【作者】 彭曙；

【导师】 卢锦堂；

【作者基本信息】 华南理工大学 ， 材料物理与化学， 2010， 博士

【摘要】 钢铁的腐蚀造成巨大的损失,热镀锌是钢铁最有效的防大气腐蚀的方法之一。热镀锌工业应用时,为了获得需要的镀层,常在锌浴中添加其他的合金元素,Al是最常用的元素之一。而当锌浴中添加少量的Pb、Sb、Bi、Sn等元素时,热镀锌层表面会出现一种肉眼可见的树枝状晶粒,即“锌花(spangle)"。锌花是热镀锌特有的现象,引起众多机构和学者的广泛关注。然而到目前为止,锌花的形成机制以及很多细节尚未形成统一的认识,国内对锌花的深入研究也不多见。本文以批量热浸Zn-Al-Sb合金锌层表面形成的锌花为研究对象,利用SEM、BSE、EDS、AFM、XRD、XPS、NSS、EIS等测试手段系统地研究了锌花的形貌、形成机制,表面合金元素的偏析和第二相粒子的析出、锌花的耐蚀性以及腐蚀机制。研究表明：锌花是热镀锌层中的自由锌层以树枝晶生长的结果,每一个锌花代表一个单晶体的锌晶粒。根据锌花的外观形貌,可将锌花分为亮锌花、羽毛状锌花和暗锌花。锌晶体基平面(0001)相对于钢基表面可能的位向决定了锌花最终的几何形貌,按照基平面(0001)相对于钢基平面的倾斜角β值的不同,可将锌花分为三类：1)β=0°,锌花具有60°对称的正六边形树枝晶状结构；2)0°<β<90°,锌花为倾斜六边形结构；3)β=90°,锌花具有垂直树枝晶状结构。锌花一次和二次树枝晶晶臂的方向均为沿锌密排六方晶格的(1010)方向择优生长,与热流方向越接近越能持续生长。晶体择优取向和温度梯度择优取向相互作用决定了在同一个锌花内会形成亮锌花、羽毛状锌花和暗锌花的形貌特征。热镀锌镀层表面形成锌花时,每个锌晶粒的尺寸异常粗大。对Zn-0.2Al和Zn-0.2Al-0.1Sb合金的定向凝固试验表明,在相同的定向凝固冷却条件下,两种合金的柱状晶固相前沿的定向生长速度基本相同,但在Zn-0.2Al合金锭中出现了柱状晶向等轴晶的转变(CET),而Zn-0.2Al-0.1Sb合金锭则全部为柱状晶,没有出现CET。这表明添加合金元素Sb溶解了熔融合金中的活性晶核,显著地抑制了Zn-0.2Al合金锌液凝固过程中有效晶核的均匀形核,Sb抑制形核是导致热浸镀锌层生成大晶粒的锌花的原因。Sb和Pb是产生锌花镀层最常用的添加元素,但由于Pb不利于环保,而加Sb的工艺更值得研究。然而过多的Sb会形成脆性的AlSb相,影响镀层的质量。本文首次研究了不同冷却条件下的热镀锌用Zn-0～6.0Al-4Sb中间合金的凝固组织变化规律,结果表明：提高冷却速度,将细化合金的凝固组织,且较快的冷却速度可以抑制亚稳态ζ-Sb2Zn3相向稳态相β-Sb3Zn4的共析分解：ζ→β+η.炉冷和空冷时,ζ相粒子完全转变为β相,水冷时ζ相则被保留下来。平衡条件下,对Zn-0～6.0Al-4Sb合金的凝固组织分析可知出现4中组织状态,即β-Sb3Zn4+Eβ+Zn(β和Zn组成的共晶)；AlSb+Eβ+Zn；AlSb+ Zn；Zn+AlSb+EZn+Al(Zn和Al组成的共晶),这与提出的Zn-Al-Sb三元合金的室温等温截面对应的富锌角完全符合。锌浴中加入的合金元素会发生表面偏析,不同形貌的锌花表面存在不同程度的的合金元素偏析。在批量钢结构件热镀锌条件下,浸锌时间较长,镀层较厚,冷却速度较慢,合金元素的偏析和析出、以及析出的第二相由亚稳态向稳态(或非平衡态向平衡态)的转变都应更加充分,本文研究了典型的具有锌花的批量热浸Zn-0.05Al-0.2Sb合金锌花镀层的表面偏析规律。结果表面：锌花表面的偏析程度由亮锌花、羽毛状锌花到暗锌花依次增大。Al和Sb的偏析均集中在锌花镀层表面30nm以内,其中Al主要是以Al2O3形式存在,而Sb则是以薄片状的Sb3Zn4粒子存在。自由锌层中锌晶体基平面(0001)相对于钢基平面的结晶位向和枝晶的生长方向共同决定了第二相β-Sb3Zn4粒子的最终分布。亮锌花表面的β-Sb3Zn4粒子无规则分布,羽毛状锌花表面β-Sb3Zn4粒子主要分布在二次枝晶晶臂间隙,而暗锌花整个表面以及羽毛状锌花二次枝晶晶臂间隙内除了有层片状β-Sb3Zn4粒子外,还出现了雪花状Sb的枝晶偏析。不同形貌的锌花,耐蚀性能存在差异。本文研究了锌花在5％NaCl溶液中耐蚀性,塔菲尔极化测试和浸泡不同时间的电化学阻抗谱(EIS)测试结果表明,亮锌花、羽毛状锌花、暗锌花的极化电阻Rp和低频阻抗ZLF都依次减小,亮锌花、羽毛状锌花、暗锌花在5％NaCl溶液中的耐蚀性依次下降。影响锌花耐蚀性的因素包括晶面位向、表面粗糙度和表面合金元素的偏析程度。锌花在5％NaCl溶液中的腐蚀过程分为三个阶段：浸泡初期,主要是发生金属氧化物／氢氧化物膜的溶解和反应粒子的传质过程；浸泡中期,主要是发生腐蚀产物的溶解破坏过程；浸泡后期,主要发生基体锌层的腐蚀反应,在浸泡后期扩散过程起主要作用。更多还原

