圖(tu)3.7(a)~(d)為(wei)(wei)(wei)(wei)(wei)(wei)鐵(tie)素(su)體(ti)(ti)和(he)(he)(he)奧(ao)氏(shi)(shi)(shi)(shi)體(ti)(ti)相(xiang)(xiang)中(zhong)(zhong)(zhong)(zhong)的(de)Cr、Mo、Ni元素(su)含(han)(han)(han)量(liang)(liang)(liang)(liang)與固溶(rong)(rong)(rong)(rong)溫(wen)度(du)(du)的(de)關系圖(tu)。圖(tu)3.7(a)~(d)證實了(le)鐵(tie)素(su)體(ti)(ti)相(xiang)(xiang)中(zhong)(zhong)(zhong)(zhong)的(de)Cr和(he)(he)(he)Mo含(han)(han)(han)量(liang)(liang)(liang)(liang)更(geng)高,而奧(ao)氏(shi)(shi)(shi)(shi)體(ti)(ti)相(xiang)(xiang)中(zhong)(zhong)(zhong)(zhong)的(de)Ni和(he)(he)(he)Mn含(han)(han)(han)量(liang)(liang)(liang)(liang)更(geng)高。從(cong)圖(tu)中(zhong)(zhong)(zhong)(zhong)可看出,鐵(tie)素(su)體(ti)(ti)相(xiang)(xiang)中(zhong)(zhong)(zhong)(zhong)的(de)Cr含(han)(han)(han)量(liang)(liang)(liang)(liang)為(wei)(wei)(wei)(wei)(wei)(wei)23.77%~25.16%,比奧(ao)氏(shi)(shi)(shi)(shi)體(ti)(ti)相(xiang)(xiang)中(zhong)(zhong)(zhong)(zhong)Cr含(han)(han)(han)量(liang)(liang)(liang)(liang)高2%左右(you);鐵(tie)素(su)體(ti)(ti)相(xiang)(xiang)中(zhong)(zhong)(zhong)(zhong)Mo含(han)(han)(han)量(liang)(liang)(liang)(liang)為(wei)(wei)(wei)(wei)(wei)(wei)3.86%~4.37%,比奧(ao)氏(shi)(shi)(shi)(shi)體(ti)(ti)相(xiang)(xiang)中(zhong)(zhong)(zhong)(zhong)的(de)Mo含(han)(han)(han)量(liang)(liang)(liang)(liang)高1.7%左右(you);奧(ao)氏(shi)(shi)(shi)(shi)體(ti)(ti)相(xiang)(xiang)中(zhong)(zhong)(zhong)(zhong)的(de)Ni含(han)(han)(han)量(liang)(liang)(liang)(liang)為(wei)(wei)(wei)(wei)(wei)(wei)5.42%、6.7%,比鐵(tie)素(su)體(ti)(ti)相(xiang)(xiang)中(zhong)(zhong)(zhong)(zhong)的(de)Ni含(han)(han)(han)量(liang)(liang)(liang)(liang)高2%;當(dang)固溶(rong)(rong)(rong)(rong)處理溫(wen)度(du)(du)為(wei)(wei)(wei)(wei)(wei)(wei)1050℃時(shi)(shi),鐵(tie)素(su)體(ti)(ti)相(xiang)(xiang)中(zhong)(zhong)(zhong)(zhong)的(de)Cr、Mo和(he)(he)(he)Ni含(han)(han)(han)量(liang)(liang)(liang)(liang)分別(bie)為(wei)(wei)(wei)(wei)(wei)(wei)23.77%、3.97%、4.24%,奧(ao)氏(shi)(shi)(shi)(shi)體(ti)(ti)相(xiang)(xiang)中(zhong)(zhong)(zhong)(zhong)的(de)Cr、Mo和(he)(he)(he)Ni含(han)(han)(han)量(liang)(liang)(liang)(liang)分別(bie)為(wei)(wei)(wei)(wei)(wei)(wei)23.53%、2.63%、5.42%.