圖3.7(a)~(d)為(wei)(wei)(wei)(wei)(wei)鐵(tie)(tie)素(su)(su)(su)(su)體(ti)(ti)(ti)(ti)和(he)(he)奧(ao)(ao)氏體(ti)(ti)(ti)(ti)相(xiang)(xiang)中(zhong)(zhong)(zhong)(zhong)的(de)(de)(de)(de)Cr、Mo、Ni元(yuan)(yuan)素(su)(su)(su)(su)含(han)(han)(han)量(liang)(liang)與固溶(rong)(rong)溫(wen)(wen)(wen)度的(de)(de)(de)(de)關系(xi)圖。圖3.7(a)~(d)證實了鐵(tie)(tie)素(su)(su)(su)(su)體(ti)(ti)(ti)(ti)相(xiang)(xiang)中(zhong)(zhong)(zhong)(zhong)的(de)(de)(de)(de)Cr和(he)(he)Mo含(han)(han)(han)量(liang)(liang)更高，而奧(ao)(ao)氏體(ti)(ti)(ti)(ti)相(xiang)(xiang)中(zhong)(zhong)(zhong)(zhong)的(de)(de)(de)(de)Ni和(he)(he)Mn含(han)(han)(han)量(liang)(liang)更高。從圖中(zhong)(zhong)(zhong)(zhong)可看出，鐵(tie)(tie)素(su)(su)(su)(su)體(ti)(ti)(ti)(ti)相(xiang)(xiang)中(zhong)(zhong)(zhong)(zhong)的(de)(de)(de)(de)Cr含(han)(han)(han)量(liang)(liang)為(wei)(wei)(wei)(wei)(wei)23.77%~25.16%,比(bi)奧(ao)(ao)氏體(ti)(ti)(ti)(ti)相(xiang)(xiang)中(zhong)(zhong)(zhong)(zhong)Cr含(han)(han)(han)量(liang)(liang)高2%左右；鐵(tie)(tie)素(su)(su)(su)(su)體(ti)(ti)(ti)(ti)相(xiang)(xiang)中(zhong)(zhong)(zhong)(zhong)Mo含(han)(han)(han)量(liang)(liang)為(wei)(wei)(wei)(wei)(wei)3.86%~4.37%,比(bi)奧(ao)(ao)氏體(ti)(ti)(ti)(ti)相(xiang)(xiang)中(zhong)(zhong)(zhong)(zhong)的(de)(de)(de)(de)Mo含(han)(han)(han)量(liang)(liang)高1.7%左右；奧(ao)(ao)氏體(ti)(ti)(ti)(ti)相(xiang)(xiang)中(zhong)(zhong)(zhong)(zhong)的(de)(de)(de)(de)Ni含(han)(han)(han)量(liang)(liang)為(wei)(wei)(wei)(wei)(wei)5.42%、6.7%,比(bi)鐵(tie)(tie)素(su)(su)(su)(su)體(ti)(ti)(ti)(ti)相(xiang)(xiang)中(zhong)(zhong)(zhong)(zhong)的(de)(de)(de)(de)Ni含(han)(han)(han)量(liang)(liang)高2%;當(dang)固溶(rong)(rong)處(chu)理(li)溫(wen)(wen)(wen)度為(wei)(wei)(wei)(wei)(wei)1050℃時(shi)，鐵(tie)(tie)素(su)(su)(su)(su)體(ti)(ti)(ti)(ti)相(xiang)(xiang)中(zhong)(zhong)(zhong)(zhong)的(de)(de)(de)(de)Cr、Mo和(he)(he)Ni含(han)(han)(han)量(liang)(liang)分(fen)別(bie)為(wei)(wei)(wei)(wei)(wei)23.77%、3.97%、4.24%,奧(ao)(ao)氏體(ti)(ti)(ti)(ti)相(xiang)(xiang)中(zhong)(zhong)(zhong)(zhong)的(de)(de)(de)(de)Cr、Mo和(he)(he)Ni含(han)(han)(han)量(liang)(liang)分(fen)別(bie)為(wei)(wei)(wei)(wei)(wei)23.53%、2.63%、5.42%.