金相試樣切割機的設(shè)計,金相,試樣,切割機,設(shè)計 河南科技學院 2009屆本科畢業(yè)設(shè)計（論文）,設(shè)計題目：金相試樣切割機的設(shè)計 學生姓名： 張靜 所在院系： 機電學院 所學專業(yè)： 機電技術(shù)教育 導師姓名： 劉貫軍,【摘 要】： 金相試樣切割機主要用于金相試樣的截取和各種材料的下料、切口等，在冶金、汽車、航空航天等制造業(yè)中應(yīng)用極為廣泛。20世紀90年代后，金相制樣技術(shù)發(fā)展極為迅速，金相試樣切割機作為金相取樣設(shè)備也取得了很大的進步。 本設(shè)計通過對金相試樣切割機的整體造型、機械結(jié)構(gòu)和控制系統(tǒng)進行了分析，完成了切割機主體結(jié)構(gòu)的設(shè)計，控制系統(tǒng)采用了銑床導軌原理，實現(xiàn)了低成本和手動化。 最后確定切割機的裝配總圖。通過此次設(shè)計，掌握了相關(guān)設(shè)計的主要步驟，并對于Pro—E軟件應(yīng)用方面有了進一步的提高。,1 引言 金屬零件的力學性能不僅與它的化學成分有關(guān)，也與它的金相組織密切有關(guān)。金相檢驗是控制和評定產(chǎn)品質(zhì)量不可缺少的重要手段，是科學研究中研究新材料、新工藝和提高金屬制品內(nèi)在質(zhì)量的重要方法。 要進行金相分析，就必須制備能用于微觀檢驗的樣品— 金相試樣。通常，金相試樣的制備要經(jīng)過取樣、鑲嵌、磨光和拋光幾個步驟。每個步驟都應(yīng)該細心操作，因為任何階段上的失誤都可能影響最后的結(jié)果，因為這可能會造成組織假象，從而得出錯誤的結(jié)論。金相試樣的制備是通過切割機、鑲嵌機、磨／拋光機來完成。金相試樣的截取是金相試樣制備過程中一個重要環(huán)節(jié)。截取試樣的方法有手鋸、鋸床、砂輪切割機和線切割機等等。根據(jù)零件的形狀和材料，選擇適當?shù)姆椒▉砬懈睢?目前砂輪切割機廣泛應(yīng)用于金相試樣的截取上，主要原因是其適應(yīng)性強，樹脂砂輪片可切割軟的金屬零件如銅、鋁及合金和硬的金屬零件如淬火后的碳鋼、高速鋼；金剛石切割機可切割超硬材料如硬質(zhì)合金、陶瓷等。另外其切割速度快、勞動強度低、操作簡便和切割成本低。選擇可靠性高的金相試樣切割機，可以提高制樣效率和質(zhì)量，降低成本，提高經(jīng)濟效益。,金相試樣切割機主要特點：本切割機的切割砂輪直接固定在與電動機的軸同軸線相連接的軸上，利用滑板箱的橫向和縱向的移動來切割固定在鉗口中的試樣 電動機固定在底座上，軸套套在電動機的軸上，砂輪片由螺母和夾片加以固定。固定在電動機的前面的滑板箱上裝有可沿縱向移動的加緊裝置，由手柄的轉(zhuǎn)動來移動鉗口把試樣夾緊在鉗座中，當轉(zhuǎn)動手柄時，就可以進行試樣切割了。機器工作時，由罩殼將砂輪片等檔住，以防冷卻液飛濺和砂輪片碎裂時飛出傷人,2 設(shè)計要求 金相試樣切割機的具體設(shè)計要求為： （1）利用Pro--E軟件設(shè)計 （2）確定結(jié)構(gòu)的尺寸 （3）繪制相應(yīng)的零件圖、實體圖及總裝配圖,3 切割機的總體設(shè)計過程 3.1 電動機的選擇,,3.2 傳動機構(gòu)的設(shè)計 3.2.1 軸的計算 3.2.2 軸的結(jié)構(gòu)設(shè)計,,3.3 控制系統(tǒng)的設(shè)計 3.3.1夾具的主要結(jié)構(gòu)與使用,,3.3.2 進給機構(gòu)的設(shè)計,,,,上 滑 板,,,中 滑 板,,,下 滑 板,,,進 給 裝 置,4 用Pro—E軟件對切割機進行實 體造型和裝配 4.1 切割機各主要零件的實體造型,,軸 的 實 體 圖,,上 滑 板 的 實 體 圖,,中 滑 板 的 實 體 圖,,下 滑 板 的 實體 圖,,進 給 系 統(tǒng) 的 實體 圖,4. 2 切割機的裝配,,切 割 機 的 內(nèi) 部 實 體 圖,5 結(jié)束語 在此次設(shè)計的過程中，培養(yǎng)了我的綜合運用所學知識的能力，分析和解決實際中所遇到問題的能力，并且能鞏固和深化我所學的專業(yè)知識，使我在調(diào)查研究和收集資料等方面有了顯著的提高，同時在理解分析能力、制定設(shè)計計算和繪圖能力方面有較大的進步；另外我的技術(shù)分析和組織工作的能力也有一定程度的提高。,致謝 非常感謝學院領(lǐng)導和老師給我提供了這次良好的深入學習的機會和寬松的學習環(huán)境。通過這次畢業(yè)設(shè)計，不但使我將大學期間所學的專業(yè)知識再次回顧學習，而且也使我學到了專業(yè)領(lǐng)域中一些前沿的知識。非常感謝在本次設(shè)計中曾給予我耐心指導和親切關(guān)懷的老師及幫助過我的同學，正是由于他們的幫助和鼓勵才使我能夠在畢業(yè)設(shè)計過程中克服種種困難，最終順利完成論文，他們的學識和為人也深深地影響著我。在此，請允許我再次向曾直接給予我多次指導的導師表示最忠誠的敬意！同時也感謝百忙之中前來參加答辯的各位老師、專家和教授!,敬請指導批正!!!,謝謝!!! 答辯人：張靜,河南科技學院本科生畢業(yè)論文（設(shè)計）課題審核表 院（系）名稱 機電學院 專業(yè)名稱 機電技術(shù)教育042 指導教師姓名 劉貫軍 課題名稱 金相試樣切割機設(shè)計 課題來源 自擬課題 立題理由 和所具備 的條件 　　 金相試樣切割機種類很多，但適合本院專業(yè)實驗室條件的用于制做透射電鏡樣品精密切割的切割機卻很少見，國外有符合要求的此類產(chǎn)品，但價格昂貴。隨著科研工作的深入，設(shè)計制做一種高精度低成本的金相試樣切割機很有必要，而且設(shè)計條件已經(jīng)具備。 教研室 審批意見 教研室主任簽字： 年 月 日 畢業(yè)論文（設(shè) 計）工作領(lǐng)導 小組審批意見 組長簽字：： 年 月 日 注：本表存院（系）備查。 學生姓名 張靜 班級 機教042 指導教師 劉貫軍 論文（設(shè)計）題目 金相試樣切割機的設(shè)計 目前已完成任務(wù) 1.制定畢業(yè)設(shè)計計劃 2.查找相關(guān)文獻 3.完成畢業(yè)論文開題報告 是否符合任務(wù)書要求進度：符合 尚需完成的任務(wù) 1.繼續(xù)對論文材料進行組織和整理； 2.按照論文提綱，有步驟有計劃的開展論文工作，存在問題要及時與老師溝通； 3.對已完成的論文內(nèi)容進行檢查審核，力求把問題降到最少； 4.到規(guī)定的時間完成論文初稿； 5.根據(jù)指導老師的指導意見和全部材料完成論文； 能否按期完成論文（設(shè)計）：能 存在問題和解決辦法 存 在 問 題 閱讀資料不足，對論文主題的研究不夠透徹，且相關(guān)的理論知識還不夠全面； 與指導老師的交流不夠充分。 擬 采 取 的 辦 法 繼續(xù)查找資料，加強對相關(guān)理論知識的理解和掌握，應(yīng)多和老師交流，在老師的指導下更好完成設(shè)計。 