JP2004191186A

JP2004191186A - Method for monitoring local corrosion of carbon steel and local corrosion preventing method

Info

Publication number: JP2004191186A
Application number: JP2002359540A
Authority: JP
Inventors: Yutaka Yoneda; 裕米田; Shiro Taya; 史郎田家
Original assignee: Kurita Water Industries Ltd
Current assignee: Kurita Water Industries Ltd
Priority date: 2002-12-11
Filing date: 2002-12-11
Publication date: 2004-07-08
Anticipated expiration: 2022-12-11
Also published as: JP4207553B2

Abstract

<P>PROBLEM TO BE SOLVED: To quantitatively monitor the scale of the local corrosion (pitting) sign of carbon steel in an aqueous system to take a proper measure before the occurrence of the local corrosion sign of a scale reaching progressive local corrosion on the basis of the monitoring result. <P>SOLUTION: The potential noise of a specific amplitude and a specific potential change speed superposed on the natural immersion potential of carbon steel coming into contact with the aqueous system is measured and the measured potential noise is applied to a separate immobilized sample piece to measure an anode responding current to estimate the scale of the local corrosion sign of carbon steel. On the basis of the monitoring result, the injection amount of a water treatment chemical agent for suppressing the occurrence of the pitting of carbon steel or a water treatment chemical agent for removing dissolved oxygen is controlled or the deoxidation amount of a deoxidation device for removing dissolved oxygen in the aqueous system is controlled. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

【０００１】
【発明の属する技術分野】
本発明は、水系における炭素鋼の局部腐食（孔食）萌芽の規模を的確にモニタリングする方法と、このモニタリング結果に基いて炭素鋼の局部腐食を防止する方法に関する。
【０００２】
【従来の技術及び先行技術】
ボイラ水系や冷却水系等の各種水系においては、熱交換器や配管の局部腐食を防止するために、これらを構成する金属材料の腐食を抑制する薬剤の添加や、腐食の原因となる水中の溶存酸素を除去するための脱酸素剤の添加、或いは脱気装置等による水中の溶存酸素の除去などの対策が講じられている。このような局部腐食防止対策により、局部腐食を確実に防止するためには、局部腐食を的確にモニタリングし、局部腐食に到る前に局部腐食防止対策を有効に施すことが望まれる。
【０００３】