可見在1050℃溫(wen)(wen)(wen)度下進行固溶(rong)(rong)時(shi)，兩(liang)(liang)(liang)相(xiang)(xiang)中(zhong)(zhong)(zhong)(zhong)的(de)(de)(de)(de)Cr、Mo、Ni含(han)(han)(han)量(liang)(liang)差(cha)異最小(xiao)，此(ci)時(shi)鐵(tie)(tie)素(su)(su)(su)(su)體(ti)(ti)(ti)(ti)含(han)(han)(han)量(liang)(liang)為(wei)(wei)(wei)(wei)(wei)51.9%,奧(ao)(ao)氏體(ti)(ti)(ti)(ti)含(han)(han)(han)量(liang)(liang)為(wei)(wei)(wei)(wei)(wei)48.1%.當(dang)固溶(rong)(rong)溫(wen)(wen)(wen)度改變時(shi)，鐵(tie)(tie)素(su)(su)(su)(su)體(ti)(ti)(ti)(ti)／奧(ao)(ao)氏體(ti)(ti)(ti)(ti)兩(liang)(liang)(liang)相(xiang)(xiang)比(bi)例相(xiang)(xiang)差(cha)變大，且兩(liang)(liang)(liang)相(xiang)(xiang)中(zhong)(zhong)(zhong)(zhong)的(de)(de)(de)(de)Cr、Mo、Ni含(han)(han)(han)量(liang)(liang)差(cha)異也(ye)變大。當(dang)固溶(rong)(rong)溫(wen)(wen)(wen)度為(wei)(wei)(wei)(wei)(wei)1000℃,兩(liang)(liang)(liang)相(xiang)(xiang)中(zhong)(zhong)(zhong)(zhong)Cr為(wei)(wei)(wei)(wei)(wei)3.69%、Mo為(wei)(wei)(wei)(wei)(wei)1.51%、Ni為(wei)(wei)(wei)(wei)(wei)3.37%;當(dang)固溶(rong)(rong)溫(wen)(wen)(wen)度為(wei)(wei)(wei)(wei)(wei)1150℃時(shi)，兩(liang)(liang)(liang)相(xiang)(xiang)中(zhong)(zhong)(zhong)(zhong)Cr為(wei)(wei)(wei)(wei)(wei)2.29%、Mo為(wei)(wei)(wei)(wei)(wei)1.34%、Ni為(wei)(wei)(wei)(wei)(wei)2.09%,可見1000℃固溶(rong)(rong)試(shi)樣(yang)的(de)(de)(de)(de)兩(liang)(liang)(liang)相(xiang)(xiang)中(zhong)(zhong)(zhong)(zhong)的(de)(de)(de)(de)Cr、Mo、Ni元(yuan)(yuan)素(su)(su)(su)(su)含(han)(han)(han)量(liang)(liang)差(cha)大于(yu)1150℃固溶(rong)(rong)試(shi)樣(yang)的(de)(de)(de)(de)兩(liang)(liang)(liang)相(xiang)(xiang)中(zhong)(zhong)(zhong)(zhong)Cr、Mo、Ni元(yuan)(yuan)素(su)(su)(su)(su)含(han)(han)(han)量(liang)(liang)差(cha)。


