吳海江，楊飛英，彭成章，郭文敏，王小明

(1.邵陽學(xué)院 機(jī)械與能源工程系，湖南 邵陽，422000；2.湖南科技大學(xué) 機(jī)械設(shè)備健康維護(hù)湖南省重點(diǎn)實(shí)驗(yàn)室，湖南 湘潭，411201)

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鎂合金表面碳納米管/硅烷復(fù)合膜的耐蝕性能*

吳海江1，楊飛英1，彭成章2，郭文敏1，王小明1

(1.邵陽學(xué)院 機(jī)械與能源工程系，湖南 邵陽，422000；2.湖南科技大學(xué) 機(jī)械設(shè)備健康維護(hù)湖南省重點(diǎn)實(shí)驗(yàn)室，湖南 湘潭，411201)

為了進(jìn)一步改善AZ91D鎂合金表面單一硅烷膜的耐蝕性能，將不同含量碳納米管添加到γ-氨丙基三乙氧基硅烷(KH-550)溶液中，借助簡單浸漬法在其表面制備了碳納米管/硅烷復(fù)合膜。通過點(diǎn)滴試驗(yàn)、中性鹽霧試驗(yàn)、全浸腐蝕試驗(yàn)和E-t曲線測試評(píng)價(jià)了復(fù)合膜的耐蝕性能。結(jié)果表明:與單一硅烷膜相比，碳納米管/硅烷復(fù)合膜的致密性較高，有效地阻擋了侵蝕性介質(zhì)向基體的滲透，顯著提高了AZ91D鎂合金的耐蝕能力；隨著碳納米管含量的增加，碳納米管/硅烷復(fù)合膜的耐蝕性能先上升后下降，當(dāng)碳納米管含量為0.3 g/L時(shí)復(fù)合膜的耐蝕性能最佳。

AZ91D鎂合金；γ-氨丙基三乙氧基硅烷膜；碳納米管；耐蝕性能

作為最輕的金屬結(jié)構(gòu)材料，鎂合金因其密度低、比強(qiáng)度高、生物相容性好、電子屏蔽能力強(qiáng)和可回收性而廣泛應(yīng)用于航空航天、汽車、3C產(chǎn)品、生物醫(yī)學(xué)等領(lǐng)域[1-3]。但由于鎂合金化學(xué)性質(zhì)非?；顫?，極易與氧、水汽等發(fā)生作用，耐蝕性較差[4,5]，成為鎂合金發(fā)展的瓶頸，大大限制了其實(shí)際應(yīng)用。傳統(tǒng)上采用工藝簡單、經(jīng)濟(jì)效益高的鉻酸鹽鈍化膜為鎂合金提供保護(hù)，但由于六價(jià)鉻毒性高且致癌而受到嚴(yán)格限制使用[6,7]。因此，鎂合金無鉻鈍化技術(shù)日益成為科研人員的關(guān)注熱點(diǎn)。

國內(nèi)外研究者先后研發(fā)了鉬酸鹽[8-11]、錫酸鹽[12-14]、稀土金屬鹽[14-17]、高錳酸鹽[18-20]等主要轉(zhuǎn)化處理工藝，但是此類無鉻鈍化工藝的腐蝕防護(hù)效果很大程度上依賴于轉(zhuǎn)化膜層的厚度，而它們達(dá)到一定厚度后就容易出現(xiàn)開裂脫落現(xiàn)象[21]。于是研究者們把目光轉(zhuǎn)向有機(jī)物處理，其中硅烷化處理以其工藝簡單、對(duì)環(huán)境友好而受到越來越多的重視。自美國Cincinnati大學(xué)van Ooij教授帶領(lǐng)的團(tuán)隊(duì)于上世紀(jì)90年代率先嘗試以來，應(yīng)用于鋼鐵[22-24]、鋁合金[25-27]、鍍鋅鋼[27-29]、鎂合金[30,31]基體取得了令人驚喜的研究成果。然而，傳統(tǒng)意義上的硅烷膜耐蝕性能還不能令人非常滿意，其中的重要原因是硅烷膜自身的厚度很小，所起的阻擋作用有限，同時(shí)機(jī)械強(qiáng)度不夠，最終限制了防護(hù)性能的發(fā)揮[32]。碳納米管具有獨(dú)特的線狀結(jié)構(gòu)、大的比表面積和良好的導(dǎo)電性能，能夠在復(fù)合涂層內(nèi)部形成良好的導(dǎo)電網(wǎng)絡(luò)，有助于消除金屬基體各部位間的電位差異，從而抑制金屬的電化學(xué)腐蝕[33,34]，同時(shí)利用其優(yōu)良的力學(xué)特性改善硅烷膜的機(jī)械強(qiáng)度，發(fā)揮其防護(hù)性能。