塩化物水溶液中に浸漬したステンレス鋼やＮｉ基合金などの耐食性材料、或いは、不働態化させた純鉄については、電位ノイズが局部腐食萌芽に対応していることが知られている（井上博之，材料と環境，45,717(1996) ; M. Hashimoto, Corros. Sci., 33,885(1992), 33,905(1992)）。そして、ステンレス鋼では、卑側の電位にて一定の停滞期間を有する電位ノイズが局部腐食萌芽に対応しており、不働態化させた純鉄では、卑側の電位における停滞期間のない電位ノイズが局部腐食萌芽に対応していることが明らかにされている。また、ステンレス鋼で発生する卑側の電位にて一定の停滞期間を有する電位ノイズについては、該電位ノイズをポテンショスタットを用いて別の試験片に逆設定し、それより得られた局部アノード応答電流の積分値から推測される食孔半径と、試験片表面に残された局部腐食萌芽の半径とは良い一致を示すことが明らかにされている。
【０００４】
本出願人は、水系における炭素鋼の局部腐食（孔食）萌芽を的確にモニタリングする方法として、先に、水系の水と接触する炭素鋼の自然浸漬電位に重畳した特定の振幅かつ特定の電位変化速度の電位ノイズを測定することにより、該炭素鋼の局部腐食萌芽をモニタリングする方法を提案した（特願２００１−２６６０３６。以下「先願」という。）。
【０００５】
しかし、先願の方法では、局部腐食萌芽の発生数をモニタリングすることは可能であるが、発生した局部腐食萌芽のそれぞれの規模について定量的な情報を得ることは不可能である。
【０００６】
【特許文献１】
特願２００１−２６６０３６
【非特許文献１】
井上博之，材料と環境，45,717(1996)
【非特許文献２】
M. Hashimoto, Corros. Sci., 33,885(1992)
【非特許文献３】
M. Hashimoto, Corros. Sci., 33,905(1992)
【０００７】
【発明が解決しようとする課題】
本発明は、水系における炭素鋼の局部腐食（孔食）萌芽の規模を定量的にモニタリングする方法と、このモニタリング結果に基づいて、進展性の局部腐食に到達する規模の局部腐食萌芽が発生する前に適切な対策を講じる局部腐食防止方法を提供することを目的とする。
【０００８】
【課題を解決するための手段】
本発明の炭素鋼の局所腐食モニタリング方法は、水系の水と接触する炭素鋼の局部腐食をモニタリングする方法において、該炭素鋼の自然浸漬電位に重畳した特定の振幅かつ特定の電位変化速度の電位ノイズを測定し、その測定された電位ノイズを、不働態化させた別の試験片に印加することにより、該炭素鋼の局部腐食萌芽の規模を推定することを特徴とする。
【０００９】
かかる本発明に従って、水系の水と接触する炭素鋼の自然浸漬電位に重畳した特定の振幅かつ特定の電位変化速度の電位ノイズを測定し、その測定された電位ノイズを、不働態化させた別の試験片に印加し、例えば、そのときに発生するアノード溶解電流（アノード応答電流）の時間積分から通過電気量を求め、この結果から、局部腐食萌芽の規模（実際の局部腐食萌芽半径）を定量的に評価することができる。なお、以下において、測定された電位ノイズを別の試験片に印加する方法を「電位ノイズの逆設定法」と称す場合がある。
【００１０】
即ち、塩化物水溶液に炭素鋼の試験片を浸漬させると、図１（ａ）及び図１（ａ）のＢ部の拡大図である図１（ｂ）に示すように、局部腐食（孔食）萌芽の生成と再不働態化に対応して自然浸漬電位に重畳成分（電位ノイズ）が生じる。この試験終了後、試験片表面のＳＥＭ観察を行った結果、電位ノイズ発生量と孔食の痕跡とがほぼ一致していることが確認された。また、本発明の電位ノイズの逆設定法により、この電位ノイズを不働態化させた別の試験片に逆設定した際のアノード応答電流は図２に示す通りであり、この応答電流の積分値から局部腐食萌芽半径を推定したところ、実際の食孔痕跡の半径とほぼ一致した。これらは、電位ノイズが局部腐食萌芽の生成と死滅に対応していることを裏付ける結果である。
【００１１】
本発明において、不働態化させた別の試験片は、当該水系の水を防食性アニオンを担持したアニオン交換体と接触させた後の水に、該炭素鋼を浸漬して不働態化させたものが好適である。
【００１２】
本発明（請求項３，４）の炭素鋼の局部腐食防止方法は、水系の水と接触する炭素鋼の局部腐食を防止する方法において、本発明の炭素鋼の局部腐食モニタリング方法のモニタリング結果に基いて、炭素鋼の孔食発生を抑制するための水処理薬剤の薬注量又は水系の溶存酸素を除去するための水処理薬剤の薬注量を制御する。
【００１３】
また、本発明（請求項５）では、水系の水と接触する炭素鋼の局部腐食を防止する方法において、上記モニタリング結果に基いて、水系の溶存酸素を除去するための脱酸素装置の脱酸素量を制御する。
【００１４】