可見在1050℃溫(wen)度(du)(du)下進(jin)行固溶(rong)(rong)(rong)(rong)時(shi)(shi),兩相(xiang)(xiang)中(zhong)(zhong)(zhong)(zhong)的(de)Cr、Mo、Ni含(han)(han)(han)量(liang)(liang)(liang)(liang)差異最小,此(ci)時(shi)(shi)鐵(tie)素(su)體(ti)(ti)含(han)(han)(han)量(liang)(liang)(liang)(liang)為(wei)(wei)(wei)(wei)(wei)(wei)51.9%,奧(ao)氏(shi)(shi)(shi)(shi)體(ti)(ti)含(han)(han)(han)量(liang)(liang)(liang)(liang)為(wei)(wei)(wei)(wei)(wei)(wei)48.1%.當(dang)固溶(rong)(rong)(rong)(rong)溫(wen)度(du)(du)改變(bian)(bian)(bian)時(shi)(shi),鐵(tie)素(su)體(ti)(ti)/奧(ao)氏(shi)(shi)(shi)(shi)體(ti)(ti)兩相(xiang)(xiang)比例相(xiang)(xiang)差變(bian)(bian)(bian)大,且兩相(xiang)(xiang)中(zhong)(zhong)(zhong)(zhong)的(de)Cr、Mo、Ni含(han)(han)(han)量(liang)(liang)(liang)(liang)差異也(ye)變(bian)(bian)(bian)大。當(dang)固溶(rong)(rong)(rong)(rong)溫(wen)度(du)(du)為(wei)(wei)(wei)(wei)(wei)(wei)1000℃,兩相(xiang)(xiang)中(zhong)(zhong)(zhong)(zhong)Cr為(wei)(wei)(wei)(wei)(wei)(wei)3.69%、Mo為(wei)(wei)(wei)(wei)(wei)(wei)1.51%、Ni為(wei)(wei)(wei)(wei)(wei)(wei)3.37%;當(dang)固溶(rong)(rong)(rong)(rong)溫(wen)度(du)(du)為(wei)(wei)(wei)(wei)(wei)(wei)1150℃時(shi)(shi),兩相(xiang)(xiang)中(zhong)(zhong)(zhong)(zhong)Cr為(wei)(wei)(wei)(wei)(wei)(wei)2.29%、Mo為(wei)(wei)(wei)(wei)(wei)(wei)1.34%、Ni為(wei)(wei)(wei)(wei)(wei)(wei)2.09%,可見1000℃固溶(rong)(rong)(rong)(rong)試樣的(de)兩相(xiang)(xiang)中(zhong)(zhong)(zhong)(zhong)的(de)Cr、Mo、Ni元素(su)含(han)(han)(han)量(liang)(liang)(liang)(liang)差大于(yu)1150℃固溶(rong)(rong)(rong)(rong)試樣的(de)兩相(xiang)(xiang)中(zhong)(zhong)(zhong)(zhong)Cr、Mo、Ni元素(su)含(han)(han)(han)量(liang)(liang)(liang)(liang)差。
由公式“PREN.=[Cr]+3.3[Mo]”計算鐵(tie)素(su)體的PREN值(zhi)(zhi),由公式“PREN,=[Cr]+3.3[Mo]+16[N]-[Mn]”計算奧(ao)氏體的PREN值(zhi)(zhi),可得PREN值(zhi)(zhi)與固溶(rong)(rong)溫(wen)度(du)的關系圖,如圖3.7(e)所示。從圖3.7(e)可看出,在不(bu)同(tong)固溶(rong)(rong)狀態下(xia),兩(liang)相的PREN值(zhi)(zhi)有所不(bu)同(tong),但PRENa>PRENy.當固溶(rong)(rong)溫(wen)度(du)為1050℃時(shi),PREN.最小、PREN,最大,分別為36.9和30.6,兩(liang)者相差最小。
