管(guan)(guan)道(dao)(dao)(dao)的(de)(de)(de)(de)(de)(de)(de)腐(fu)(fu)蝕(shi)會發生在防(fang)腐(fu)(fu)層(ceng)(ceng)缺陷(xian)點處，如(ru)果防(fang)腐(fu)(fu)層(ceng)(ceng)是完整的(de)(de)(de)(de)(de)(de)(de)，管(guan)(guan)道(dao)(dao)(dao)就不會腐(fu)(fu)蝕(shi)。我(wo)們真正關注的(de)(de)(de)(de)(de)(de)(de)是管(guan)(guan)道(dao)(dao)(dao)防(fang)腐(fu)(fu)層(ceng)(ceng)缺陷(xian)點處的(de)(de)(de)(de)(de)(de)(de)保護(hu)水平(ping)。為了(le)測(ce)量防(fang)腐(fu)(fu)層(ceng)(ceng)缺陷(xian)點處的(de)(de)(de)(de)(de)(de)(de)保護(hu)電位(wei)(wei)，我(wo)們用試(shi)(shi)(shi)片(pian)(pian)來模擬管(guan)(guan)道(dao)(dao)(dao)防(fang)腐(fu)(fu)層(ceng)(ceng)缺陷(xian)點。該(gai)試(shi)(shi)(shi)片(pian)(pian)平(ping)時(shi)通過測(ce)試(shi)(shi)(shi)樁與(yu)管(guan)(guan)道(dao)(dao)(dao)連(lian)接，得(de)到(dao)同樣的(de)(de)(de)(de)(de)(de)(de)陰極保護(hu)。測(ce)量時(shi)，將試(shi)(shi)(shi)片(pian)(pian)與(yu)管(guan)(guan)道(dao)(dao)(dao)的(de)(de)(de)(de)(de)(de)(de)連(lian)接瞬時(shi)斷開，這樣就可(ke)(ke)以測(ce)得(de)試(shi)(shi)(shi)片(pian)(pian)的(de)(de)(de)(de)(de)(de)(de)斷電電位(wei)(wei)而不需(xu)要對(dui)管(guan)(guan)道(dao)(dao)(dao)上的(de)(de)(de)(de)(de)(de)(de)所有電源進(jin)行中斷。試(shi)(shi)(shi)片(pian)(pian)的(de)(de)(de)(de)(de)(de)(de)電位(wei)(wei)就是同一水平(ping)位(wei)(wei)置的(de)(de)(de)(de)(de)(de)(de)管(guan)(guan)道(dao)(dao)(dao)防(fang)腐(fu)(fu)層(ceng)(ceng)與(yu)試(shi)(shi)(shi)片(pian)(pian)面積相等(deng)的(de)(de)(de)(de)(de)(de)(de)缺陷(xian)點處的(de)(de)(de)(de)(de)(de)(de)保護(hu)電位(wei)(wei)。該(gai)方法(fa)同時(shi)消除了(le)雜散(san)電流(liu)等(deng)各(ge)種干擾因素的(de)(de)(de)(de)(de)(de)(de)影響，也可(ke)(ke)得(de)到(dao)較為準(zhun)確(que)的(de)(de)(de)(de)(de)(de)(de)保護(hu)電位(wei)(wei)。
保(bao)護(hu)電(dian)位測(ce)量方法
①試(shi)(shi)片(pian)(pian)法(fa)斷電(dian)(dian)(dian)電(dian)(dian)(dian)位測(ce)量有兩種(zhong)方法(fa)，一種(zhong)是(shi)將(jiang)參(can)(can)比(bi)(bi)(bi)(bi)電(dian)(dian)(dian)極(ji)(ji)與試(shi)(shi)片(pian)(pian)組(zu)裝在(zai)一起組(zu)成極(ji)(ji)化(hua)(hua)探(tan)頭進(jin)行(xing)(xing)測(ce)量，稱為(wei)極(ji)(ji)化(hua)(hua)探(tan)頭法(fa)(極(ji)(ji)化(hua)(hua)探(tan)頭結構(gou)見圖(tu)2)；另(ling)一種(zhong)是(shi)將(jiang)試(shi)(shi)片(pian)(pian)固定在(zai)參(can)(can)比(bi)(bi)(bi)(bi)管(guan)上，利用(yong)便攜(xie)式參(can)(can)比(bi)(bi)(bi)(bi)電(dian)(dian)(dian)極(ji)(ji)進(jin)行(xing)(xing)測(ce)量，稱為(wei)參(can)(can)比(bi)(bi)(bi)(bi)管(guan)試(shi)(shi)片(pian)(pian)法(fa)(見圖(tu)3)。