指導教師簽 字 日期 年 月 日 教學院長（系主任） 意 見 簽字： 年 月 日 河南科技學院本科畢業(yè)論文（設(shè)計）中期進展情況檢查表 河南科技學院本科生畢業(yè)論文（設(shè)計）任務(wù)書 題目名稱 金相試樣切割機的設(shè)計 學生姓名 張靜 所學專業(yè) 機教 學號 20040315049 指導教師姓名 劉貫軍 所學專業(yè) 機械設(shè)計 職稱 教授 完成期限 2008 年 11 月 01 日 至 2009 年 05 月 24 日 一、論文（設(shè)計）主要內(nèi)容及主要技術(shù)指標 1、連接金相試樣切割機的主要用途，國內(nèi)外研究及使用狀況（包括選擇國內(nèi)市場上此類產(chǎn)品的性能及不足）； 2、研究制定設(shè)計方案； 3、對受力構(gòu)件進行受力分析并有必要計算后方可進行設(shè)計制圖； 二、 畢業(yè)論文（設(shè)計）的基本要求 1、 通過互聯(lián)網(wǎng)、校內(nèi)期刊數(shù)據(jù)庫等途徑了解切割機的工作原理、分析存在問題，提出改進方案； 2、 學習并熟練使用Pro—E繪圖軟件，并用其進行零件和產(chǎn)品設(shè)計（重要部件應(yīng)有受力分析），提交任務(wù)內(nèi)的全部零件圖及部件總成圖。 3、 完成不少于2000字（單詞）的專業(yè)英文資料翻譯。 三、畢業(yè)論文（設(shè)計）進度安排 2008年11月1日—12月30日 查找相關(guān)專業(yè)資料，熟悉Pro—E繪圖軟件的使用，提交開題報告，論證設(shè)計方案、完成不少于2000單詞英文資料翻譯稿。 2009年2月16日—5月16日 基本完成畢業(yè)設(shè)計規(guī)定的繪圖任務(wù)。 2009年5月17日—5月24日 撰寫畢業(yè)論文（設(shè)計說明書）。 2009年5月24日交齊全部畢業(yè)設(shè)計資料。 畢業(yè)設(shè)計（論文）開題報告 題目名稱： 金相試樣切割機 學生姓名 張靜 專業(yè) 機電技術(shù)教育 班級 042 一、選題的目的意義 目前,正處在科學技術(shù)飛速發(fā)展的信息時代,自動化、最優(yōu)化、集成化、智能化和精密化等使現(xiàn)代機械制造行業(yè)正經(jīng)歷著巨大的變化,也是其今后發(fā)展的必然趨勢.金相取樣設(shè)備作為其中一個重要分支,正在由原來的手工操作逐漸走向半自動化和自動化.為此,我設(shè)計了對金相切割的半自動化控制系統(tǒng). 與此同時在設(shè)計的過程中，能培養(yǎng)我綜合運用所學知識，分析和解決實際中所遇到的問題，并且能鞏固和深化我所學的專業(yè)知識，使我在調(diào)查研究和收集資料等方面有了顯著的提高，同時在理解分析能力、制定設(shè)計或試驗方案能力、設(shè)計計算和繪圖能力方面有較大的進步；另外我的技術(shù)分析和組織工作的能力也有一定程度的提高。 希望在此次畢業(yè)設(shè)計中，充分發(fā)揮出我們的創(chuàng)新能力，樹立良好的學術(shù)思想和工作作風，牢牢把握住這次走上崗位之前的實踐機會，充分鍛煉出自己的工作能力。 二、國內(nèi)外研究綜述 金屬零件的力學性能不僅與它的化學成分有關(guān)，也與它的金相組織密切有關(guān)。金相檢驗是控制和評定產(chǎn)品質(zhì)量不可缺少的重要手段，是科學研究中研究新材料、新工藝和提高金屬制品內(nèi)在質(zhì)量的重要方法。 要進行金相分析，就必須制備能用于微觀檢驗的樣品— — 金相試樣。通常，金相試樣的制備要經(jīng)過取樣、鑲嵌、磨光和拋光幾個步驟。每個步驟都應(yīng)該細心操作，因為任何階段上的失誤都可能影響最后的結(jié)果，因為這可能會造成組織假象，從而得出錯誤的結(jié)論。金相試樣的制備是通過切割機、鑲嵌機、磨／拋光機來完成。金相試樣的截取是金相試樣制備過程中一個重要環(huán)節(jié)。截取試樣的方法有手鋸、鋸床、砂輪切割機和線切割機等等。根據(jù)零件的形狀和材料，選擇適當?shù)姆椒▉砬懈?。目前砂輪切割機廣泛應(yīng)用于金相試樣的截取上，主要原因是其適應(yīng)性強，樹脂砂輪片可切割軟的金屬零件如銅、鋁及合金和硬的金屬零件如淬火后的碳鋼、高速鋼；金剛石切割機可切割超硬材料如硬質(zhì)合金、陶瓷等。另外其切割速度快、勞動強度低、操作簡便和切割成本低。選擇可靠性高的金相試樣切割機，可以提高制樣效率和質(zhì)量，降低成本，提高經(jīng)濟效益。 金相試樣切割機主要特點：本切割機的切割砂輪直接固定在與電動機的軸同軸線相連接的軸上，利用滑板箱的橫向和縱向的移動來切割固定在鉗口中的試樣 電動機固定在底座上，軸套套在電動機的軸上，砂輪片由螺母和夾片加以固定。固定在電動機的前面的滑板箱上裝有可沿縱向移動的加緊裝置，由手柄的轉(zhuǎn)動來移動鉗口把試樣夾緊在鉗座中，當轉(zhuǎn)動手柄時，就可以進行試樣切割了。機器工作時，由罩殼將砂輪片等檔住，以防冷卻液飛濺和砂輪片碎裂時飛出傷人。 三、畢業(yè)設(shè)計主要研究內(nèi)容 1、研究切割機的切割原理； 2、利用Pre-E軟件繪制切割機模型； 3、繪制相應(yīng)的零件圖及總裝配圖 四、畢業(yè)設(shè)計（論文）的研究方法和技術(shù)路線 1．采用理論和實際操作相結(jié)合的方式再結(jié)合現(xiàn)代設(shè)計理念的基礎(chǔ)上，利用現(xiàn)有的條件來進研究。 2．結(jié)合指導教師的教學經(jīng)驗來重新完善和提高自己新的認識和研究。 3．大量查閱有關(guān)書籍和資料來擴充自己視野與認識，提高理論成果的技術(shù)含量。 4．充分利用互連網(wǎng)來查找最新技術(shù)成果，提高自身的創(chuàng)新意識。 五、主要參考文獻與資料獲得情況 ［1］成大先。機械設(shè)計手冊[Ｍ]。北京：化學工業(yè)出版社，2004 ［2］成大先。機械設(shè)計手冊 第四版[Ｍ]。北京：化學工業(yè)出版社，2002 ［3］毛謙德,李振清。袖珍機械設(shè)計手冊 第三版[Ｍ]。北京：機械工業(yè)出版社，2007 ［4］機械設(shè)計實用手冊編委會。 機械設(shè)計實用手冊[Ｍ] 。北京：機械工業(yè)出版社，2008 ［5］陳立德。 機械設(shè)計基礎(chǔ)課程設(shè)計[Ｍ]。北京：高等教育出版社，2006 ［6］濮良貴，紀名剛。機械設(shè)計 第八版[Ｍ]。北京：高等教育出版社，2007 ［7］朱金波。Pro—E 3.0 工業(yè)產(chǎn)品設(shè)計完全掌握[Ｍ]。北京：兵器工業(yè)出版社，2007 ［8］金鑫，陳雪梅，賈長治。Pro—E 3.0中文版機械設(shè)計專家指導教程[Ｍ]。 北京：機械工業(yè)出版社，2007 ［9］ 曹巖。Pro—E 3.0 機械設(shè)計實例精解[Ｍ]。北京：機械工業(yè)出版社，2007 ［10］朱文堅，黃平，吳昌林。機械設(shè)計[Ｍ]。北京：機械工業(yè)出版社，2005 ［11］朱龍根。機械設(shè)計[Ｍ]。北京：機械工業(yè)出版社，2006 ［12］吳克堅，于曉紅，錢瑞明。機械設(shè)計[Ｍ]。