully modeled using a simple Taylor-like polycrystalline approach. The recrystallized coating behaves much like the bulk material. The interpretation of this grain-size effect between the NSK and SK coating, on the one hand, and the recrystallized coating, on the other hand, requires a full three-dimensional finite-element analysis of the multicrystalline coating provided in this work. The simulations show that strong strain gradients can develop in the recrystallized coating from the interface to the surface, which is not the case in the NSK and SK coatings.     

冲压“积瘤”对汽车外板涂装影响及对策分析

评价热镀锌汽车外板表面质量的参数包括粗糙度、波纹度和PC值,而评价汽车板涂装后表面质量的重点参数包括桔皮、鲜映性、长波、短波等。冷轧钢板表面粗糙度的波长在油漆前后有明显的差别,油漆前冷轧钢板表面的起伏以短波长的比例占优势,而油漆后短波长的部分几乎消失,长波长部分仍然保留并且占优势。以汽车公司反映最为强烈的影响涂装的表面积瘤缺陷为切入点,全面分析了积瘤的类别及缺陷形成的根本原因,重点制定了包括降低入锌锅板温,优化入锌锅板温与锌锅温度的匹配,通过提高锌锅铝含量来抑制锌锅中的铁,保持铁含量在极低的范围内,从根本上抑制锌粒的形成,从而达到有效降低锌粒缺陷比例,降低汽车公司返修率,满足2C1B涂装工艺要求的目的。     

Effects of zinc coatings on the stress corrosion cracking and hydrogen embrittlement of low-alloy steel

The effects of electroplated and hot-dip zinc coatings on the fracture of low-alloy steel AISI 4140 bars tempered to hardnesses in the range Rc 33 to 49 were studied. Either electroplated or hot-dip zinc coatings decrease resistance to stress corrosion cracking,i.e., they reduceK _sc, the threshold stress intensity for stress corrosion cracking in 3.5 wt pct NaCl solution. AboveK _scelectroplated-zinc coatings do not appear to affect the crack-growth rate, although the incubation period prior to the onset of crack growth is reduced. Hot-dip zinc coatings increase stress corrosion crack growth rates slightly because of the additive effect of internal dissolved hydrogen. Hot-dip zinc coatings reduce the critical stress intensity for fracture in the absence of a corrosive environment because of embrittlement by internal hydrogen which is released from traps during hot-dip coating and confined by the inter metallic coatings which form on the steel surface in the hot dip bath. A simple fracture mechanics analysis indicates that either increasing diameter or the presence of a zinc coating lowers the critical hardness at which the stress corrosion cracking of structural bolts can occur.     

Distribution of aluminum in hot-dip galvanized coatings

Hot-dip galvanized panels of low-carbon (LC) and interstitial-free (IF) steels were produced in a laboratory simulator with an average coating mass of 60 g/m². Three pot aluminum levels were used, viz., 0.10 pct (by wt), 0.15 pct, and 0.18 pct. Metallography, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were used to characterize coating and base steel microstructures. Wet chemical analysis and scanning transmission electron microscopy (STEM) were employed for compositional analyses. The aluminum content of the melt was found to be the predominant factor influencing the distribution of Al in the coating. At 0.18 pct melt aluminum, Al is partitioned between the aluminide inhibition layer at the coating-steel interface (∼80 pct) and the zinc overlay (∼20 pct). At 0.15 pct, it is partitioned among the aluminide layer (∼75 pct to 80 pct), zinc-iron (FeZn₁₃, ζ) intermetallic layer (∼5 pct to 15 pct), and the coating overlay (∼10 pct). At 0.10 pct, the aluminum is divided almost equally between the overlay and the zinc-iron intermetallics. At the two lower aluminum levels is the distribution marginally influenced by the steel grade. The ζ was found to not preferentially nucleate at the ferrite grain boundaries. When both the aluminide and ζ occurred at the coating-steel interface, the ζ particles appeared near discontinuities and thinner regions in the aluminide layer. The coating, relative to the melt, is enriched in aluminum because of its concentration in the aluminide and in the zinc-iron intermetallics. This enrichment increases with melt aluminum through an increase in the aluminum content of the aluminide layer and not of its thickness. In addition, a few tens-of-nanometers-thick layer enriched in aluminum, oxygen, and iron is observed on the outer surface of all coatings. The aluminum content in this layer also increases with an increase in the melt aluminum, but it contributes negligibly to the coatings’s content because of its extreme thinness.     

Analysis of zinc coatings on the basis of the iron-zinc phase diagram

The structure of zinc coatings is studied on the basis of the iron-zinc phase diagram, and the coating properties are compared. Corrosion tests indicate that diffusional zinc coatings are preferable in challenging operating conditions. Their structure consists mainly of intermetallides (iron-zinc phases) whose corrosion resistance is greater than that of pure zinc, primarily on account of passivation by zinc ferrites (ZnFeO₂, ZnFe₂O₄), as well as ZnO. The coating life in various aggressive media is considered, together with the main applications of zinc coatings.     

