无锡钢上镀铬层的晶体织构与其机械寿命的关系

GIRIN O B*, OVCHARENKO V I

【电镀】

无锡钢上镀铬层的晶体织构与其机械寿命的关系

GIRIN O B*, OVCHARENKO V I

采用X射线衍射研究了在接近工业条件下制备的无锡钢上薄镀铬层(0.03 μm或每面0.22 g/m2)的晶体织构特征。实验发现，铬镀层的晶体结构含以下组分：以垂直于镀层表面的[111]为轴心的轴向分量，(111)[112]受限分量以及一个自由分量。其中[111]轴向分量为主要织构分量，是初始阶段的生长织构；(111)[112]受限分量为次要织构分量，代表了外延生长。无锡钢上镀铬层织构与亚结构的定量特征之间的规律性变化取决于沉积参数。影响无锡钢上薄镀铬层机械寿命的主要因素之一是镀层中[111]轴向织构的形成。

无锡钢；铬镀层；晶体织构；X射线衍射；机械寿命

1 Introduction

In the work[1]it was shown that formation of the axial texture (101)[hkl] in electrochemical tin coatings on non-reflown tinplate is one of the major factors influencing their protective ability. As tin-free steel is also used for packaging of dry and granular foodstuffs as well as tinplate, it is interesting to continue the investigation of interdependence between crystallographic texture and properties of coatings on tin-free steel.

Today the development of manufacturing of tin-free steel is hindered by two causes. First, high concentration of chromium anhydride (220-280 g/L) is the main obstacle for obtaining of tin-free steel. To reduce environmental hazard of chromium-plating process, the concentration of chromium anhydride in the electrolyte, obviously, should be decreased significantly.

The second cause is the wear of thin (0.05-0.07 µm) chromium coating during the process of cans manufacturing due to the friction by the instrument, which deforms and delivers a blank. Therefore, the solution of the problem for the enhancement of quality of tin-free steel with simultaneous reduction of the production cost[2](by the reduction of thickness of chromium coating to 0.03 µm) requires intent attention.

It is known that the formation of texture in electrodeposited chromium coatings causes the increase of their wear resistance[3-4]. But the data on the formation of texture in thin chromium coatings on tin-free steel so far is missing. The factor hindering the investigations in this field is the uncertainty of the influence of features of industrial chromium-plating process on texture of chromium coatings as a result of transportation of steel strip through the electrolyte with a velocity of 10 m/s and more.

In this regard the discovering of features of texture formation of thin (0.03 μm) electrochemical chromium coatings on tin-free steel in low-concentration electrolyte under conditions close to the industrial ones and the determination of interdependence between crystallographic texture and mechanical durability of coatings are the actual tasks.

2 Experimental

To approach the industrial conditions of the process of chromium-plating of tin-free steel in the laboratory, an electrochemical unit with moving cathode imitating the motion of steel strip with linear velocity up to 10 m/s was developed and constructed[5]. Precise determination of thickness of thin chromium coatings on tin-free steel was accomplished by the method[5].

Thin chromium coatings 0.03 μm thick (or 0.22 g/m2per side) were deposited on blackplate of steel 08kp in the electrolyte of the following composition: chromium anhydride 80-120 g/L, calcium sulfate 8-10 g/L, and additive “UP-Cr-1” 3-5 g/L.

Previous electrochemical investigations determined that quality coatings form on tin-free steel at two types of chromium-plating conditions: the first one (electrolyte temperature 30-40 °С and current density 25-55 А/dm2) provides current yield up to 35% and the second one (electrolyte temperature 50-60 °С and current density 40-70 A/dm2) provides current yield up to 25%[6].

To perform X-ray textural investigations, separate layers of electrodeposited chromium (0.03 µm) were combined into multilayered coatings, the thickness of which was completely sufficient for obtaining of reliable X-ray diffraction results. Multilayered coatings were separated from blackplate by the dissolution of the latter in nitric acid. The investigation of texture was done at the side of the coating adjacent to blackplate. Texture and substructure analysis of chromium coatings were carried out at an innovatively improved X-ray diffractometer according to the methods[7-9]. For quantitative estimation of the degree of perfection of texture of chromium coatings on tin-free steel, the average dispersion angle of the axial component of texture, the fraction of the axial component of texture, and the fraction of a random component of texture were chosen.