北京：高等教育出版社，2003 六、指導教師審批意見 年 月 日 The electroless nickel-plating on magnesium alloy using NiSO4d6H2O as the main salt Jianzhong Lia,*, Zhongcai Shaob, Xin Zhanga, Yanwen Tiana aSchool of materials and metallurgy, Northeastern University, Shenyang 110004, China bFaculty of Environment and Chemical Engineering, Shenyang Institute of Technology, Shenyang 110168, China Received 23 July 2004; accepted in revised form 19 December 2004 Available online 26 January 2005 Abstract In this paper, the electroless nickel-plating on magnesium alloy was studied, using NiSO4d 6H2O as the main salt in the electroless plating alkaline solutions. The effects of the buffer agent and plating parameters on the properties and structures of the plating coatings on magnesium alloy were investigated by means of scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and Xray diffraction (XRD). In addition, the weight loss/gain of the specimens immersed in the test solution and plating bath was measured by using the electro-balance, to evaluate the erosion of the alloy in the plating solutions. The adhesion between the electroless plating coatings and the substrates was also evaluated. The compositions of the non-fluoride and environmentally friendly plating bath were optimized through Latin orthogonal experiment. The buffer agent (Na2CO3) added to the plating bath was found to be useful in increasing the growth rate of the plating coating, adjusting the adhesion between the electroless plating coatings and the substrates, and maintaining the pH value within the range of 8.5–11.5, which is required for the successful electroless nickel-plating on magnesium alloy with NiSO4d 6H2O as the main salt. Trisodium citrate dihydrate was found to be an essential component of the plating bath to plate magnesium alloy, with an optimum concentration of 30 g L_1. The obtained plating coatings are crystalline with preferential orientation of (111), having advantages such as lowphosphorus content, high density, low-porosity, good corrosion resistance and strengthened adhesion. D 2004 Elsevier B.V. All rights reserved. Keywords: Magnesium alloy; Electroless plating; Buffer; Corrosion resistance; Adhesion 1. Introduction The use of magnesium alloys in a variety of applications, particularly in aerospace, automobiles, and mechanical and electronic components, has increased steadily in recent years as magnesium alloys exhibit an attractive combination of low density, high strength-to-weight ratio, excellent castability, and good mechanical and damping characteristics. However, magnesium is intrinsically highly reactive and its alloys usually have relatively poor corrosion resistance, which is actually one of the main obstacles to the application of magnesium alloys in practical environments [1–3]. Hence, the application of a surface engineering technique is the most appropriate method to further enhance the corrosion resistance. Among the various surface engineering techniques that are available for this purpose, coating by electroless nickel is of special interest especially in the electronic industry, due to the possession of a combination of properties, such as good corrosion and wear resistance, deposit uniformity, electrical and thermal conductivity, and solderability etc. As far as magnesium alloys are concerned, the main salts of electroless plating solutions mostly focus attentions on basic nickel carbonate or nickel