【Abstract】 The corrosion of steel causes much loss. Hot-dip galvanizing is the most effective way to protect steel from corrosion. In order to obtain a required coating in this process, minor aluminium is usually added into the metal bath. The hot-dip-coated surface often exhibits a structure consisting of very large grains, termed ’spangles’, when lead, antimony, bismuth, tin is/are added into the bath alloyed with aluminium. Spangles are special phenomenon of hot-dip galvanizing and have been received widely attention from institutes and researchers. Up to now, however, from the point of view of the formation mechanism and its details of were speculative, and the knowledge of spangle was only fragmentary in domestic studies.In this thesis, the spangles on batch hot-dipped coating were investigated. Spangle morphologies, formation mechanism, surface segregation of alloying elements, precipitation of second phase particles, corrosion resistance and corrosion mechanism were investigated using SEM, BSE, EDS, AFM, XRD, XPS, NSS and EIS. The results show that:Spangle is the result of dendritic growth of zinc layer, and each spangle represents a single-crystal zinc grain. On the basis of macroscopic surface appearance, spangles may be classified intod three types:shiny spangle, feathery spangle and dull spangle. The spangle crystallographic classification lies into three types according to the correlation between the basal plane (0001) and the inclined angleβwith respect to the steel sheet plane:1)β=0°, the spangle presents 60°symmetry dendrite structure with snow-flake or six-fold star structure.2) 0°<β<90°, the spangle presents inclined hexagonal structure or shiny/dull divided morphology.3)β=90°, the spangle presents dull and rough orthogonal-dendrite morphology. Both dendritic primary and secondary arms of spangle grow along the preferred direction (1010) of zinc crystal, the ones which are parallel to the thermal direction will posess sustained growth. The preferred direction and thermal gradent determine the formation of shiny spangle, feathery spangle and dull spangle in a single spangle area. Columnar-to-equiaxed transition (CET) Zn-0.2Al alloy was derived, whenas, no CET was found in Zn-0.2Al-0.1Sb alloy under the same directional solidification condition. This indicated that the addition of Sb (or Pb) restrains CET and reduces the amount of nucleus in unit volume and that Sb (or Pb) poisons these available nucleation sites, resulting in large spangle grains on hot-dip galvanized coatings.The coating with spangle can be produced by the addition of Sb (or Pb). As we know however, Pb is poisonous, and the research of application of Sb is more valuable. But overmuch of Sb addition would affect the quality of the coating by forming brittle AlSb phase. In this paper, solidification structures of Zn-Al-Sb master alloys under different cooling condition were investigated. The results show that solidification microstructure of the alloys are refined significantly as the cooling rate increases, and the eutectoid decomposition of metastableζtoβ:ζ→β+ηis inhibited under water cooling. Under equilibrium solidification condition, Zn-Al-Sb ternary phase diagram at ambient temperature can be divided into 4 phase zone:β-Sb3Zn4+Eβ+Zn; AlSb+Eβ+Zn; AlSb+Zn; Zn+AlSb+EZn+Al·The coating surface usually exhibited three kinds of spangles:shiny, feathery and dull spangle, of which extensively antimony surface segregation was detected. Batch hot dip galvanized coatings hold longer dipping time, thicker coating and slower coating solidification rate, thus resulting in a rougher coating surface and a greater surface segregation, and the transition of metastable intermetallic precipitate to stable state occurs more completed. In this paper, surface segregation rule of alloying elements on typical coating with spangle obtained in batch hot dipped Zn-0.05Al-0.2Sb bath was investigated. The results show that the segregation degree increased from shiny, feathery to dull spangle. Aluminium segregated toward the outer spangle surface in the form of Al2O3 layer with the thickness of 20nm, while antimony segregated toward the surface in the form ofβ-Sb3Zn4 particles with the thickness of 29nm. Final distribution of the precipitate is correlated with spangle morphology, which depends on interaction of crystal orientation and growth direction of the nucleated zinc grain in the zinc layer. Theβ-Sb3Zn4 particles randomly distribute on the shiny spangle surface;β-Sb3Zn4 particles and snow-like dendritic segregation of antimony exists in the dendritic secondary arm spacings of feathery spangle surface;β-Sb3Zn4 particles and dendritic segregation of antimony spread extensively on the whole dull spangle surface.Corrosion resistance in 5%NaCl solution differs from spangles. From the results of Tafel polarization and EIS under different immersion time it was concluded that both the polarization resistance Rp and low-frequency impedance ZLF decreased from shiny, feathery to dull spangle, and corrosion resistance of spangle in 5%NaCl solution decreased from shiny, feathery to dull spangle in turn. The difference of corrosion resistance of spangle follows into three factors:the crystallographic orientation of crystal plane, surface roughness, the amount of alloying elements and precipitates segregated on the spangle surface. There are three stages during the corrosion process of spangle immersed in 5%NaCl solution, they are:the initial immersion stage involves the dissolution of superficial metallic oxide/hydroxide layer and the mass transfer of reaction particles; the middle immersion stage of which the relaxation of corrosion products layer takes place and, the later immersion stage involves the dissolution of zinc associated with the diffusion process, which controls the zinc dissolution.更多还原