Mechanical durability of chromium coatings on tin-free steel, that is their ability to resist wear and fracture during the process of tin-free steel cans manufacturing, was tested by the method[10]. The wear rate IVwas used as a quantitative characteristic of mechanical durability of the coatings.

The estimation of protective ability of passivated chromium coatings on tin-free steel was carried out in the most widely used simulated environments that are mostly typical for numerous foodstuffs environments and for canned foods (3% solution of common salt and 3% solution of acetic acid), and in the foodstuffs environments that are considered standard overseas (100% grapefruit juice and tomato paste diluted with deionized water in a 1:1 ratio). Adhesion strength, porosity, and continuity were tested by the standard method at an Erichsen press. Reflecting ability of the coatings was measured by a photoelectric lustermeter relative to a silver mirror.

As a result of previous electron microscopy investigations, it was found that the structure of thin (0.01-0.03 µm) chromium electrocoating on tin-free steel is nanocrystal, and this nanostructure is texturized[11]. At that, chromium electrocoatings are characterized by textural inhomogeneity of nanostructure. The obtained results are compatible with the data of the work[12], where the fact of textural inhomogeneity of nano- and microstructure of bulk chromium electrochemical coatings was determined by the electron microscopy investigations.

3 Results and discussion

3. 1 Crystallographic texture of chromium coatings on tin-free steel

As a result of the completed texture investigations, it was found that at the electrolyte temperature ranging from 35 °C to 65 °С and the current density from 30 A/dm2to 70 A/dm2the texture of coatings being formed on tin-free steel are characterized by the axial orientation of grains with the axis [111], perpendicular to the surface of the coating, the restricted orientation of grains (111)[112], and a random orientation of grains (Figure 1).

Figure 1 Typical pole figure (110) of chromium coatings on tin-free steel图1 无锡钢上铬镀层的典型(110)极图

At the electrolyte temperature 35 °С and the current density ranging from 25 A/dm2to 35 A/dm2, the additional axial orientation of grains with the axis [100] is being formed in coatings on tin-free steel. It should be mentioned that the axial orientation of grains [111] is typical for electrochemical chromium coatings regardless to their thickness and the type of a base where they are deposited[13-14].

The results of quantitative analysis of texture of chromium coatings obtained on tin-free steel indicate that the most texturized coatings with the axis [111] are being deposited at the electrolyte temperature 55 °С in the range of current density 45-55 A/dm2. In this range of current density, the coatings have the highest fraction of the axial component [111] of texture, the least fraction of a random component of texture (Figure 2), and the least average dispersion angle of the axial component [111] of texture (Figure 3).

Figure 2 Influence of current density on fraction of texture components in chromium coatings produced on tin-free steel at the electrolyte temperatures 35 °C (curves 1, 2 and 3) and 55 °C (curves 4 and 5)图2 电流密度对在镀液温度分别为35 °C(曲线1、2和3)和55 °C(曲线4和5)时制备的无锡钢上铬镀层中不同织构分量所占分数的影响

Figure 3 Average dispersion angle of the axial component [111] of texture in chromium coatings produced on tin-free steel at the electrolyte temperatures 35 °С (curve 1) and 55 °С (curve 2) as a function of current density图3 在镀液温度分别为35 °C(曲线1)和55 °C(曲线2)时制备的无锡钢上铬镀层中[111]轴向织构的平均色散角随电流密度的变化

As it is seen in Figures 2 and 3, the average dispersion angle of the axial component [111] of texture of chromium coatings decreases with the increase of the fraction of this component, which causes a conclusion that texture of the axial component [111] is a texture of growth at the stage of its initiation. The obtained results are verified by the data of the work[15], where an enlargement of blocks and a decrease of the angles of disorientation between adjacent blocks with an increase of thickness of chromium coatings on tin-free steel were found by the method of transmission electron microscopy.

In order to verify the validity of the obtained data, the X-ray investigations of substructure of chromium coatings in the components of their texture were performed. For that, the size of blocks of the crystal lattice of grains of the axial and random components of texture of electrodeposited chromium on tin-free steel was investigated. Experimental results indicate that the size of the blocks of a random component of texture is 5-7 nm, and the size of the blocks of the axial component [111] of texture is 24-37 nm.