acetate [4–9], which result in high-cost, low-efficiency, instability of electroless plating solutions and little applications. In addition, the basic nickel carbonate or nickel acetate of plating solutions yet including fluoride, are harmful to the environment, therefore, it is urgently needed to develop new environmentally friendly plating bath. It is difficult to carry 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.12.009 * Corresponding author. Tel.: +86 24 8368 7731; fax: +86 24 2398 1731. E-mail address: mengsuo66@163.com (J. Li). Surface & Coatings Technology 200 (2006) 3010– 3015 www.elsevier.com/locate/surfcoat out electroless plating on magnesium alloys due to the highcorrosion rate of magnesium alloys in the plating bath with NiSO4d 6H2O or NiCl2d 6H2O as the main salt. It is reported [10] that the corrosion rate of magnesium and its alloys in NaCl solutions solely depends on the pH of the buffered chloride solutions. The objective of this study was to find a buffer agent and determine how the buffer agent affects the dissolution of magnesium alloy in NiSO4d 6H2O alkaline solutions, and the non-fluoride plating solutions for magnesium alloy with NiSO4d 6H2O as the main salt. The microstructure, compositions and corrosion behavior of the coatings were investigated in detail. 2. Experimental The substrate material used in the research was AZ91D ingot-cast alloy. The chemical composition of the alloy is given in Table 1. Substrates with a size of 50 mm_40 mm_20 mm were used in the research. The substrates were mechanically polished with emery papers up to 1000 grit to ensure similar surface roughness. The polished substrates were thoroughly washed with distilled water before passing through the precleaning procedure as shown in Table 2. The substrates were air-dried after the fluoride activation (the last step in the pre-cleaning procedure). In a typical experiment, the initial weight of a air-dried substrate was measured and then quickly transferred to the plating bath (1000 mL) in a glass container placed in a water bath with a constant temperature of 80 8C. A fresh bath was used for each experiment to avoid any change in concentration of bath species. The bath compositions and other parameters used in these experiments are given through Latin orthogonal experiment in Table 3. Final weights of the specimens were determined and the coating rates in micrometer per hour were calculated from the weight gain. At the same time, in order to study the each buffer’s influence on the substrates and find a buffer appropriate for the electroless plating on magnesium alloy, test solutions with compositions similar to those of the plating bath except that sodium hypophosphite was not added, were prepared to simulate the corrosion rates of magnesium alloy in plating bath and the behaviors of the buffers. Duplicate experiments were conducted in each case, and the coating rate reported is the average of two experiments. The growth rates of the plating coating were measured using the electro-balance made in America, which is the 0.1 mg precision. In the research, the pH value of plating bath was monitored by a pHS-25C model of precision pH/mV meter. Morphology of the