7.jpg


  由(you)公(gong)式(shi)“PREN.=[Cr]+3.3[Mo]”計算(suan)鐵素體的(de)PREN值(zhi)，由(you)公(gong)式(shi)“PREN,=[Cr]+3.3[Mo]+16[N]-[Mn]”計算(suan)奧氏體的(de)PREN值(zhi)，可得PREN值(zhi)與固(gu)溶溫(wen)度(du)的(de)關系圖(tu)，如圖(tu)3.7(e)所(suo)示。從(cong)圖(tu)3.7(e)可看出，在不同固(gu)溶狀態下，兩(liang)相的(de)PREN值(zhi)有(you)所(suo)不同，但(dan)PRENa>PRENy.當固(gu)溶溫(wen)度(du)為(wei)1050℃時，PREN.最小(xiao)、PREN,最大，分別為(wei)36.9和30.6,兩(liang)者相差(cha)最小(xiao)。


  圖(tu)3.8為(wei)(wei)不同(tong)固溶試樣的極(ji)化曲線。可(ke)見(jian)，在自(zi)(zi)腐(fu)蝕電(dian)位(wei)下，材料開(kai)始發生(sheng)鈍(dun)化；當(dang)極(ji)化電(dian)位(wei)升高到(dao)一定值時，不同(tong)固溶的材料都發生(sheng)點蝕，電(dian)流密度急劇增大。根據(ju)GB 4334.9-1984中電(dian)流密度為(wei)(wei)0.1mA/c㎡所對(dui)應的電(dian)位(wei)為(wei)(wei)點蝕電(dian)位(wei)和(he)Tafel擬合，分別得到(dao)點蝕電(dian)位(wei)、自(zi)(zi)腐(fu)蝕電(dian)流密度與固溶溫度的關系圖(tu)，如圖(tu)3.11(a)所示。


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  圖(tu)(tu)(tu)(tu)3.9(a)、（b)分(fen)別(bie)為(wei)不(bu)同(tong)固溶(rong)試樣(yang)的(de)Nyquist 圖(tu)(tu)(tu)(tu)和Bode圖(tu)(tu)(tu)(tu)。從(cong)圖(tu)(tu)(tu)(tu)3.9(a)可見(jian)，試樣(yang)在低頻和高頻區分(fen)別(bie)存(cun)在一個容抗弧。從(cong)圖(tu)(tu)(tu)(tu)3.9(b)可見(jian)，試樣(yang)在低頻和高頻處分(fen)別(bie)存(cun)在一個時間常數。根據曹楚南的(de)《電(dian)化學阻(zu)(zu)抗譜導(dao)論》可知(zhi)。雙相不(bu)銹鋼在NaCl溶(rong)液中(zhong)的(de)阻(zu)(zu)抗譜中(zhong)存(cun)在兩個時間常數，常以圖(tu)(tu)(tu)(tu)3.10所示的(de)等效電(dian)路（R1為(wei)電(dian)荷(he)轉移電(dian)阻(zu)(zu)，R2為(wei)鈍化膜電(dian)阻(zu)(zu)）進行阻(zu)(zu)抗擬合。經阻(zu)(zu)抗擬合，得到鈍化膜電(dian)阻(zu)(zu)、電(dian)荷(he)轉移電(dian)阻(zu)(zu)與(yu)固溶(rong)處理溫度(du)的(de)關系(xi)圖(tu)(tu)(tu)(tu)，如圖(tu)(tu)(tu)(tu)3.11(b)所示。


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 從圖3.11(a)可見，1050℃固溶試樣的點蝕電位最高，約為0.89V,且1050℃固溶試樣的自腐蝕電流密度最低，約為2.34×10-6A/c㎡,說明該狀態下試樣的耐點蝕性能最好。由圖3.11(b)可知，1050℃固溶試樣的鈍化膜電阻值最高，約為1300Ω,且其電荷轉移電阻值也最高，約為9500Ω,說明該狀態下試樣的鈍化膜較穩定，耐點蝕性能較好。


  圖3.12為不同固溶的2205雙(shuang)相(xiang)不銹鋼極化后的點蝕形貌圖，圖中黑色為蝕坑，深色為鐵素體，淡色為奧氏體。從圖3.12可見，四種固溶試樣的點蝕都發生于鐵素體與奧氏體相界面處，且易向鐵素體相中長大。當固溶溫度為1050℃時，試樣的點蝕尺寸最小，點蝕敏感性最低。