Effect of Process Variables on the Grain Size and Crystallographic Texture of Hot-Dip Galvanized Coatings

A galvanizing simulator was used to determine the effect of galvanizing bath antimony (Sb) content, substrate surface roughness, and cooling rate on the microstructural development of metallic zinc coatings. Substrate surface roughness was varied through the use of relatively rough hot-rolled and relatively smooth bright-rolled steels, cooling rates were varied from 0.1 to 10 K/s, and bulk bath Sb levels were varied from 0 to 0.1 wt pct. In general, it was found that increasing bath Sb content resulted in coatings with a larger grain size and strongly promoted the development of coatings with the close-packed {0002} basal plane parallel to the substrate surface. Increasing substrate surface roughness tended to decrease the coating grain size and promoted a more random coating crystallographic texture, except in the case of the highest Sb content bath (0.1 wt pct Sb), where substrate roughness had no significant effect on grain size except at higher cooling rates (10 K/s). Increased cooling rates tended to decrease the coating grain size and promote the {0002} basal orientation. Calculations showed that increasing the bath Sb content from 0 to 0.1 wt pct Sb increased the dendrite tip growth velocity from 0.06 to 0.11 cm/s by decreasing the solid–liquid interface surface energy from 0.77 to 0.45 J/m². Increased dendrite tip velocity only partially explains the formation of larger zinc grains at higher Sb levels. It was also found that the classic nucleation theory cannot completely explain the present experimental observations, particularly the effect of increasing the bath Sb, where the classical theory predicts increased nucleation and a finer grain size. In this case, the “poisoning” theory of nucleation sites by segregated Sb may provide a partial explanation. However, any analysis is greatly hampered by the lack of fundamental thermodynamic information such as partition coefficients and surface energies and by a lack of fundamental structural studies. Overall, it was concluded that the fundamental mechanisms behind the microstructural development of solidified metallic zinc coatings have yet to be completely elucidated and require further investigation.     

The use of titanium aluminides to form electric-spark coatings

Titanium aluminides (TiAl₃, TiAl, Ti₃Al) fabricated by powder metallurgy were used as alloying electrodes for the formation of electric-spark coatings. Intermetallic coatings were deposited on steel substrates in argon or nitrogen. The microstructure and composition of fabricated coatings were investigated by scanning electron microscopy, X-ray structural analysis, and electron probe microanalysis. It is established that initial Ti–Al intermetallic phases are present in fabricated coatings; however, the ratio between Ti and Al concentrations is shifted to aluminum compared with the stoichiometric one. When depositing titanium aluminide in the nitrogen medium, titanium nitride is additionally formed in surface layers. Thermal and tribotechnical tests showed that the Ti₃Al coating deposited in nitrogen possesses high wear resistance and heat resistance.     

Corrosion behavior of rare earth cerium based conversion coating on aluminum alloy

The present paper focused on the use of the salt of rare earth cerium as corrosion inhibitor of aluminum by using cathodic electrolytic passivation method. The corrosion resistance and the microphology of the cerium passivation film were studied by the methods of electrochemical method, scanning electron microscopy (SEM), and energy dispersive spectroscopic analysis. From the results, it was shown that good corrosion resistance of cerium-based passive coating was obtained when the compositions were as follows: CeCl₃·7H₂O, 0.05 mol/L; H₂O₂, 30 ml/L; current density, 1.1 mA/cm²; temperature, 40 °C; time, 9 min. SEM and EDS revealed that the cerium conversion coatings formed on the surface of aluminum alloy were related to cerium hydroxide/hydrated oxide depositions.     

Copper coatings for minimization of retention and permeation of implanted tritium in aluminum alloy 6061

Copper coatings deposited on Al-6061 substrates by radio frequency magnetron sputtering, to prevent the retention and permeation of energetically implanted tritium in Al-6061, were evaluated by a variety of characterization techniques. The coatings, weighing in the 0.03 to 0.088 kg/m² range, were smooth and had a fine grain structure. They contained the intermetallic phases Cu₉Al₄ and CuAl₂ as well as copper. The fractions of Al and Cu in any coating increased and decreased, respectively, with increasing depth below the surface. Furthermore, the fractions of Al and Cu on the coating surface decreased and increased, respectively, with increasing coating weight. There was no texture or preferred orientation in the Cu phase of the coatings. A significant amount of oxygen was also detected at the original substrate surface. Residual stress measurements revealed that, in both Cu and CuAl₂, the stresses in the coating plane were compressive, while the stress normal to the coating plane was zero in Cu but tensile in CuAl₂. The shear-stress components were, however, negligible in both the Cu and CuAl₂ phases. In the coating plane, the residual stress in Cu was always much smaller than that in the CuAl₂ phase. Bond-strength measurements using tensile-pull testing provided a lower limit of the bond strength of about 2 MPa.     

Oxidation and corrosion resistance of TiAl3 coatings

Titanium aluminides based on TiAl, TiAl₃ and Ti₃Al are potential materials for high temperature aerospace applications. Their low density, high temperature creep resistance, high temperature strength and high oxidation resistance make them excellent coating materials. However these coatings are likely to be subjected to high temperature and corrosive environments during service. Hence it is aimed to study the oxidation and corrosion resistance of TiAl₃ coatings on various types of substrates. In the present work, TiAl₃ is coated on high speed steel, stainless steel 304, stainless steel 316, copper and aluminum substrates by physical vapor deposition technique. X-ray diffraction analysis confirms the presence of TiAl₃ phase. The hardness studies reveal that better hardness can be achieved with thick coatings. The oxidation behavior of the coatings is studied by carrying out step stress experiments at elevated temperatures. Coated samples are heated up from 400°C in the steps of 100°C for 1h in each step to 1000°C. The mass gain caused by oxidation was determined. The oxidation curve drawn as a function of mass gain versus temperature reveals that TiAl₃ film started to oxidize above 800°C, where as oxidation of the uncoated substrates began at a much lower temperature of 550°C. The excellent oxidation resistance of the coatings can be attributed to the formation of an amorphous Al₂O₃ film. Scanning electron microscope (SEM) and EDAX analysis confirm the presence of an amorphous Al₂O₃ film. The corrosion behavior of TiAl₃ coatings are investigated by the polarization resistance experiments in NaCl aqueous solution at ambient temperature. According to the Tafel plot analysis, the coatings show lower corrosion rate than the untreated substrates. The major corrosion in the coatings arose from electrolyte penetration into the pores of the coatings. In fact, a dense coating showed a high corrosion resistance in an aqueous medium.