當(dang)使用(yong)極(ji)(ji)化(hua)(hua)探(tan)頭法(fa)時，參(can)(can)比(bi)(bi)(bi)(bi)電(dian)(dian)(dian)極(ji)(ji)中的(de)電(dian)(dian)(dian)解液(ye)會逐漸滲漏(lou)直至參(can)(can)比(bi)(bi)(bi)(bi)電(dian)(dian)(dian)極(ji)(ji)失效(xiao)，同(tong)時，滲漏(lou)出(chu)來的(de)電(dian)(dian)(dian)解液(ye)會改變試(shi)(shi)片(pian)(pian)所處(chu)環(huan)境，當(dang)硫酸銅電(dian)(dian)(dian)解液(ye)與試(shi)(shi)片(pian)(pian)接觸時，會發(fa)生置(zhi)換反(fan)應，試(shi)(shi)片(pian)(pian)表面被銅覆(fu)蓋，改變了其(qi)極(ji)(ji)化(hua)(hua)特性，參(can)(can)比(bi)(bi)(bi)(bi)電(dian)(dian)(dian)極(ji)(ji)電(dian)(dian)(dian)位漂移(yi)時也不易察覺。采用(yong)參(can)(can)比(bi)(bi)(bi)(bi)管(guan)，由于參(can)(can)比(bi)(bi)(bi)(bi)管(guan)的(de)屏蔽(bi)作用(yong)，利用(yong)便攜(xie)式參(can)(can)比(bi)(bi)(bi)(bi)電(dian)(dian)(dian)極(ji)(ji)在(zai)地表就可以進(jin)行(xing)(xing)測(ce)量，避免漏(lou)液(ye)帶來的(de)問題。
②試(shi)(shi)片(pian)(pian)(pian)面(mian)積的(de)(de)選擇(ze)。試(shi)(shi)片(pian)(pian)(pian)面(mian)積根據管(guan)道(dao)的(de)(de)防腐(fu)層類型(xing)、施工(gong)質(zhi)量、管(guan)道(dao)所處環境及運(yun)行年限(xian)確定(ding)，試(shi)(shi)片(pian)(pian)(pian)面(mian)積一般為6.5～50.0cm2。如(ru)果(guo)試(shi)(shi)片(pian)(pian)(pian)面(mian)積太(tai)小(xiao)(xiao)，它與(yu)土壤的(de)(de)接觸電阻和(he)管(guan)道(dao)涂(tu)層缺(que)陷(xian)處與(yu)土壤的(de)(de)接觸電阻差異過大，腐(fu)蝕特性也(ye)會不同。如(ru)果(guo)試(shi)(shi)片(pian)(pian)(pian)電位測量結果(guo)說明該點保(bao)護(hu)滿足規范要求，只代(dai)表管(guan)道(dao)上面(mian)積小(xiao)(xiao)于試(shi)(shi)片(pian)(pian)(pian)面(mian)積的(de)(de)涂(tu)層缺(que)陷(xian)點滿足保(bao)護(hu)要求，不能(neng)說明面(mian)積大于試(shi)(shi)片(pian)(pian)(pian)面(mian)積的(de)(de)涂(tu)層缺(que)陷(xian)點的(de)(de)保(bao)護(hu)是否充分。因此要選擇(ze)面(mian)積大于涂(tu)層缺(que)陷(xian)點面(mian)積的(de)(de)試(shi)(shi)片(pian)(pian)(pian)，才能(neng)保(bao)證測量的(de)(de)準確性。
③試(shi)片(pian)的(de)埋深及回填狀態與管(guan)道(dao)相同(tong)。在(zai)測(ce)量之前，應(ying)確認陰極保護運(yun)行(xing)正常，試(shi)片(pian)與管(guan)道(dao)已連通，試(shi)片(pian)與管(guan)道(dao)充分極化。測(ce)量中，將直流(liu)數(shu)字電(dian)壓(ya)表的(de)正極接試(shi)片(pian)，負極接硫酸銅參比電(dian)極。測(ce)量并(bing)記錄試(shi)片(pian)的(de)通電(dian)電(dian)位。