12.jpg


  通過計算得(de)到不(bu)(bu)同(tong)相(xiang)比例(li)下(xia)的(de)(de)鐵素體(ti)PREN值(zhi)(zhi)和(he)奧氏體(ti)PREN值(zhi)(zhi)，可(ke)(ke)見不(bu)(bu)同(tong)相(xiang)比例(li)下(xia)的(de)(de)PREN.都(dou)大于PREN,.雙相(xiang)不(bu)(bu)銹(xiu)(xiu)鋼(gang)的(de)(de)耐(nai)(nai)(nai)點(dian)蝕(shi)(shi)(shi)性(xing)(xing)能(neng)可(ke)(ke)通過點(dian)蝕(shi)(shi)(shi)電(dian)位衡量。點(dian)蝕(shi)(shi)(shi)電(dian)位越(yue)高，耐(nai)(nai)(nai)點(dian)獨性(xing)(xing)能(neng)越(yue)好。前人(ren)認(ren)為(wei)雙相(xiang)不(bu)(bu)銹(xiu)(xiu)鋼(gang)的(de)(de)耐(nai)(nai)(nai)點(dian)蝕(shi)(shi)(shi)性(xing)(xing)能(neng)由B能(neng)N值(zhi)(zhi)較區的(de)(de)相(xiang)決(jue)定(ding)，且PEN值(zhi)(zhi)越(yue)高，耐(nai)(nai)(nai)點(dian)蝕(shi)(shi)(shi)性(xing)(xing)能(neng)越(yue)好，從(cong)圖(tu)(tu)3.71e)可(ke)(ke)矩，不(bu)(bu)同(tong)海溶(rong)試(shi)(shi)樣的(de)(de)PREN.都(dou)大于PREN,,當固溶(rong)溫度為(wei)1050℃時，PHEN,最(zui)大，材料的(de)(de)耐(nai)(nai)(nai)點(dian)蝕(shi)(shi)(shi)性(xing)(xing)能(neng)應最(zui)好。從(cong)圖(tu)(tu)3.11(a)、（b)可(ke)(ke)知，1050℃固溶(rong)試(shi)(shi)樣的(de)(de)點(dian)蝕(shi)(shi)(shi)電(dian)位最(zui)高，鈍化膜阻(zu)抗(kang)值(zhi)(zhi)最(zui)大，電(dian)荷轉移(yi)電(dian)阻(zu)值(zhi)(zhi)最(zui)商；且從(cong)圖(tu)(tu)3.12(b)可(ke)(ke)見，1050℃固溶(rong)試(shi)(shi)樣的(de)(de)點(dian)蝕(shi)(shi)(shi)坑尺寸(cun)最(zui)小，表現出最(zui)好的(de)(de)耐(nai)(nai)(nai)點(dian)蝕(shi)(shi)(shi)性(xing)(xing)能(neng)。綜(zong)上(shang)可(ke)(ke)知，雙相(xiang)不(bu)(bu)銹(xiu)(xiu)鋼(gang)的(de)(de)耐(nai)(nai)(nai)點(dian)蝕(shi)(shi)(shi)性(xing)(xing)能(neng)由 PREN 值(zhi)(zhi)較小相(xiang)決(jue)定(ding)的(de)(de)理論是有實驗依(yi)據的(de)(de)。


  從圖3.12(a)~(d)可(ke)見，在不同固溶(rong)狀態(tai)(tai)下，鐵素體(ti)相都(dou)更易(yi)發(fa)生點(dian)(dian)蝕；而(er)從圖3.7(e)可(ke)見，在不同固溶(rong)狀態(tai)(tai)下，PREN.都(dou)大于PREN,,鐵素體(ti)的耐點(dian)(dian)蝕性能應優于奧(ao)氏體(ti)相，可(ke)見兩者(zhe)存在矛盾。