coatings was analyzed using a scanning electron microscope. The energy dispersive X-ray spectroscopy analysis was used for determining the content of phosphorus in the coatings. Crystallinity of the coatings was investigated by Rigaku D/ max-rA X-ray diffractometer with Cu K-alpha radiation. The adhesion strength of the electrolessly deposited nickel coatings to the magnesium alloy substrates was determined by scratch test. During the scratch test, the specimen was moved at a constant speed of approximately 11.4 mm/min. Scratches were generated on the specimen using a diamond indenter with a spherical tip of 300 Am in diameter. Corrosion potential measurement in 3.5 wt.% NaCl solution was carried out to comparatively investigate the corrosion behaviors of the bare substrate and the nickel-plated substrates. The electrochemical cell used for corrosion potential measurement consisted of a bare substrate or a nickel-plated substrate as the working electrode (exposed area: 1 cm2), a saturated calomel electrode (SCE), and a platinum-foil counter electrode. Table 1 Chemical composition of the AZ91D alloy (in wt.%) Al Mn Ni Cu Zn Ca Si K Fe Mg 9.1 0.17 0.001 0.001 0.64 b0.01 b0.01 b0.01 b0.001 Bal Table 2 Optimized pre-cleaning procedure Table 3 Optimized bath composition and parameters Bath species and parameters Quantity NiSO4d 6H2O 25 g/L NaH2PO2d H2O 30 g/L C6H5Na3O7d 2H2O 30 g/L Na2CO3 30 g/L NH3d H2O Adjusting pH pH value 11 Temperature 80F2 8C J. Li et al. / Surface & Coatings Technology 200 (2006) 3010–3015 3011 3. Results and discussion 3.1. The buffers’ behaviors in the test NiSO4 solutions and the choice of an appropriate buffer Fig. 1 shows the variation of weight loss of magnesium alloy as a function of the immersion time with different buffers in the test solutions. The compositions and the controlled temperature of the test solutions were similar to those of the plating bath except that sodium hypophosphite was not included. The pH values of the test solutions were adjusted by NH3d H2O to fix at 11. The weight loss increases linearly with the immersion time increasing of magnesium alloys in the Na2CO3, Na2 B4O7, and CH3COONa test solutions. It is revealed in Fig. 1 that the corrosion rates were constant throughout the examined immersion time. As recognized from the slope of each solid line in Fig. 1, corrosion rate in the test solution containing Na2CO3 buffer is the lowest among the three tested buffers. The obtained slopes are 0.015, 0.022 and 0.056 mg cm_2 min_1 for Na2CO3, Na2 B4O7 and CH3COONa buffers, respectively. These results can be explained in terms of dissociation constants of the corresponding acids, which are k 2=4.7_10_11 ( k 1=4.4_10_7 ) , k 2=1_10_9 (k1=1_10_4), and k=1.75_10_5 for H2CO3, H2B4O7 and CH3COOH, respectively. The second dissociation constant of a binary acid decides the buffer capability of the buffer. Obviously, the Na2CO3 buffer has the lowest cost and best buffer capability among the tested buffers. Fig. 2 shows the weight loss of the substrates versus immersion time in the test solutions with pH values at 9, 10 and 11, using Na2CO3 as the buffer. Corrosion of the specimens in non-buffered test solutions with pH values at 