本発明の電位ノイズの逆設定法により推定される局部腐食萌芽の規模が大きく、進展性の局部腐食に到達する規模の局部腐食萌芽が発生していると判断される場合には、局部腐食を抑制する薬剤を適正量投入する、或いは、適切な脱酸素処理を講じることにより、局部腐食萌芽が進展性の局部腐食へと成長することを未然に防止することができる。
【００１５】
【発明の実施の形態】
以下に本発明の炭素鋼の局部腐食モニタリング方法及び炭素鋼の局部腐食防止方法の実施の形態を詳細に説明する。
【００１６】
本発明の炭素鋼の局部腐食モニタリング方法では、まず各種の水系の炭素鋼の自然浸漬電位を測定し、自然浸漬電位に重畳した特定の振幅かつ特定の電位変化速度の電位ノイズを測定する。
【００１７】
この炭素鋼の電位ノイズは、炭素鋼よりなる試験片（以下、「電位ノイズ測定用試験片」と称す場合がある。）をモニタリング対象の水系と同等の条件に置き、電位測定装置で測定することができる。このとき、発生する電位ノイズを測定するためのサンプリング間隔は０．５秒以下とし、電圧計は１μＶ程度の精度のものを用いることが好ましい。自然浸漬電位に重畳する電位ノイズとは、電位が上下に振動する電位ノイズであり、例えば、振幅１０ｍＶ以上で電位変化速度１ｍＶ／ｓｅｃ以上の電位ノイズをとらえることができる。
【００１８】
なお、モニタリング対象の実環境においては電位ノイズが発生しない条件で操業することが好ましい。
【００１９】
このようにして測定した電位ノイズを、不働態化した試験片（以下「逆電位設定用試験片」と称す場合がある。）に印加する。ここで用いる不働態化した試験片は、完全に不働態化しており、腐食電位が安定に推移しているものである必要がある。そのために、該水系の水を、ＨＣＯ_３ ⁻、ＯＨ⁻などの防食性アニオンを担持したアニオン交換体と接触させることにより、孔食発生の要因となるＣｌ⁻、ＳＯ_４ ^２−などの水中の腐食性イオンをＨＣＯ_３ ⁻、ＯＨ⁻などの防食性アニオンとイオン交換した水に、電位ノイズ測定用試験片と同一の炭素鋼よりなる試験片を浸漬することで不働態化した試験片を用いることが好ましい。
【００２０】
このような逆電位設定用試験片に逆設定する電位ノイズ波形は、例えば、電位ノイズ測定用試験片で測定した電位ノイズデータを最小桁が２５μＶのＤ／Ａコンバーターを用いて１０ｍｓｅｃ間隔で再生し、１０ｍｓｅｃ毎の電位値を０．５秒間隔の測定データ直線に補完することにより再生することができる。そして、再生した電位ノイズをポテンショスタットを用いて、逆電位設定用試験片に印加し、その際得られるアノード応答電流を測定する。即ち、例えば、アノード応答電流出力を１２ビットの分解能を有する多チャンネルのＡ／Ｄ変換器を用いて、０．５秒間隔で測定する。
【００２１】
このようにして測定された逆電位設定用試験片のアノード応答電流に基いて、水系における局部腐食萌芽の規模を推定することができる。即ち、例えば、このアノード応答電流を時間積分することにより得られた積分値Ｓは、実水系での局部腐食萌芽の半径と関係するため、この関係を予め求めておき、積分値Ｓを予め求めた関係式に代入することにより局部腐食萌芽の推定半径ｒを算出することができる。
【００２２】
本発明の炭素鋼の局部腐食防止方法において、水系に添加する、孔食発生を抑制するための水処理薬剤や溶存酸素を除去するための水処理薬剤の薬注量を上記モニタリング結果に基いて制御する場合、具体的には次のような薬注制御を行うことができる。
（１）算出された局部腐食萌芽の推定半径が予め設定した設定値以上の場合には、薬注を開始し、算出された局部腐食萌芽の推定半径が該設定値未満となるまで、又は一定の時間、若しくは算出された局部腐食萌芽の推定半径が該設定値未満となった後一定の時間、水処理薬剤を連続的又は間欠的に添加し、その後水処理薬剤の添加を停止する。
（２）水処理薬剤の薬注量について、定常時の第１の薬注量とこの第１の薬注量よりも多い第２の薬注量を予め設定し、定常時には第１の薬注量で連続的又は間欠的に薬注を行い、算出された局部腐食萌芽の推定半径が予め設定した設定値以上である場合には、算出された局部腐食萌芽の推定半径が該設定値未満となるまで、又は一定の時間、若しくは算出された局部腐食萌芽の推定半径が該設定値未満となった後一定の時間、第２の薬注量で連続的又は間欠的に薬注を行い、その後は再び第１の薬注量で連続的又は間欠的に薬注を行う。
【００２３】
このように、電位ノイズの逆設定法により求めた局部腐食萌芽の規模に基いて水処理薬剤の薬注制御を行うことにより、水処理薬剤の過不足を防止して、効率的な薬剤処理を行い、局部腐食を確実に防止することが可能となる。
【００２４】