With the increase of current density, the size of blocks of a random component of texture of chromium coatings becomes somewhat higher (this fact is also valid for bulk chromium coatings[16]). And the character of changes of the size of blocks of the axial component [111] of the texture completely correlates to the change of the fraction of the axial texture component [111] (Figure 2). The obtained interdependence proves the conclusion about the axial texture with the axis [111] of chromium coatings on tin-free steel as a texture of growth.

The formation of texture of growth in chromium electrocoatings of minor thickness can be explained by the following. According to the model of texture formation in electrocoatings[17-20], the main factors determining the process of the formation of crystallographic texture in electrochemical coatings are the thermodynamic factors: grain boundaries energy (for fine-grained structure of a random component) or surface energy (for coarse-grained structure) at the stage of formation of nuclei of the main texture component. The axis of texture depends on the ratio of the energies. And at the stage of growth of grains of the main texture component, the main thermodynamic factor is the volume energy influencing the degree of perfection of texture. As the structure of randomly oriented grains of chromium coatings is very fine, the texture formation at the stage of initiation of grains of the axial component [111] is caused by a gain of grain boundaries energy of chromium being electrodeposited. Because of the nanocrystal structure of chromium coatings on tin-free steel[11], the formation of their texture begins in the layers of minor thickness.

Taking into account the fact that restricted texture of blackplate consists of the restricted components (111)[112], (100)[110] and (112)[110], the found restricted orientation of grains (111)[112] of thin chromium coatings on tin-free steel should be classified as a texture of epitaxial initiation.

3. 2 Interdependencies between texture and mechanical durability of chromium coatings on tin-free steel

As a result of joint investigations of texture and mechanical durability of chromium coatings on tin-free steel, it was found that the coatings with the least dispersed texture component [111] have the highest mechanical durability. Indeed, as it is seen at Figure 4, the coatings with the average dispersion angle α of the axial component [111] equal to 11.2° have a wear rate of 1.30 × 10−4µm/s, and the coatings with the angle α = 15.8° under the same conditions have 1.65 × 10−4µm/s.

Figure 4 Influence of texture of thin (0.03 μm) electrochemical chromium coatings of tin-free steel on their mechanical durability图4 无锡钢上镀铬薄层(厚度0.03 μm)的织构对其机械寿命的影响

The performed tests of mechanical durability of thin chromium coatings on tin-free steel with regard to the texture shown that with the increase of the average dispersion angle of texture from 10.6° to 17.4°, the wear rate of the coatings grows almost 1.5 times (from 1.26 × 10−4μm/s to 1.81 × 10−4µm/s). The dependence of wear rate (IV, µm/s) of thin chromium coatings on tin-free steel on the average dispersion angle of the axial component [111] of their texture (α, degrees) is described by the following expression:

The obtained result correlates with the data of the work[3], where the influence of texture on wear resistance of bulk chromium electrocoatings was determined.

The influence of texture on mechanical durability of thin chromium coatings on tin-free steel can be explained by the following. The wear of a material decreases when plastic deformation is hindered in its surface layer due to favorable orientation of grains relative to the friction plane. It is known that at the formation of the axial texture with the axis [111] in metals with bcc structure, plastic deformation under the influence of friction forces is hindered in comparison with other preferred orientations of grains. The decrease of the angle α of the axial component [111] of texture of chromium coatings on tin-free steel indicates the increase of the number of grains with the plane {111} strictly parallel to the surface of the coating (or the friction plane) and, hence, with the direction ＜111＞ strictly perpendicular to the surface of the coating. Therefore the increase of the degree of perfection of the axial component [111] of texture of thin chromium coatings on tin-free steel should cause the decrease of their ware rate.

Thus, it is experimentally discovered that one of the main factors influencing mechanical durability of thin chromium coatings on tin-free steel is the formation of the axial texture with the axis [111].

Experimental results indicate that thin chromium coatings on tin-free steel have sufficient adhesion strength and satisfied porosity (the highest level of porosity was not more than 0.2 pores/cm2). It was also found that chromium coatings increase reflecting ability of tin-free steel by 2%-3%.

The determination of the continuity of the coatings with nanocrystal structure after tensile and flexural deformation of tin-free steel at an Erichsen press shows that there are no pores at the elongation area at the depth of penetration of tin-free steel 2 mm, which is in compliance with the requirements for standard tests. Therefore the formation of nanostructure in thin chromium coatings provided adequate continuity during deformation of tin-free steel.

