摘要
镁及镁合金(如AZ91D)具有密度小、比强度高、尺寸稳定性好及良好的磁屏蔽性等一系列特性,正成为现代高新技术领域中最有希望的材料,其性能优于铝基合金。它以其优异的力学性能、物理性能已经广泛应用于航空航天、汽车工业、电子工业等领域。然而,镁合金的耐蚀性和耐磨性差使其应用受到了制约。如何解决这个问题,一直是人们所关注的课题。化学镀镍是近年来广泛运用的一种表面处理方法,其镀层具有结构致密、厚度均匀、硬度高、耐蚀耐磨性好等优点。但在镁合金化学镀镍的工艺中,酸性和碱性镀液都会对镁合金基体造成严重的腐蚀,直接影响镀层的性能。因此,怎么样减小镀液对基体的腐蚀是一个很重要的研究方向,然而对镁基体腐蚀较小的中性化学镀液报道却不多。本文研究了PH值为6.5～7的中性镀液中的镁合金化学镀Ni-P的工艺、镀层性能和相结构,并且在前处理液中没有加入CrO_3,不会产生对人体有害的Cr~(6+),减轻了对环境的压力。
本文研究了无铬预处理和中性镀液中化学镀Ni-P合金工艺,利用金相显微镜和Axiovert25CA(Zeiss)光学图像分析仪观察镀层的组织和表面形貌及其显微组织,发现镀层表面呈球胞状堆积,胞与胞之间结合较紧密,大胞由若干个“变形”的小胞组成。用WS-97型声发射划痕仪进行划痕试验测定镀层的结合强度,可以看出镁合金基体上化学镀Ni-P镀层的F_c大约为40-60N,镀层与基体结合良好。用电子天平称重并计算镀速,发现主盐浓度、还原剂的浓度、络合剂和温度都会影响镀层的沉积速度。用GDS(辉光放电光谱分析仪)测定镀层成分分布和X射线衍射(XRD)检测相结构,得出镀层磷含量在9.5%～11%之间,X射线衍射分析曲线呈“馒头”状,具有典型非晶态合金的衍射图样,说明镀层呈非晶态结构。
用磨损试验仪测得Ni-P合金镀层与GCr15钢珠对磨的平均摩擦系数较AZ91D镁合金基摩擦系数有较大减小,经过回火后镀层硬度提高,摩擦系数进一步减小。磨痕形貌表明,镁合金基体在干摩擦条件下的承载能力很差,磨损比较严重,磨痕较宽,并且有多处小块剥落和大量的平行犁沟。经化学镀镍磷后,点蚀略多,磨痕浅而窄,犁沟则浅而细小,耐磨性有所提高。经过400℃回火,犁沟和粘着脱落现象不明显,只是轻微的擦伤,说明Ni-P镀层回火后耐磨性能良好。
电化学测试表明经过化学镀镍处理后的镁合金试样,在3.5%NaCl水溶液中,其耐蚀性得到了很大的提高,镀镍层的自腐蚀电位为-615mv,比镁合金基体的自腐蚀电位上升了843mv。与原始镀层相比,回火热处理后随着温度的提高,自腐蚀电位呈现先上升后急剧下降的特征。200℃回火后自腐蚀电位提高,钝化现象也很明显,耐腐蚀性能加强;经过300℃、400℃回火后自腐蚀电位下降,耐腐蚀性明显减弱。盐雾测试发现镁合金化学镀镍层连续喷雾8小时未见有明显的腐蚀斑点,耐腐蚀性能良好。喷雾时间达到20小时,镀层局部出现了腐蚀,尤其是试样边缘,有很多白色棉花状腐蚀产物出现。
Magnesium and magnesium alloy(such as AZ91D)had low density,high strength,good size stability and good magnetic shield,which became the most hopeful material in modern high-tech fields.Its performance was superior to aluminum alloy.Because of its mechanical properties and physical properties, it was widely used in aerospace,automotive industry,electronics industry and so on.However,poor corrosion resistance and wear resistance of magnesium and magnesium alloy constraint its use.How to solve this problem,had been a subject of concern to people all the time.Chemical nickel plating was widely used in recent years as a surface treatment methods.Its coating had the benefits of tight structure,uniform thickness,hardness,good corrosion resistance and good wear resistance.However,in the processes of chemical nickel on magnesium alloy,acidic or alkaline solution would cause serious corrosion to magnesium alloy matrix,impacting on the performance of coating directly.So how to reduce corrosion of solution to matrix was also a very important research direction.However,the neutral chemical bath which had small corrosions to magnesium matrix are not widely reported.In this paper,we studied the magnesium alloy electroless Ni-P process in the PH value of 6.5 to 7 neutral bath、the coating performance and structure,and we added none of the CrO_3 to pre-treatment bath,not producing Cr~(6+)which was harmful to human body and reducing the pressure on the environment.
We had studied the pretreatment on the chromium-free and neutral in the chemical bath Ni-P alloy plating process and use of metallographic microscope and Axiovert25CA(Zeiss)optical image to analyize and observe the coating surface topography and microstructure,finding that the coating surface with the ball cellular accumulation,cellular and cellular combine closely,every large cell was consitiuted of several deformed small cells.From the WS-97-AE-scratch coating for scratch test of the combination of strength,we could see that the matrix of magnesium alloy plating on the chemical coating of Ni-P FC approximately 40-60 N,coating and the matrix combining well.Using electronic weighing scales and calculate plating rate,we found main salt concentration,reductant concentration chelators and temperature would affect the coating deposition rate.Useing of GDS(glow discharge spectrometer)to determine the coating composition and distribution while using of X-ray diffraction(XRD)detect Structures,we came to the conclusion that the phosphorus content in the Coating is in the range of 9.5%to 11%,X-ray diffraction analysis curve was like steamed bread,possessing diffraction pattern of the typical of amorphous alloy,showing that the coating was amorphous structure.
By wear tester we found the friction coefficient of Ni-P coating and Steel ball's mutual frictions was larger than magnesium alloy AZ91D.After tempering coating hardness reduce,friction coefficient reduced more.Wear track morphology indicate that under the conditions of the dry friction the carrying capacity of magnesium alloy matrix was poor,friction was more serious,trace of friction was wider.Furthermore various small spalling and a large number of parallel plow ditch.After chemical nickel phosphorus,there were slightly more pittings trace of friction is shallow and narrow,plow ditch shallow and small, wear resistance had increased.
After tempering 400℃,plow ditch and loss of adhesion was not obvious, only minor abrasions,showing after tempering the wear resistance of Ni-P coating had increased.
Electrochemical tests showed that the sample of magnesium alloy which was done after chemical nickel processing whose corrosion resistance has been greatly improved.Nickel layer of corrosion potential was-615,which had increase by 843 mv compared with magnesium alloys.Compared with the original coating,after tempering heat Treatment,the corrosion potential at first rise after the sharp decline characteristics.
After tempering at 200℃,the corrosion potential increase,Passivation was also very clear,corrosion resistance has strengthened.Aftertempering at 300℃and 400℃the corrosion potential decline,corrosion resistance significantly weakened。Salt spray test found that magnesium alloy of nickel plating chemical spray for eight hours with no significant corrosion spots,well corrosion resistance.When the spray time was up to 20 hours,Corrosion appeared in local coating,especially in the edge of the sample,a lot of corrosion product like white cotton appeared.
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