9, 10 and 11 was also investigated. The corresponding weight loss curves are shown in Fig. 2. All test solutions used for these experiments had compositions similar to those in the plating bath except that sodium hypophosphite was not included. The weight loss linearly changes with the increase of the immersion time in all cases shown in Fig. 2. Under the same pH value, the corrosion rate of the substrates in the buffer solution is obviously lower than that of the substrates in the non-buffered solution, as shown by the slopes of the curves in Fig. 2. This suggests that the buffer solution has a considerable effect on the corrosion rate of magnesium alloy. In both the Na2CO3 buffered and non-buffered test solutions, the corrosion rates of magnesium alloy decrease with the increase of the pH value. This indicates the weight-loss of the substrates is related to the reaction between the substrate metal and the hydrogen ions. But the corrosion reaction between the substrate metal and the hydrogen ions goes gradually on, because the low concentration of hydrogen ions is presented in the plating alkaline solutions. And then, the concentration of hydrogen ions is weakly decreased during the test progress. This leads to the constant corrosion rates in the short test time, which is shown in Figs. 1 and 2. At the same time, knowing that for Mg(OH)2 Ksp at 25 8C=8.9_10_12 at pH 9, [OH_]=10_5 M, most Mg2+ diffused into plating solution to form up to 10_2 M. At pH 11, [OH_]=10_3 M, the [Mg2+] couldn’t exceed 10_6 M, thus most Mg2+ formed Mg(OH)2 and stayed near the substrate. Mg(OH)2 could increase the adsorption energy barrier and reduce the corrosion rate. Therefore, higher pH resulted in lower corrosion rate. As to the Na2CO3 buffered solutions, for MgCO3 Ksp at 25 8C=10_15, in test solutions, [Na2CO3]N0.1 M, thus the possible [Mg2+]b10_14 M. This means that the driving force for Mg to form Mg2+ was very low. Instead of dissolving Mg, the CO3 2_ ion would bond or be adsorbed to the substrate surface to form local Mg—CO3 2_. In this case, the substrate surface area exposed to H2O or H+ was reduced a lot, 0 5 10 15 20 25 30 35 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Na(CH3COO) Na2B4O7 Na2CO3 Weight loss/mg.cm-2 Time/min Fig. 1. The variation of weight loss of magnesium alloy in test solutions with different buffers. 0 5 10 15 20 25 30 35 0 1 2 3 4 5 solution pH=9 pH=10 pH=11 pH=9 pH=10 pH=11 Weight loss/mg.cm-2 Time/min in non-buffered solution in Na2CO3 buffered Fig. 2. The variation of weight loss of magnesium alloy in test solutions with different pH values. 3012 J. Li et al. / Surface & Coatings Technology 200 (2006) 3010–3015 leading to lower corrosion rates. The pKa2 for Na2CO3 is 10.33, at pH lower than 10.33 some CO3 2_ ions formed HCO3 _. Reaction Mg+2HCO3 _=MgCO3+H2 potentially existed. At pH higher than 10.33, [HCO3 _] is negligible. Therefore in Fig. 2, we can see that the corrosion rate at pH 11 was not reduced as much, compared the rate at 10. H2B4O7 and CH3COOH don’t have such advantages. 