また、水系の溶存酸素を除去するための脱気膜装置等の脱酸素装置の脱酸素量を上記モニタリング結果に基いて制御する場合には、具体的には、次のような運転制御を行えば良い。
（１）算出された局部腐食萌芽の推定半径が予め設定した設定値以上である場合には、脱酸素装置の運転を開始し、算出された局部腐食萌芽の推定半径が該設定値未満となるまで、又は一定の時間、若しくは算出された局部腐食萌芽の推定半径が該設定値未満となった後一定の時間、脱酸素処理し、その後、脱酸素装置の運転を停止する。
（２）脱酸素装置について定常時の第１の運転条件（電力量、真空度、通水量、ガス流量等）とこの第１の運転条件よりも脱酸素量の多い第２の運転条件を予め設定し、定常時は第１の運転条件で脱酸素装置を運転し、算出された局部腐食萌芽の推定半径が予め設定した設定値以上である場合には、算出された局部腐食萌芽の推定半径が該設定値未満となるまで、又は一定の時間、若しくは算出された局部腐食萌芽の推定半径が該設定値未満となった後一定の時間、第２の運転条件で運転を行い、その後は再び第１の運転条件で運転を行う。
【００２５】
このように、電位ノイズの逆設定法により求めた局部腐食萌芽の規模に基いて脱酸素装置の運転を制御することにより、脱酸素装置の過負荷を防止して、効率的な脱酸素処理を行い、局部腐食を確実に防止することが可能となる。
【００２６】
なお、本発明においてモニタリング対象とする水系としては、ボイラ水系、冷却水系等の水系等が挙げられる。
【００２７】
【実施例】
以下に実施例を挙げて本発明をより具体的に説明する。
【００２８】
実施例１
図３に示した実機開放冷却水系において、炭素鋼の局部腐食モニタリングを実施した。
【００２９】
この冷却水系では、冷却塔１０のピット１１の水が送水配管１２により熱交換器１３に送給され、戻り配管１４より冷却塔１０に戻される。冷却塔１０のピット１１には、薬液タンク１６の水処理薬剤が薬注ポンプ１７により添加される。薬注ポンプ１７による薬注量は制御機器１８により制御される。
【００３０】
なお、水処理薬剤としては、リン酸・亜鉛系の防食剤、アクリル酸系のスケール防止剤、及び非ハロゲン系のスライムコントロール剤の水溶液を用いている。
【００３１】
この冷却水系の冷却水の送水配管１２から枝管を介して分取した冷却水を試験水とし、送水ポンプ６及び定流量弁５を介してカラム４に定流量通水した。カラム４内の冷却水流速は、定流量弁５により実機熱交換器チューブ内の冷却水流速と同じ０．５ｍ／ｓｅｃに調整した。このカラム４には電位ノイズ測定用の炭素鋼製試験片（冷間圧延鋼板（ＳＰＣＣ））１及び参照電極（ＫＣｌ飽和銀・塩化銀）２が設置されており、該試験片１と参照電極２との電位差が電位測定装置３によって測定される。この電位測定装置３により、自然浸漬電位に重畳した振幅１０ｍＶ以上かつ電位変化速度１ｍＶ／ｓｅｃ以上の電位ノイズを計測する。電位ノイズの計測値は、逆電位設定装置８に入力される。
【００３２】
別に、同様に、送水配管１２から枝管を介して分取した冷却水を試験水とし、送水ポンプ６Ａ及び定流量弁５Ａによりアニオン交換塔７で処理した後カラム４Ａに定流量通水した。カラム４Ａ内の冷却水流速は、定流量弁５Ａにより実機熱交換器チューブ内の冷却水流速と同じ０．５ｍ／ｓｅｃに調整した。このカラム４Ａにはアノード応答電流測定用の炭素鋼製試験片（冷間圧延鋼板（ＳＰＣＣ））１Ａ及び参照電極（ＫＣｌ飽和銀・塩化銀）２Ａが設置されている。
【００３３】
このカラム４Ａにおいて、アニオン交換塔７で、防食性アニオンを担持したアニオン交換体と接触させ、腐食性アニオンがＨＣＯ_３ ⁻、ＯＨ⁻などの防食性アニオンとアニオン交換された後の冷却水に浸漬されることで不働態化された試験片１Ａに、電位測定装置３で測定された電位ノイズが逆電位設定装置８により再現され、印加される。この電位ノイズを印加したときの試験片１Ａのアノード応答電流を計測し、この値を制御機器１８に入力して時間積分し、この結果から、局部腐食萌芽半径を推定した。即ち、この積分値Ｓから、局部腐食萌芽の半径（ｒ）を下記［Ｉ］式で算出し、局部腐食萌芽の推定半径が１μｍ以上と算出された場合、制御機器１８から薬注ポンプ１７に水処理薬剤の薬注量の制御信号を出力した。
【００３４】
【数１】

【００３５】
図４に示す通り、この冷却水系についてモニタリングを開始した当初は、水処理薬剤の薬注量が適正ではなかったため、局部腐食萌芽の発生を示す電位ノイズが確認されていた。また、電位ノイズ測定装置３で測定した電位ノイズを逆電位設定装置８によって、不働態化させた別の試験片１Ａに印加した結果、発生した電位ノイズはすべて半径１μｍ以上の規模の局部腐食萌芽の発生に対応することが分かった。そこで、この結果に基づいて、電位ノイズの逆設定法で求めた局部腐食萌芽の推定半径が１μｍ未満になるまで水処理薬剤の薬注量を現状の２倍量とする薬注制御を行った結果、約３０分後には、半径１μｍ以上に相当する電位ノイズは発生しなくなった。