  隨固(gu)溶(rong)溫度變(bian)(bian)化(hua)，雙(shuang)相(xiang)不(bu)(bu)銹鋼中(zhong)(zhong)(zhong)的(de)(de)(de)(de)(de)鐵素(su)(su)(su)(su)體(ti)和奧(ao)氏(shi)體(ti)相(xiang)的(de)(de)(de)(de)(de)比例改變(bian)(bian)，且兩(liang)相(xiang)形態也(ye)發(fa)生(sheng)變(bian)(bian)化(hua)。Cr是鐵素(su)(su)(su)(su)體(ti)形成(cheng)元(yuan)(yuan)(yuan)(yuan)素(su)(su)(su)(su)，可(ke)提(ti)高材(cai)料(liao)的(de)(de)(de)(de)(de)耐(nai)(nai)蝕(shi)(shi)性(xing)能；Mo是鐵素(su)(su)(su)(su)體(ti)形成(cheng)元(yuan)(yuan)(yuan)(yuan)素(su)(su)(su)(su)，可(ke)提(ti)高點蝕(shi)(shi)電(dian)位(wei)，降低腐(fu)蝕(shi)(shi)速率；Ni是奧(ao)氏(shi)體(ti)形成(cheng)元(yuan)(yuan)(yuan)(yuan)素(su)(su)(su)(su)，可(ke)維持兩(liang)相(xiang)平衡，提(ti)高耐(nai)(nai)蝕(shi)(shi)性(xing)能。并且材(cai)料(liao)中(zhong)(zhong)(zhong)存(cun)(cun)在(zai)(zai)一定量的(de)(de)(de)(de)(de)N,其為(wei)(wei)奧(ao)氏(shi)體(ti)形成(cheng)元(yuan)(yuan)(yuan)(yuan)素(su)(su)(su)(su)，提(ti)高局部(bu)腐(fu)蝕(shi)(shi)抗力(li)。從圖3.7(a)~(c)可(ke)見，隨固(gu)溶(rong)溫度變(bian)(bian)化(hua)，兩(liang)相(xiang)中(zhong)(zhong)(zhong)的(de)(de)(de)(de)(de)Cr、Mo、Ni元(yuan)(yuan)(yuan)(yuan)素(su)(su)(su)(su)含量發(fa)生(sheng)變(bian)(bian)化(hua)。由(you)于(yu)Cr、Mo、Ni元(yuan)(yuan)(yuan)(yuan)素(su)(su)(su)(su)之間(jian)(jian)的(de)(de)(de)(de)(de)腐(fu)蝕(shi)(shi)電(dian)位(wei)存(cun)(cun)在(zai)(zai)差異，勢必(bi)造成(cheng)兩(liang)相(xiang)之間(jian)(jian)存(cun)(cun)在(zai)(zai)電(dian)化(hua)學差異，使腐(fu)蝕(shi)(shi)更易(yi)發(fa)生(sheng)。從圖3.7(e)可(ke)知，不(bu)(bu)同(tong)(tong)固(gu)溶(rong)狀(zhuang)態下材(cai)料(liao)的(de)(de)(de)(de)(de)PREN,大(da)(da)小(xiao)關(guan)系為(wei)(wei)1050℃>1000℃>1150℃>1100℃,因(yin)此(ci)(ci)根據(ju)前人(ren)的(de)(de)(de)(de)(de)研(yan)究，不(bu)(bu)同(tong)(tong)固(gu)溶(rong)態材(cai)料(liao)的(de)(de)(de)(de)(de)耐(nai)(nai)點蝕(shi)(shi)性(xing)能優(you)(you)劣關(guan)系應為(wei)(wei)：1050℃優(you)(you)于(yu)1000℃優(you)(you)于(yu)1150℃.當固(gu)溶(rong)溫度為(wei)(wei)1050℃時，PREN,值較大(da)(da)，兩(liang)相(xiang)中(zhong)(zhong)(zhong)的(de)(de)(de)(de)(de)Cr、Mo、Ni元(yuan)(yuan)(yuan)(yuan)素(su)(su)(su)(su)含量差異最小(xiao)，材(cai)料(liao)的(de)(de)(de)(de)(de)點蝕(shi)(shi)坑較小(xiao)，材(cai)料(liao)的(de)(de)(de)(de)(de)耐(nai)(nai)點蝕(shi)(shi)性(xing)能最優(you)(you)。1000℃固(gu)溶(rong)試樣(yang)(yang)(yang)的(de)(de)(de)(de)(de)點蝕(shi)(shi)坑尺寸大(da)(da)于(yu)1150℃固(gu)溶(rong)試樣(yang)(yang)(yang)的(de)(de)(de)(de)(de)點蝕(shi)(shi)坑尺寸，因(yin)此(ci)(ci)，前者的(de)(de)(de)(de)(de)耐(nai)(nai)點蝕(shi)(shi)性(xing)能劣于(yu)后者的(de)(de)(de)(de)(de)耐(nai)(nai)點蝕(shi)(shi)性(xing)能。由(you)此(ci)(ci)可(ke)見，雙(shuang)相(xiang)不(bu)(bu)銹鋼中(zhong)(zhong)(zhong)的(de)(de)(de)(de)(de)相(xiang)腐(fu)蝕(shi)(shi)不(bu)(bu)能僅僅由(you)PREN值來解釋。由(you)圖3.7(a)~(c)可(ke)知，1000℃固(gu)溶(rong)試樣(yang)(yang)(yang)的(de)(de)(de)(de)(de)兩(liang)相(xiang)中(zhong)(zhong)(zhong)的(de)(de)(de)(de)(de)Cr、Mo、Ni元(yuan)(yuan)(yuan)(yuan)素(su)(su)(su)(su)含量差大(da)(da)于(yu)1150℃固(gu)溶(rong)試樣(yang)(yang)(yang)的(de)(de)(de)(de)(de)兩(liang)相(xiang)中(zhong)(zhong)(zhong)的(de)(de)(de)(de)(de)Gr、Mo、Ni元(yuan)(yuan)(yuan)(yuan)素(su)(su)(su)(su)含量差，因(yin)此(ci)(ci)，雙(shuang)相(xiang)不(bu)(bu)銹鋼中(zhong)(zhong)(zhong)的(de)(de)(de)(de)(de)相(xiang)腐(fu)蝕(shi)(shi)還與兩(liang)相(xiang)中(zhong)(zhong)(zhong)的(de)(de)(de)(de)(de)元(yuan)(yuan)(yuan)(yuan)素(su)(su)(su)(su)分布有(you)關(guan)，并且還需考慮(lv)到材(cai)料(liao)中(zhong)(zhong)(zhong)點蝕(shi)(shi)敏感性(xing)較強的(de)(de)(de)(de)(de)區域，如晶界和相(xiang)界。