3.2. The effects of plating parameters on coatings The coating rate, surface appearance, and adhesion of the coatings at different concentrations of Na2CO3 buffer are listed in Table 4. The critical load (LC) was measured under progressive loading conditions, which can be used to accurately characterize the adhesion strength of the deposit/ substrate system [13]. The adhesion between the coatings and substrates decreases obviously with the increase of the concentration of Na2CO3. Surface appearance of the plating coatings becomes gradually shining with the increase of the Na2CO3 concentration. Grave corrosion of the substrates was found in the non-buffered plating bath. The growth rate of the coatings noticeably increases with the increase of the Na2CO3 concentration. Considering the combination of growth rate, surface appearance, and adhesion of the coatings, the optimum concentration of the Na2CO3 buffer was determined to be 30 g L_1.With this concentration, the purpose of adding Na2CO3 in plating bath is commendably achieved. In the research, it was found that the pH value of plating bath had a considerable effect on the growth rate and the surface appearance of the coatings. The hydrogen ions in plating bath were not only astricted by the CO3 2_ ions dissociated from the buffer Na2CO3, but linked with the OH_ ions. When the pH value of the plating bath was below 8.5, point corrosion or dark gray coatings were obtained and the coating growth rate was low. When the pH value of the plating bath was above 11.5, the adhesion between coatings and substrates were deteriorated, although the growth rate and the surface appearance of the coatings were satisfying. During the electroless plating, the pH value of the plating bath was monitored with a pHS-25C model of precision pH/mV meter. In this research, the preferred pH range of the plating bath for electroless plating on magnesium alloy is 8.5–11.5. Table 4 Coating rate, surface appearance and adhesion of the coatings obtained from the plating bath with different amounts of Na2CO3 Concentration of Na2CO3 (g L_1) Coating rate (Am/h) Surface appearance LC (N) 0 – Grave corrosion – 10 12.32 Point corrosion 81 20 16.41 Dark gray 76 30 18.32 Shining 73 40 18.91 Shining 61 50 19.26 Shining 51 20 30 40 50 60 70 13 14 15 16 17 18 19 20 The coating thickness/ìm The trisodium citrate dihydrate content/g.L-1 Fig. 3. Relationship between the coating thickness and the trisodium citrate dihydrate concentration. 30 40 50 60 1000 2000 3000 4000 5000 6000 7000 8000 Intensity 2è /( o ) Fig. 4. XRD patterns of the electroless plating coating. Fig. 5. Surface morphology of a plating coating. J. Li et al. / Surface & Coatings Technology 200 (2006) 3010–3015 3013 Fig. 3 shows the variation of coating thickness on magnesium alloy at same plating time as a function of the trisodium citrate dihydrate concentration at constant temperature and pH. The coating thickness decreases with the increase of the trisodium citrate dihydrate concentration. According to De Minjer and Brenner’s explanation [11], at low concentrations the low adsorption of ligand on the catalytic surface of the substrate accelerates the plating reaction. At higher concentration, there is a high adsorption of ligand on the surface, which slows down the plating reaction. But when the concentration was below 20 g L_1, the plating bath became destabilized and nickel precipitate was observed. 