【００３６】
試験後、実機熱交換器の炭素鋼チューブを詳しく調査した結果、腐食が発生していないことを確認した。
【００３７】
実施例２
図５に示したボイラのブロー水において、炭素鋼の局部腐食モニタリングを実施した。
【００３８】
図５において、図３と同じく、１は電位ノイズ測定用試験片、１Ａはアノード応答電流測定用試験片、２，２Ａは参照電極（ＫＣｌ飽和銀・塩化銀）、３は電位測定装置、４，４Ａはカラム、７は水から腐食性アニオンを取り除きＨＣＯ_３ ⁻、ＯＨ⁻などの防食性アニオンと交換するアニオン交換塔、８は電位測定装置において測定された電位ノイズを不働態化されたアノード応答電流測定用試験片１Ａに印加する逆電位設定装置、１６は薬液タンク、１７は薬注ポンプ、１８は制御機器である。これらの機器によりなる電位ノイズ測定システム、逆電位設定システム、及び薬注制御システムの構成は図３と同様であり、試験片１，１Ａの材料も図３と同一のＳＰＣＣである。
【００３９】
図５では、ボイラ缶体２０へ給水タンク２１から軟化水がボイラ給水として配管２２より供給されている。ボイラ缶体２０に設けられた吹き出し弁２３を介してボイラ水が分取され、このボイラ水が前記カラム４，４Ａに導入される。そして、電位測定装置３において測定された電位ノイズを逆電位設定装置８によって逆電位設定して得た結果に基づいて、実施例１と同様にして給水タンク２１からボイラ缶体２０に送られるボイラ給水に対し、水処理薬剤（本実施例では脱酸素剤の水溶液）の薬注量が制御される。
【００４０】
図６に示す通り、この水系においてモニタリングを開始した当初は、水処理薬剤の添加量が適正ではなかったため、局部腐食萌芽の発生を示す電位ノイズが確認されていた。また、逆電位設定法によってアノード応答電流の積分値Ｓから前述の［Ｉ］式により算出したアノード応答電流に対応した局部腐食萌芽の推定半径ｒは１μｍ以上であった。そこで、アノード応答電流に対応した局部腐食萌芽の推定半径が１μｍ未満になるまで水処理薬剤をボイラ給水に注入する薬注濃度管理に変更した結果、約１時間後には電位ノイズそのものの発生が消失し、局部腐食萌芽の発生が抑制された。
【００４１】
試験後、ボイラ缶内を詳しく調査したが、腐食の発生は認められなかった。
【００４２】
【発明の効果】
以上詳述した通り、本発明の炭素鋼の局部腐食モニタリング方法によれば、水系における炭素鋼の自然浸漬電位に重畳する電位ノイズを測定し、不働態化させた別の試験片に、この電位ノイズを逆電位設定することで、炭素鋼の局部腐食萌芽の規模をモニタリングすることが可能となり、進展性の局部腐食に到達する規模の局部腐食萌芽が生起するまでに、迅速かつ効果的な腐食防止対策を講じることが可能となる。
【００４３】
また、本発明の炭素鋼の局部腐食防止方法によれば、このモニタリング結果を基にして、孔食を抑制する水処理薬剤を薬注することにより、適切に薬注管理することが可能となる。また、溶存酸素の除去処理を講じる場合にも、脱酸素装置の適切な運転管理を行うことが可能となる。
【図面の簡単な説明】
【図１】炭素鋼の自然浸漬電位の電位ノイズを示すグラフである。
【図２】電位ノイズを不働態化した試験片に逆設定したときのアノード応答電流を示すグラフである。
【図３】実施例１で用いた試験装置を示す系統図である。
【図４】実施例１における自然浸漬電位の経時変化を示すグラフである。
【図５】実施例２で用いた試験装置を示す系統図である。
【図６】実施例２における自然浸漬電位の経時変化を示すグラフである。
【符号の説明】
１電位ノイズ測定用試験片
１Ａアノード応答電流測定用試験片
２，２Ａ参照電極
３電位測定装置
４，４Ａカラム
５，５Ａ定流量弁
６，６Ａ送水ポンプ
７アニオン交換塔
８逆電位設定装置
１０冷却塔
１１ピット
１３熱交換器
１６薬液タンク
１７薬注ポンプ
１８制御機器
２０ボイラ缶体
２１給水タンク[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for accurately monitoring the scale of local corrosion (pitting) germination of carbon steel in an aqueous system, and a method for preventing local corrosion of carbon steel based on the monitoring result.