圖3.8為不同固溶試樣的極化(hua)(hua)曲線。可見,在自(zi)腐蝕電(dian)位(wei)(wei)下,材料(liao)開始發生鈍化(hua)(hua);當極化(hua)(hua)電(dian)位(wei)(wei)升高到一定值時,不同固溶的材料(liao)都發生點蝕,電(dian)流密度(du)(du)急劇增大。根據GB 4334.9-1984中電(dian)流密度(du)(du)為0.1mA/c㎡所(suo)對(dui)應的電(dian)位(wei)(wei)為點蝕電(dian)位(wei)(wei)和Tafel擬合,分(fen)別得到點蝕電(dian)位(wei)(wei)、自(zi)腐蝕電(dian)流密度(du)(du)與固溶溫(wen)度(du)(du)的關系圖,如圖3.11(a)所(suo)示。
圖(tu)(tu)3.9(a)、(b)分別(bie)為不同固溶(rong)試(shi)樣的(de)Nyquist 圖(tu)(tu)和(he)Bode圖(tu)(tu)。從(cong)圖(tu)(tu)3.9(a)可見,試(shi)樣在低頻和(he)高頻區分別(bie)存(cun)(cun)在一個容(rong)抗弧。從(cong)圖(tu)(tu)3.9(b)可見,試(shi)樣在低頻和(he)高頻處分別(bie)存(cun)(cun)在一個時間(jian)常(chang)數。根據曹楚南(nan)的(de)《電(dian)(dian)(dian)(dian)化學(xue)阻抗譜導論》可知。雙相不銹(xiu)鋼(gang)在NaCl溶(rong)液中(zhong)的(de)阻抗譜中(zhong)存(cun)(cun)在兩個時間(jian)常(chang)數,常(chang)以圖(tu)(tu)3.10所示的(de)等(deng)效(xiao)電(dian)(dian)(dian)(dian)路(lu)(R1為電(dian)(dian)(dian)(dian)荷轉移電(dian)(dian)(dian)(dian)阻,R2為鈍化膜電(dian)(dian)(dian)(dian)阻)進行阻抗擬(ni)合(he)。經(jing)阻抗擬(ni)合(he),得到鈍化膜電(dian)(dian)(dian)(dian)阻、電(dian)(dian)(dian)(dian)荷轉移電(dian)(dian)(dian)(dian)阻與固溶(rong)處理溫(wen)度的(de)關系圖(tu)(tu),如圖(tu)(tu)3.11(b)所示。
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雙相不銹鋼(gang)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。
通過計(ji)算得(de)到不同(tong)相(xiang)比例(li)下的(de)(de)鐵(tie)素體PREN值(zhi)(zhi)和(he)奧(ao)氏(shi)體PREN值(zhi)(zhi),可(ke)(ke)見不同(tong)相(xiang)比例(li)下的(de)(de)PREN.都大于(yu)PREN,.雙相(xiang)不銹(xiu)鋼的(de)(de)耐(nai)點(dian)(dian)蝕(shi)(shi)(shi)性(xing)(xing)(xing)能(neng)(neng)(neng)可(ke)(ke)通過點(dian)(dian)蝕(shi)(shi)(shi)電位(wei)(wei)衡量。點(dian)(dian)蝕(shi)(shi)(shi)電位(wei)(wei)越高(gao),耐(nai)點(dian)(dian)獨性(xing)(xing)(xing)能(neng)(neng)(neng)越好。前(qian)人(ren)認為(wei)雙相(xiang)不銹(xiu)鋼的(de)(de)耐(nai)點(dian)(dian)蝕(shi)(shi)(shi)性(xing)(xing)(xing)能(neng)(neng)(neng)由B能(neng)(neng)(neng)N值(zhi)(zhi)較(jiao)區的(de)(de)相(xiang)決(jue)定,且(qie)PEN值(zhi)(zhi)越高(gao),耐(nai)點(dian)(dian)蝕(shi)(shi)(shi)性(xing)(xing)(xing)能(neng)(neng)(neng)越好,從圖(tu)3.71e)可(ke)(ke)矩,不同(tong)海溶(rong)試樣(yang)的(de)(de)PREN.都大于(yu)PREN,,當固溶(rong)溫度為(wei)1050℃時,PHEN,最(zui)大,材料的(de)(de)耐(nai)點(dian)(dian)蝕(shi)(shi)(shi)性(xing)(xing)(xing)能(neng)(neng)(neng)應最(zui)好。