本文將AZ91D鎂合金簡單浸漬在不同含量碳納米管的γ-氨丙基三乙氧基硅烷(KH-550)溶液中，取出后經(jīng)過固化處理，獲得了碳納米管/硅烷復(fù)合膜，應(yīng)用點(diǎn)滴試驗(yàn)、中性鹽霧試驗(yàn)、全浸腐蝕試驗(yàn)和電化學(xué)測試等手段評(píng)價(jià)了膜層的耐蝕性能。

1 試驗(yàn)

1.1 基體前處理

選取AZ91D鎂合金作為基體材料，加工成50mm×40mm×2mm大小，其化學(xué)成分見表1。試樣依次采用360～1500號(hào)水砂紙逐級(jí)打磨→水洗→在丙酮溶液中超聲波清洗5min→去離子水洗→堿洗(60g/L NaOH+10g/L Na3PO4，70℃，10min)→去離子水洗，然后用吹風(fēng)機(jī)吹干后置于干燥器中備用。

表1 AZ91D鎂合金的化學(xué)成分(質(zhì)量分?jǐn)?shù)，%)

1.2 碳納米管/硅烷復(fù)合膜制備

在室溫條件下，γ-氨丙基三乙氧基硅烷(KH-550)體積分?jǐn)?shù)為5%，無水乙醇去離子水體積比85∶15，pH值9，水解時(shí)間2h；隨之將0g/L、0.1g/L、0.3g/L、0.5g/L、0.7g/L碳納米管加入到適量無水乙醇溶液中磁力攪拌30min分散，然后取分散溶液添加到水解好的硅烷溶液中，接著磁力攪拌5～10min；最后將預(yù)處理好的AZ91D鎂合金試樣浸入配制好的碳納米管摻雜改性KH-550硅烷溶液中90s，緩慢提拉出液面后用壓縮空氣吹掉鎂合金表面殘留的溶液，馬上置于ZK-82BB型電熱真空干燥箱(上海市實(shí)驗(yàn)儀器總廠)中加熱固化，固化溫度為120℃，固化時(shí)間為60min[35]。

1.3 性能檢測

(1)點(diǎn)滴試驗(yàn) 依據(jù)HB5061277標(biāo)準(zhǔn)，點(diǎn)滴溶液由1mL HNO3+0.05g KMnO4+100mL H2O配制而成，滴至試樣表面適當(dāng)?shù)奈恢茫^察點(diǎn)滴溶液由紅色轉(zhuǎn)變?yōu)闊o色的時(shí)間，以變色時(shí)間的長短來評(píng)價(jià)試樣的耐蝕性能好壞。每個(gè)試樣測試5次，取其平均值。

(2)中性鹽霧試驗(yàn) 按照ASTMB117-2003進(jìn)行，溶液為5%NaCl水溶液，箱內(nèi)溫度為35±2℃，鹽霧沉降速度為1～2mL/(h·80cm2)，氯化鈉收集溶液濃度50g/L±5g/L，pH值6.5～7.2。將試樣放入鹽霧箱中，表面與垂直方向成30°角，每天連續(xù)噴8h停16h作為一個(gè)試驗(yàn)周期，記錄試樣的腐蝕面積隨噴霧周期的變化情況。試驗(yàn)時(shí)采用5片平行試樣，取其平均值。

(3)全浸腐蝕試驗(yàn) 按照J(rèn)B/T6073-1992進(jìn)行，浸漬液為5%NaCl水溶液，浸泡168h。先稱量浸泡前的試樣質(zhì)量m1，并測定其表面積S。浸泡試驗(yàn)結(jié)束后取出試樣并清除腐蝕產(chǎn)物，烘干稱重，質(zhì)量為m2，精確至0.1mg，則其失重△m=m1-m2，腐蝕速率v=△m/(St)，t為浸泡時(shí)間。同時(shí)用數(shù)碼相機(jī)觀察試樣宏觀腐蝕形貌。試驗(yàn)時(shí)采用5片平行試樣，取其平均值。

(4)E-t曲線測量 測試采用常規(guī)的三電極體系，以飽和甘汞電極(SCE)作為參比電極、鉑電極作為輔助電極、待測試樣作為工作電極，在CHI660E電化學(xué)工作站(上海辰華儀器公司)上進(jìn)行。試樣用環(huán)氧樹脂涂封后暴露10mm×10mm大小區(qū)域，腐蝕介質(zhì)為5%NaCl水溶液，在室溫、不除氣的條件下測量試樣開路電位隨浸泡時(shí)間的變化曲線。