The results of the test of protective ability of passivated chromium coatings on tin-free steel show the ability of the coatings to preserve their protective properties during the whole period of testing (24 hours), and on the unprotected blackplate the spots of corrosion appear after 1-2 hours after the beginning of the test.

4 Conclusions

(1) The texture of nanostructural chromium coatings on tin-free steel consists of the following components: the axial component with the axis [111], perpendicular to the surface of the coating, the restricted component (111)[112], and a random component of texture. It is shown that the main texture component is the axial component with the axis [111], which is the texture of growth (at the stage of its initiation), and the secondary one is the restricted component of texture (111)[112], which is the texture of epitaxial initiation.

(2) Regular changes of quantitative characteristics of texture and substructure of chromium coatings on tin-free steel depending on deposition parameters are found. As a result, the parameters for obtaining the most texturized chromium coatings were determined: the electrolyte temperature 55 °С and the current density 45-55 A/dm2. The interdependence between quantitative characteristics of texture and substructure of chromium coatings on tin-free steel is found.

(3) It is discovered that one of the main factors influencing the mechanical durability of thin chromium coatings on tin-free steel is the formation of the axial texture with the axis [111] in the coatings.

(4) It is found that the formation of crystallographic texture with the axial component [111] in thin electrochemical chromium coatings provides sufficient level of their mechanical durability during the manufacturing of tin-free steel cans, and the formation of nanostructure provides adequate continuity of the coatings during deformation, which causes preservation of characteristics of the protective ability of deformed tin-free steel in various foodstuff environments.

Acknowledgments

The authors acknowledge the Science and Technology Center in Ukraine (STCU) for financial support to this research, Prof. J. A. Szpunar, McGill University of Canada, Prof. N. S. Spyrellis, National Technical University of Athens of Greece, and Dr. G. R. Stafford, National Institute of Standards and Technology of USA for the discussion of preliminary results of the work as well as industrial company Companhia Siderurgica Nacional of Brazil for the discussion of possible ways of commercialization of the obtained results.

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Crystallographic texture of electrochemical chromium coatings on tin-free steel as related to their mechanical durability

O. B. Girin*, V. I. Ovcharenko
( Materials Science Department, Ukrainian State University of Chemical Technology, Dnipropetrovsk 49005, Ukraine )

The features of crystallographic texture of thin (0.03 μm or 0.22 g/m2per side) electrodeposited chromium coatings, obtained on tin-free steel under conditions close to the industrial ones, were investigated by X-ray diffraction. It was found that the texture of chromium coatings consists of the following components: the axial component with the axis [111], perpendicular to the surface of the coating, the restricted component (111)[112] and a random component of texture. It was shown that the main texture component is the axial component with the axis [111], which is texture of growth (at the stage of its initiation), and the secondary one is the restricted component of texture (111)[112], which is texture of epitaxial initiation. Regular changes of quantitative characteristics of texture and substructure of chromium coatings on tin-free steel depending on deposition parameters were observed. There exists an interdependence between quantitative characteristics of texture and substructure of chromium coatings on tin-free steel. It was discovered that one of the main factors influencing mechanical durability of thin chromium coatings on tin-free steel is the formation of the axial texture with the axis [111] in the coatings.

tin-free steel; chromium coating; crystallographic texture; X-ray diffraction; mechanical durability

TQ153.11

A

1004 – 227X (2012) 08 – 0001 – 06

date:2012–03–09

Oleg B. Girin, (E-mail) girin@ua.fm.

Biography:Oleg B. Girin (1952–), Professor, D.Sc. (Engineering), the Head of the Materials Science Department, the Vice-Rector of Science of Ukrainian State University of Chemical Technology (Dnipropetrovsk, Ukraine). Main fields of his scientific activities – studying of electrochemical phase formation of metallic materials and developing of advanced technologies for producing coatings with enhanced properties. Under the scientific supervision of Prof. Oleg B. Girin a previously unknown phenomenon of electrochemical phase formation of metallic materials through a stage of liquid state has been discovered, the advanced technologies for producing new types of protective coatings on metal-roll have been developed and the special-property composite film materials have been obtained.

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