[0002]
[Prior Art and Prior Art]
In various water systems such as boiler water systems and cooling water systems, in order to prevent local corrosion of heat exchangers and pipes, addition of chemicals that suppress the corrosion of the metallic materials that compose them, and dissolution in water that causes corrosion Countermeasures such as addition of a deoxidizer for removing oxygen or removal of dissolved oxygen in water using a deaerator or the like have been taken. In order to surely prevent local corrosion by such local corrosion prevention measures, it is desired to accurately monitor local corrosion and to effectively perform local corrosion prevention measures before local corrosion occurs.
[0003]
For corrosion-resistant materials such as stainless steel and Ni-based alloys immersed in an aqueous chloride solution, or for passivated pure iron, it is known that potential noise corresponds to local corrosion sprouting (Hiroyuki Inoue) 45,717 (1996); M. Hashimoto, Corros. Sci., 33,885 (1992), 33,905 (1992)). In stainless steel, potential noise having a certain stagnation period at the base-side potential corresponds to local corrosion sprouting, and in passivated pure iron, potential noise without a stagnation period at the base-side potential. Correspond to localized corrosion sprouting. Further, with respect to the potential noise having a constant stagnation period at the base potential generated by stainless steel, the potential noise is reversely set to another test piece using a potentiostat, and the local anode response obtained therefrom is set. It has been shown that the pit radius estimated from the integrated current and the radius of the localized corrosion sprouting left on the specimen surface show good agreement.
[0004]
As a method of accurately monitoring local corrosion (pitting) germination of carbon steel in an aqueous system, the present applicant has previously set a specific amplitude and a specific potential superimposed on the natural immersion potential of carbon steel in contact with water in an aqueous system. A method of monitoring the local corrosion sprouting of the carbon steel by measuring the potential noise of the change rate was proposed (Japanese Patent Application No. 2001-266036, hereinafter referred to as “first application”).
[0005]
However, with the method of the prior application, it is possible to monitor the number of local corrosion sproutings generated, but it is not possible to obtain quantitative information on the scale of each of the generated local corrosion sproutings.
[0006]
[Patent Document 1]
Japanese Patent Application No. 2001-266036
[Non-patent document 1]
Hiroyuki Inoue, Materials and Environment, 45,717 (1996)
[Non-patent document 2]
M. Hashimoto, Corros. Sci., 33,885 (1992)
[Non-Patent Document 3]
M. Hashimoto, Corros. Sci., 33,905 (1992)
[0007]
[Problems to be solved by the invention]
The present invention provides a method for quantitatively monitoring the scale of local corrosion (pitting) germination of carbon steel in an aqueous system, and based on the monitoring result, generates local corrosion germination of a scale that reaches progressive local corrosion. The purpose is to provide a local corrosion prevention method that takes appropriate measures beforehand.
[0008]
[Means for Solving the Problems]
The method for monitoring local corrosion of carbon steel according to the present invention is a method for monitoring local corrosion of carbon steel in contact with water in an aqueous system, wherein the potential of a specific amplitude and a specific potential change rate superimposed on the natural immersion potential of the carbon steel. It is characterized by measuring noise and applying the measured potential noise to another passivated test piece to estimate the magnitude of local corrosion sprouting of the carbon steel.
[0009]
According to the present invention, a potential noise having a specific amplitude and a specific potential change rate superimposed on the spontaneous immersion potential of carbon steel in contact with aqueous water is measured, and the measured potential noise is passivated. The amount of passing electricity is determined from the time integration of the anodic dissolution current (anode response current) generated at that time, and the scale of the local corrosion sprouting (actual local corrosion sprouting radius) is obtained from the result. It can be evaluated quantitatively. In the following, a method of applying the measured potential noise to another test piece may be referred to as a “potential noise reverse setting method”.
[0010]
That is, when a test piece of carbon steel is immersed in an aqueous chloride solution, as shown in FIG. 1 (a) and FIG. 1 (b) which is an enlarged view of a portion B in FIG. ) A superimposed component (potential noise) is generated in the spontaneous immersion potential in response to sprouting generation and re-passivation. After the test, the surface of the test piece was observed by SEM, and as a result, it was confirmed that the amount of potential noise and the trace of pitting corrosion were almost the same. Further, the anode response current when the potential noise is reversely set to another test piece in which the potential noise is passivated by the reverse setting method of the potential noise of the present invention is as shown in FIG. From the results, the local germination sprouting radius was estimated and found to be almost the same as the actual radius of the pit trace. These results confirm that the potential noise corresponds to the generation and death of local corrosion sprouting.
[0011]
In the present invention, another passivated test piece was passivated by immersing the carbon steel in water after contacting the aqueous water with an anion exchanger carrying anticorrosive anions. Those are preferred.