2 結(jié)果與討論

2.1 點(diǎn)滴試驗(yàn)

圖1所示為經(jīng)不同含量碳納米管硅烷溶液處理的鎂合金試樣抗點(diǎn)滴變色時(shí)間?？瞻祖V合金試樣幾乎是瞬間就變色了，抗點(diǎn)滴腐蝕的時(shí)間極短。從圖1中可以看出，經(jīng)純KH-550溶液處理鎂合金試樣的抗點(diǎn)滴變色時(shí)間顯著延長；而添加碳納米管后，所制備的碳納米管/硅烷復(fù)合膜抗點(diǎn)滴變色時(shí)間又邁上了一個(gè)新臺(tái)階，且隨著碳納米管含量的增加，復(fù)合膜的抗點(diǎn)滴變色時(shí)間呈現(xiàn)先升后降的變化規(guī)律，其中在含0.3g/L碳納米管硅烷溶液中制備的復(fù)合膜抗點(diǎn)滴變色時(shí)間最長。說明此時(shí)碳納米管/硅烷復(fù)合膜對(duì)AZ91D有最佳的防護(hù)性能，它有效地阻遏了腐蝕介質(zhì)向基體的滲入。

圖1 經(jīng)不同含量碳納米管硅烷溶液處理鎂合金試樣點(diǎn)滴試驗(yàn)時(shí)間

2.2 中性鹽霧試驗(yàn)

圖2所示為經(jīng)不同含量碳納米管硅烷溶液處理的鎂合金試樣在中性鹽霧中腐蝕面積隨噴霧周期的變化曲線?？瞻譇Z91D鎂合金試樣噴霧2h就有40%左右的表面被腐蝕，8h后腐蝕斑點(diǎn)幾乎布滿整個(gè)表面[35]。由圖2可見，碳納米管的加入顯著地增強(qiáng)了硅烷膜對(duì)AZ91D鎂合金基體的防護(hù)能力。隨著碳納米管含量的增加，硅烷復(fù)合膜的腐蝕面積減小，耐蝕性能提高；當(dāng)硅烷溶液中碳納米管含量超過0.3g/L后，所得硅烷復(fù)合膜的腐蝕面積逐漸增大，耐蝕性能反而下降。原因可能是當(dāng)加入過量的碳納米管時(shí)，破壞了硅烷水解產(chǎn)物Si-OH之間的脫水縮合反應(yīng)，削弱了所形成的化學(xué)鍵鍵合力[26,34]，使得硅烷分子與AZ91D鎂合金基體之間的吸附能力變差，最終獲得致密性下降的復(fù)合膜，導(dǎo)致降低了其對(duì)AZ91D鎂合金基體的腐蝕防護(hù)能力。試驗(yàn)結(jié)果進(jìn)一步印證了點(diǎn)滴試驗(yàn)結(jié)果。

圖2 經(jīng)不同含量碳納米管硅烷溶液處理鎂合金試樣中性鹽霧試驗(yàn)后的腐蝕面積

2.3 全浸腐蝕試驗(yàn)

圖3所示為空白鎂合金、硅烷膜、最佳碳納米管/硅烷復(fù)合膜試樣浸泡在5%NaCl溶液中168h后的表面宏觀腐蝕形貌。對(duì)于空白鎂合金(圖3(a))，自浸泡開始就反應(yīng)劇烈，在試樣表面聚集生成許多氣泡，很快就出現(xiàn)了明顯的腐蝕斑點(diǎn)，168h后表面已嚴(yán)重腐蝕，布滿了大大小小的腐蝕坑，幾乎看不到完好區(qū)域，平均腐蝕速率達(dá)到了125.8g/(m2·h)；硅烷膜試樣(圖3(b))點(diǎn)蝕萌生時(shí)間約為30h，且擴(kuò)展較快，168h后表面也被嚴(yán)重腐蝕，但仍存在部分完好區(qū)域，平均腐蝕速率達(dá)71.2g/(m2·h)；而碳納米管/硅烷復(fù)合膜試樣(圖3(c))浸泡約50h后才出現(xiàn)第一個(gè)腐蝕點(diǎn)，但擴(kuò)展較慢，168h后表面僅有少量的腐蝕斑點(diǎn)，仍保留有大量完好完整區(qū)域，平均腐蝕速率僅有26.6g/(m2·h)，說明碳納米管/硅烷復(fù)合膜顯著提高了AZ91D鎂合金的耐蝕能力，它很好地阻擋了侵蝕性介質(zhì)向基體的滲透。