3.3. Properties of the plating coatings from nickel sulfate The coating obtained under optimized bath composition was probably preferentially crystallized (see Fig. 4). The only and strong diffraction observed in the XRD spectrum corresponds to the (111) peak of nickel. Fig. 5 shows the surface morphology of the plating coating. The surface is optically smooth and of low porosity. No obvious surface damage was observed. The compositions of the plating coating were determined to be 5.39 wt.% P and 94.61 wt.% Ni by energy dispersive X-ray spectroscopy. Fig. 6 shows the cross section of an electroless plating coating. The coating has a good adhesion to the substrate and no cracks or holes were observed. Fig. 7 shows the curve of the Ni–P coating free corrosion potential with time. After the sample was immersed in 3.5 wt.% NaCl solution at room temperature for 2 h, the free corrosion potential of the coated magnesium alloy approached to about _0.4 V. The steady-state working potential of magnesium electrode is generally about _1.50 V, although its standard potential is _2.43 V [14]. This indicates the improved corrosion resistance of the plating coatings prepared in this research, compared with the bare alloy. The adhesion between the coatings and the substrates was evaluated by means of quenching and the scratch test. The plated specimens were heated at a temperature of 250 8CF10 8C for 1 h, and then quenched in the cold water. This process was repeated for 20 times on each specimen. No discoloration, cracks, blisters, or peeling was observed [12]. For the scratch test, the critical load (LC) of 73 N was found for the coatings obtained in the optimized bath composition and parameters. These results suggest the excellent adhesion of the plating coating to the substrate. 3.4. Proposed mechanism of the electroless plating nickel Even under the same pH value, the magnesium alloy exhibits better corrosion resistance in the Na2CO3 buffered plating solution than in the non-buffered plating solution. Fig. 6. Cross section view of an electroless plating coating. 0 1 2 3 4 5 6 7 8 -0.46 -0.44 -0.42 -0.40 -0.38 -0.36 -0.34 -0.32 -0.30 ESCE/V × 103, time/s Fig. 7. Curve of the Ni–P coating free corrosion potential with time. 3014 J. Li et al. / Surface & Coatings Technology 200 (2006) 3010–3015 Fig. 8 gives a simple model to explain this phenomenon. Large amount of H2 gas is produced in the electroless plating process. Most of the H+ ions are taken out by the H2 gas bubbles and combine with the CO3 2_, to form HCO3 _. Therefore, a very thin layer of dilute H+ solution is formed near the surface of substrate. The Ni2+ ions react with the magnesium atoms to form the autocatalysis nickel, which leads to the deposition of the Ni–P coating. If the concentration of the CO3 2_ ions is low, more H+ ions will be free and erode the thin Ni–P coating and the substrate. If the concentration of the CO3 2_ ions is much higher, the H+ ions concentration in the thin dilute H+ solution layer near the substrate surface will be much lower. Therefore almost no corrosion process will exist in the interface between the Ni–P coating and the su