[0012]
The method for preventing local corrosion of carbon steel according to the present invention (claims 3 and 4) is a method for preventing local corrosion of carbon steel in contact with water-based water. Based on this, the injection amount of the water treatment agent for suppressing pitting corrosion of the carbon steel or the injection amount of the water treatment agent for removing dissolved oxygen in the water system is controlled.
[0013]
Further, according to the present invention (claim 5), in the method for preventing local corrosion of carbon steel coming into contact with water-based water, the deoxygenation device for removing dissolved oxygen in the water-based system based on the above monitoring results. Control the amount.
[0014]
If the scale of the local corrosion sprouting estimated by the potential noise reverse setting method of the present invention is large, and it is determined that local corrosion By adding an appropriate amount of the suppressing agent or by taking an appropriate deoxygenation treatment, it is possible to prevent local corrosion sprouting from growing into progressive local corrosion.
[0015]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the method for monitoring local corrosion of carbon steel and the method for preventing local corrosion of carbon steel according to the present invention will be described in detail.
[0016]
In the method for monitoring local corrosion of carbon steel of the present invention, first, the spontaneous immersion potential of various water-based carbon steels is measured, and the potential noise having a specific amplitude and a specific potential change rate superimposed on the spontaneous immersion potential is measured.
[0017]
The potential noise of the carbon steel is measured with a potential measurement device by placing a test piece made of carbon steel (hereinafter, sometimes referred to as a “test piece for measuring potential noise”) under the same conditions as the water system to be monitored. be able to. At this time, it is preferable that the sampling interval for measuring the generated potential noise be 0.5 seconds or less, and that the voltmeter be used with an accuracy of about 1 μV. The potential noise superimposed on the natural immersion potential is a potential noise whose potential oscillates up and down. For example, a potential noise having an amplitude of 10 mV or more and a potential change speed of 1 mV / sec or more can be captured.
[0018]
In the actual environment to be monitored, it is preferable to operate under a condition in which potential noise does not occur.
[0019]
The potential noise measured in this way is applied to a passivated test piece (hereinafter, sometimes referred to as a “reverse potential setting test piece”). The passivated test piece used here must be completely passivated and have a stable corrosion potential. Therefore, the water of the water system, HCO 3 _^-, OH ^- by contacting with an anion exchanger which corrosion anion carries such, Cl as a cause of pitting corrosion ^-, in water, such as SO ₄ ^2- A test piece that is passivated by immersing a test piece made of the same carbon steel as the test piece for potential noise measurement in water in which corrosive ions are ion-exchanged with anticorrosive anions such as HCO ₃ ⁻ and OH ⁻ is used. Is preferred.
[0020]
Such a potential noise waveform reversely set to the reverse potential setting test piece is obtained, for example, by reproducing potential noise data measured by a potential noise measurement test piece at an interval of 10 msec using a D / A converter whose minimum digit is 25 μV. It can be reproduced by complementing the potential value every 10 msec with the measurement data line at 0.5 second intervals. Then, the reproduced potential noise is applied to a test piece for setting a reverse potential using a potentiostat, and an anode response current obtained at that time is measured. That is, for example, the anode response current output is measured at 0.5 second intervals using a multi-channel A / D converter having 12-bit resolution.
[0021]
Based on the anode response current of the test piece for setting a reverse potential measured in this way, the scale of local corrosion sprouting in an aqueous system can be estimated. That is, for example, since the integral value S obtained by time-integrating the anode response current is related to the radius of the local corrosion sprouting in the actual water system, this relationship is determined in advance, and the integral value S is determined in advance. The estimated radius r of the local corrosion sprouting can be calculated by substituting into the relational expression.
[0022]
In the method for preventing local corrosion of carbon steel according to the present invention, the injection amount of a water treatment agent for suppressing pitting corrosion or a water treatment agent for removing dissolved oxygen added to an aqueous system is based on the above monitoring results. Specifically, the following chemical injection control can be performed.
(1) When the calculated estimated radius of the localized corrosion sprouting is equal to or larger than a preset value, the chemical injection is started, and the calculated estimated radius of the localized corrosion sprouting becomes smaller than the set value or until the calculated value becomes constant. The water treatment agent is continuously or intermittently added for a certain period of time after the calculated estimated radius of local corrosion sprouting becomes less than the set value, or after that, and then the addition of the water treatment agent is stopped.