(a)空白鎂合金 (b)硅烷膜(c)碳納米管/硅烷復(fù)合膜

2.4 E-t曲線

圖4給出了空白鎂合金、硅烷膜、最佳碳納米管/硅烷復(fù)合膜試樣烷膜試樣浸泡在5%NaCl溶液中7天開路電位隨浸泡時(shí)間的變化曲線。一般來說，防護(hù)膜層的開路電位越高，說明膜層在腐蝕介質(zhì)中越穩(wěn)定，反映了膜層的致密性越高，越能更好地阻止侵蝕性介質(zhì)對(duì)金屬基體的入侵[20]。從圖4中可以看出，浸泡在5%NaCl溶液中后，隨著浸泡時(shí)間的增加，空白鎂合金試樣的開路電位在較短時(shí)間內(nèi)就穩(wěn)定在-1.66V附近；而硅烷膜和碳納米管/硅烷復(fù)合膜則經(jīng)歷了較長的時(shí)間才穩(wěn)定下來，分別穩(wěn)定在-1.52V和-1.29V左右的較高水平，明顯高于空白鎂合金的開路電位。這是因?yàn)閷?duì)于未經(jīng)任何處理的空白鎂合金來說，腐蝕性介質(zhì)較容易滲透到基體表面，腐蝕反應(yīng)易于達(dá)到穩(wěn)態(tài)；而對(duì)于硅烷膜和碳納米管/硅烷復(fù)合膜，溶液很難穿透它而到達(dá)鎂合金基體表面，腐蝕反應(yīng)很難在短時(shí)間內(nèi)達(dá)到穩(wěn)態(tài)。這就意味著碳納米管的加入導(dǎo)致硅烷膜層變得更加致密，同時(shí)碳納米管獨(dú)特的長鏈結(jié)構(gòu)使得腐蝕介質(zhì)向鎂合金基體的滲透路徑更為復(fù)雜[34,36]，有效地阻礙了侵蝕性介質(zhì)向基體的滲透，從而大大提高了對(duì)鎂合金的腐蝕防護(hù)作用。

浸泡時(shí)間/h

3 結(jié)論

(1)點(diǎn)滴試驗(yàn)、中性鹽霧試驗(yàn)、全浸腐蝕試驗(yàn)和E-t曲線測試結(jié)果表明，AZ91D鎂合金經(jīng)不同含量碳納米管的γ-氨丙基三乙氧基硅烷溶液處理后，獲得的碳納米管/硅烷復(fù)合膜耐蝕性能優(yōu)異。

(2)隨著碳納米管含量的增加，AZ91D鎂合金表面碳納米管/硅烷復(fù)合膜的耐蝕性能呈現(xiàn)先升后降的變化規(guī)律，其中含0.3g/L碳納米管硅烷溶液中制備的復(fù)合膜耐蝕性能最佳。

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Corrosion resistance of carbon nanotubes/silane composite coatings on AZ91D magnesium alloy

WU Haijiang1,YANG Feiying1,PENG Chengzhang2,GUO Wenmin1,WANG Xiaoming1

(1.Department of Mechanical and Energy Engineering,Shaoyang University,Shaoyang 422000,China;2.Hunan Provincial Key Laboratory of Health Maintenance for Mechanical Equipment,Hunan University of Science and Technology,Xiangtan 411201,China)

In order to further improve the corrosion resistance of the single silane coating on AZ91D magnesium alloy,which was treated in γ-APS solution with different concentrations of the carbon nanotubes,the carbon nanotubes/silane composite coatings were prepared.The corrosion resistance of the composite coatings were evaluated by drop test,neutral salt spray test,immersion corrosion test and potential-time curve test.The results revealed that the compactness of the carbon nanotubes/silane coatings were higher than the single silane coating,which significantly improved the corrosion resistance of AZ91D magnesium alloy by effectively blocking the infiltration of corrosive medium to the substrate.With the increase of the concentrations of the carbon nanotubes,the corrosion resistance of the carbon nanotubes/silane composite coatings first increased and then decreased.The carbon nanotubes/silane composite coatings had the optimum corrosion resistance when the concentration of the carbon nanotubes was 0.3g/L.

AZ91D magnesium alloy; γ-APS coating; carbon nanotubes; corrosion resistance

1672-7010(2016)03-0069-07

2016-06-30

湖南省自然科學(xué)基金資助項(xiàng)目(2015JJ2064)；湖南省教育廳青年項(xiàng)目(15B213)；邵陽市科技計(jì)劃項(xiàng)目(2015JH36)

吳海江(1975-)，男，安徽淮南人，博士，副教授，從事金屬材料腐蝕與防護(hù)研究；E-mail: haijiang_wu@126.com

TG174.4

A