  點蝕(shi)是一種局部腐蝕現象，是由氯離子破壞鈍化膜而導致的。點蝕產生后，蝕坑處的基體被暴露在溶液中，導致材料進一步發生腐蝕，蝕坑長大。蝕坑前長大速率由材料的均勻溶解速率決定，溶解越快，蝕坑長大速度越大。因此，雙相不銹鋼的耐點蝕性能由兩部分構成：控制點蝕萌生和控制蝕坑長大的能力。本書中的點蝕電位意味著點蝕已長大。影響點蝕萌生的因素不僅包括PREN 值，還包括兩相中的元素分布和兩相的比例。從本書的分析可見，點蝕的長大與元素分布有關，兩相中的元素分布越均勻，蝕坑的長大速度越慢。因此在1050℃固溶狀態下，兩相中的元素分布最均勻，PREN,最高，合金的點蝕電位最高，材料的耐點蝕性能最好。而1000℃固溶試樣兩相中的元素分布不均勻，導致點蝕電位較低，點蝕坑尺寸較大，降低材料的耐點蝕性能。


  影(ying)響(xiang)雙相(xiang)(xiang)不(bu)銹鋼相(xiang)(xiang)腐蝕的因(yin)(yin)素不(bu)僅包(bao)括PREN值，還(huan)包(bao)括各(ge)相(xiang)(xiang)中(zhong)的元素分布(bu)和兩相(xiang)(xiang)比例(li)，并且三者(zhe)互相(xiang)(xiang)影(ying)響(xiang)、關系復雜。因(yin)(yin)此(ci)，不(bu)能僅憑PREN值的大小來判斷耐點(dian)蝕能力，應該綜合考(kao)慮各(ge)因(yin)(yin)素的影(ying)響(xiang)。