從圖(tu)3.11(a)、(b)可(ke)(ke)知,1050℃固溶(rong)試樣(yang)的(de)(de)點(dian)(dian)蝕(shi)(shi)(shi)電位(wei)(wei)最(zui)高(gao),鈍化膜阻(zu)(zu)抗值(zhi)(zhi)最(zui)大,電荷轉移電阻(zu)(zu)值(zhi)(zhi)最(zui)商(shang);且(qie)從圖(tu)3.12(b)可(ke)(ke)見,1050℃固溶(rong)試樣(yang)的(de)(de)點(dian)(dian)蝕(shi)(shi)(shi)坑尺寸最(zui)小(xiao)(xiao),表現(xian)出最(zui)好的(de)(de)耐(nai)點(dian)(dian)蝕(shi)(shi)(shi)性(xing)(xing)(xing)能(neng)(neng)(neng)。綜上可(ke)(ke)知,雙相(xiang)不銹(xiu)鋼的(de)(de)耐(nai)點(dian)(dian)蝕(shi)(shi)(shi)性(xing)(xing)(xing)能(neng)(neng)(neng)由 PREN 值(zhi)(zhi)較(jiao)小(xiao)(xiao)相(xiang)決(jue)定的(de)(de)理論是有實驗依(yi)據的(de)(de)。
從圖(tu)3.12(a)~(d)可(ke)見(jian),在(zai)不同(tong)固(gu)溶狀態下(xia),鐵素(su)體(ti)(ti)相(xiang)都(dou)更易發生點(dian)蝕(shi);而從圖(tu)3.7(e)可(ke)見(jian),在(zai)不同(tong)固(gu)溶狀態下(xia),PREN.都(dou)大于PREN,,鐵素(su)體(ti)(ti)的耐點(dian)蝕(shi)性能應(ying)優(you)于奧氏體(ti)(ti)相(xiang),可(ke)見(jian)兩者存在(zai)矛盾。
隨固(gu)(gu)(gu)溶(rong)(rong)(rong)溫度(du)(du)變(bian)化(hua),雙相(xiang)(xiang)不(bu)(bu)銹(xiu)鋼(gang)中(zhong)(zhong)(zhong)的(de)鐵(tie)素(su)(su)(su)(su)體(ti)(ti)和奧氏(shi)體(ti)(ti)相(xiang)(xiang)的(de)比(bi)例改(gai)變(bian),且兩相(xiang)(xiang)形(xing)態(tai)也發(fa)(fa)生變(bian)化(hua)。Cr是(shi)鐵(tie)素(su)(su)(su)(su)體(ti)(ti)形(xing)成(cheng)元(yuan)(yuan)素(su)(su)(su)(su),可提(ti)高(gao)材(cai)(cai)料(liao)(liao)的(de)耐(nai)蝕(shi)(shi)性(xing)能;Mo是(shi)鐵(tie)素(su)(su)(su)(su)體(ti)(ti)形(xing)成(cheng)元(yuan)(yuan)素(su)(su)(su)(su),可提(ti)高(gao)點(dian)蝕(shi)(shi)電(dian)位(wei),降低(di)腐蝕(shi)(shi)速率(lv);Ni是(shi)奧氏(shi)體(ti)(ti)形(xing)成(cheng)元(yuan)(yuan)素(su)(su)(su)(su),可維(wei)持兩相(xiang)(xiang)平(ping)衡,提(ti)高(gao)耐(nai)蝕(shi)(shi)性(xing)能。并且材(cai)(cai)料(liao)(liao)中(zhong)(zhong)(zhong)存在一定量的(de)N,其為(wei)奧氏(shi)體(ti)(ti)形(xing)成(cheng)元(yuan)(yuan)素(su)(su)(su)(su),提(ti)高(gao)局部腐蝕(shi)(shi)抗(kang)力。從圖3.7(a)~(c)可見(jian),隨固(gu)(gu)(gu)溶(rong)(rong)(rong)溫度(du)(du)變(bian)化(hua),兩相(xiang)(xiang)中(zhong)(zhong)(zhong)的(de)Cr、Mo、Ni元(yuan)(yuan)素(su)(su)(su)(su)含(han)量發(fa)(fa)生變(bian)化(hua)。由(you)(you)于Cr、Mo、Ni元(yuan)(yuan)素(su)(su)(su)(su)之間的(de)腐蝕(shi)(shi)電(dian)位(wei)存在差(cha)異,勢必造成(cheng)兩相(xiang)(xiang)之間存在電(dian)化(hua)學差(cha)異,使腐蝕(shi)(shi)更易發(fa)(fa)生。