④將試片與管道(dao)斷(duan)(duan)(duan)開，立即測量并記錄試片的(de)斷(duan)(duan)(duan)電(dian)(dian)(dian)電(dian)(dian)(dian)位(wei)(wei)。所(suo)測得的(de)斷(duan)(duan)(duan)電(dian)(dian)(dian)電(dian)(dian)(dian)位(wei)(wei)代表埋設(she)點(dian)附近(jin)與試片同(tong)等面積(ji)防腐(fu)層(ceng)缺陷點(dian)處的(de)極(ji)化電(dian)(dian)(dian)位(wei)(wei)。所(suo)測量的(de)試片斷(duan)(duan)(duan)電(dian)(dian)(dian)電(dian)(dian)(dian)位(wei)(wei)和把參比電(dian)(dian)(dian)極(ji)放(fang)置到管道(dao)正(zheng)上(shang)方測量得到的(de)斷(duan)(duan)(duan)電(dian)(dian)(dian)電(dian)(dian)(dian)位(wei)(wei)不同(tong)，因為當參比電(dian)(dian)(dian)極(ji)位(wei)(wei)于管道(dao)正(zheng)上(shang)方時，測得的(de)斷(duan)(duan)(duan)電(dian)(dian)(dian)電(dian)(dian)(dian)位(wei)(wei)是附近(jin)所(suo)有(you)防腐(fu)層(ceng)缺陷點(dian)斷(duan)(duan)(duan)電(dian)(dian)(dian)電(dian)(dian)(dian)位(wei)(wei)的(de)綜合(he)值。
⑤試片的(de)通(tong)電電位(wei)值與(yu)參比電極(ji)位(wei)于(yu)防腐層缺(que)陷點正上方(fang)時測得的(de)通(tong)電電位(wei)值接近。由(you)于(yu)試片與(yu)管道在電位(wei)讀數中所占的(de)比例(li)難以分(fen)析，因(yin)此，試片通(tong)電電位(wei)不宜作為評判管地電位(wei)的(de)依(yi)據。
⑥試片要選(xuan)用與管(guan)道類似的鋼(gang)(gang)材(cai)(cai)，鋼(gang)(gang)材(cai)(cai)的含碳量或(huo)強度對(dui)其腐(fu)蝕(shi)特(te)性影響不大。因此，試片材(cai)(cai)質(zhi)不必(bi)和(he)管(guan)材(cai)(cai)wanquan一樣。
確定自然電位
如果試片的斷電電位滿足陰極保護指標，說明管道保護充分；如果試片斷電電位比-0.85V正，要判斷管道是不是滿足100mV陰極極化值。此時，可以用試片的斷電電位減去管道的自然電位獲得陰極極化值。如果不知道管道的自然電位，可以利用試片的自然電位，試片的自然電位受試片表面光亮程度影響，應將其埋設并自然銹蝕一段時間(1～12個月)后，再測量其自然電位。
通(tong)電試(shi)(shi)片(pian)(pian)的(de)(de)(de)去極(ji)化電位和自(zi)(zi)腐(fu)蝕試(shi)(shi)片(pian)(pian)的(de)(de)(de)自(zi)(zi)然電位會有差別，通(tong)電試(shi)(shi)片(pian)(pian)的(de)(de)(de)去極(ji)化電位可能會正(zheng)于(yu)自(zi)(zi)腐(fu)蝕試(shi)(shi)片(pian)(pian)的(de)(de)(de)自(zi)(zi)然電位，因為陰(yin)極(ji)保護改變了通(tong)電試(shi)(shi)片(pian)(pian)的(de)(de)(de)環(huan)境(jing)。對于(yu)同樣的(de)(de)(de)試(shi)(shi)片(pian)(pian)，在土(tu)壤(rang)腐(fu)蝕性強的(de)(de)(de)環(huan)境(jing)中，試(shi)(shi)片(pian)(pian)的(de)(de)(de)自(zi)(zi)然電位較負(可以達到(dao)-0.80V，本文中所有的(de)(de)(de)電位值均是相對于(yu)銅(tong)／飽和硫酸銅(tong)參比電極(ji))，在土(tu)壤(rang)腐(fu)蝕性弱的(de)(de)(de)環(huan)境(jing)中，試(shi)(shi)片(pian)(pian)的(de)(de)(de)自(zi)(zi)然電位較正(zheng)(干沙中可以達到(dao)-0.30V)。可以以此來判(pan)斷土(tu)壤(rang)的(de)(de)(de)腐(fu)蝕性。