從圖3.7(e)可知(zhi),不(bu)(bu)同固(gu)(gu)(gu)溶(rong)(rong)(rong)狀態(tai)下材(cai)(cai)料(liao)(liao)的(de)PREN,大(da)(da)小(xiao)關系為(wei)1050℃>1000℃>1150℃>1100℃,因此(ci)根據前(qian)(qian)人的(de)研(yan)究,不(bu)(bu)同固(gu)(gu)(gu)溶(rong)(rong)(rong)態(tai)材(cai)(cai)料(liao)(liao)的(de)耐(nai)點(dian)蝕(shi)(shi)性(xing)能優(you)劣(lie)關系應為(wei):1050℃優(you)于1000℃優(you)于1150℃.當(dang)固(gu)(gu)(gu)溶(rong)(rong)(rong)溫度(du)(du)為(wei)1050℃時,PREN,值(zhi)較大(da)(da),兩相(xiang)(xiang)中(zhong)(zhong)(zhong)的(de)Cr、Mo、Ni元(yuan)(yuan)素(su)(su)(su)(su)含(han)量差(cha)異最小(xiao),材(cai)(cai)料(liao)(liao)的(de)點(dian)蝕(shi)(shi)坑較小(xiao),材(cai)(cai)料(liao)(liao)的(de)耐(nai)點(dian)蝕(shi)(shi)性(xing)能最優(you)。1000℃固(gu)(gu)(gu)溶(rong)(rong)(rong)試(shi)(shi)樣的(de)點(dian)蝕(shi)(shi)坑尺寸(cun)(cun)大(da)(da)于1150℃固(gu)(gu)(gu)溶(rong)(rong)(rong)試(shi)(shi)樣的(de)點(dian)蝕(shi)(shi)坑尺寸(cun)(cun),因此(ci),前(qian)(qian)者的(de)耐(nai)點(dian)蝕(shi)(shi)性(xing)能劣(lie)于后者的(de)耐(nai)點(dian)蝕(shi)(shi)性(xing)能。由(you)(you)此(ci)可見(jian),雙相(xiang)(xiang)不(bu)(bu)銹(xiu)鋼(gang)中(zhong)(zhong)(zhong)的(de)相(xiang)(xiang)腐蝕(shi)(shi)不(bu)(bu)能僅僅由(you)(you)PREN值(zhi)來(lai)解釋。由(you)(you)圖3.7(a)~(c)可知(zhi),1000℃固(gu)(gu)(gu)溶(rong)(rong)(rong)試(shi)(shi)樣的(de)兩相(xiang)(xiang)中(zhong)(zhong)(zhong)的(de)Cr、Mo、Ni元(yuan)(yuan)素(su)(su)(su)(su)含(han)量差(cha)大(da)(da)于1150℃固(gu)(gu)(gu)溶(rong)(rong)(rong)試(shi)(shi)樣的(de)兩相(xiang)(xiang)中(zhong)(zhong)(zhong)的(de)Gr、Mo、Ni元(yuan)(yuan)素(su)(su)(su)(su)含(han)量差(cha),因此(ci),雙相(xiang)(xiang)不(bu)(bu)銹(xiu)鋼(gang)中(zhong)(zhong)(zhong)的(de)相(xiang)(xiang)腐蝕(shi)(shi)還與(yu)兩相(xiang)(xiang)中(zhong)(zhong)(zhong)的(de)元(yuan)(yuan)素(su)(su)(su)(su)分布有關,并且還需考慮到材(cai)(cai)料(liao)(liao)中(zhong)(zhong)(zhong)點(dian)蝕(shi)(shi)敏感(gan)性(xing)較強的(de)區域,如晶界和相(xiang)(xiang)界。
點蝕是一種局部腐蝕(shi)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)(xiang)雙相(xiang)不(bu)銹鋼相(xiang)腐蝕(shi)的(de)因素(su)不(bu)僅包括PREN值(zhi),還(huan)包括各相(xiang)中的(de)元(yuan)素(su)分(fen)布和兩相(xiang)比例,并且三者(zhe)互相(xiang)影(ying)響(xiang)(xiang)、關系復雜。因此,不(bu)能僅憑PREN值(zhi)的(de)大(da)小來判斷耐(nai)點蝕(shi)能力(li),應該綜合考(kao)慮各因素(su)的(de)影(ying)響(xiang)(xiang)。