(2) Regarding the amount of the water treatment chemical to be injected, a first injection amount in a steady state and a second injection amount larger than the first injection amount are set in advance, and the first injection amount in a steady state. Perform the chemical injection continuously or intermittently in the amount, if the calculated estimated radius of local corrosion sprouting is equal to or larger than a preset value, the calculated estimated radius of local corrosion sprouting is less than the set value. Until or for a certain time, or a certain time after the calculated estimated radius of local corrosion sprouting is less than the set value, perform the continuous or intermittent injection at the second injection amount, and then Again or continuously or intermittently at the first dose.
[0023]
In this way, by performing the chemical injection control of the water treatment chemical based on the scale of the local corrosion sprouting obtained by the inverse setting method of the potential noise, it is possible to prevent excess or deficiency of the water treatment chemical and to efficiently perform the chemical treatment. By doing so, local corrosion can be reliably prevented.
[0024]
When controlling the amount of deoxidation of a deoxidizing device such as a degassing film device for removing dissolved oxygen in an aqueous system based on the above monitoring results, specifically, the following operation control is performed. Good.
(1) When the calculated estimated radius of the local corrosion sprouting is equal to or larger than a preset value, the operation of the deoxidizer is started, and the calculated estimated radius of the local corrosion sprouting becomes smaller than the set value. Until a certain time, or for a certain time after the calculated estimated radius of local corrosion sprouting becomes smaller than the set value, the deoxidation treatment is performed, and then the operation of the deoxidation device is stopped.
(2) The first operating conditions (electric power, degree of vacuum, water flow rate, gas flow rate, etc.) in the steady state of the deoxidizer and the second operating conditions having a greater deoxygenation than the first operating conditions are determined in advance. When the deoxidizer is operated under the first operating condition in a steady state, and the calculated estimated radius of the local corrosion sprouting is equal to or larger than a preset value, the calculated estimated radius of the local corrosion sprouting is calculated. Until the value is less than the set value, or for a certain period of time, or for a certain time after the calculated estimated radius of local corrosion sprouting becomes less than the set value, operate under the second operating condition, and thereafter again The operation is performed under the first operation condition.
[0025]
As described above, by controlling the operation of the deoxidizer based on the scale of the local corrosion sprouting obtained by the inverse setting method of the potential noise, it is possible to prevent the overload of the deoxygenator and perform an efficient deoxygenation process. By doing so, local corrosion can be reliably prevented.
[0026]
The water system to be monitored in the present invention includes water systems such as a boiler water system and a cooling water system.
[0027]
【Example】
Hereinafter, the present invention will be described more specifically with reference to examples.
[0028]
Example 1
Local corrosion monitoring of carbon steel was carried out in the actual open-circuit cooling water system shown in FIG.
[0029]
In this cooling water system, the water in the pit 11 of the cooling tower 10 is supplied to the heat exchanger 13 through the water supply pipe 12 and returned to the cooling tower 10 through the return pipe 14. The water treatment chemical in the chemical liquid tank 16 is added to the pit 11 of the cooling tower 10 by the chemical injection pump 17. The injection amount by the injection pump 17 is controlled by the control device 18.
[0030]
As the water treatment chemical, an aqueous solution of a phosphoric acid / zinc anticorrosive, an acrylic acid scale inhibitor, and a non-halogen slime control agent is used.
[0031]
The cooling water sampled from the cooling water supply pipe 12 of the cooling water system via a branch pipe was used as test water, and was supplied to the column 4 through the water supply pump 6 and the constant flow valve 5 at a constant flow rate. The cooling water flow rate in the column 4 was adjusted by the constant flow valve 5 to 0.5 m / sec which is the same as the cooling water flow rate in the actual heat exchanger tube. This column 4 is provided with a carbon steel test piece (cold rolled steel plate (SPCC)) 1 and a reference electrode (KCl saturated silver / silver chloride) 2 for measuring potential noise. 2 is measured by the potential measuring device 3. The potential measuring device 3 measures potential noise having an amplitude of 10 mV or more and a potential change rate of 1 mV / sec or more superimposed on the natural immersion potential. The measured value of the potential noise is input to the reverse potential setting device 8.
[0032]
Separately, similarly, cooling water separated from the water supply pipe 12 via a branch pipe was used as test water, treated with the anion exchange tower 7 by the water supply pump 6A and the constant flow valve 5A, and then passed through the column 4A at a constant flow rate. The flow rate of the cooling water in the column 4A was adjusted to 0.5 m / sec, the same as the flow rate of the cooling water in the actual heat exchanger tube, by the constant flow valve 5A. The column 4A is provided with a carbon steel test piece (cold rolled steel plate (SPCC)) 1A and a reference electrode (KCl saturated silver / silver chloride) 2A for anode response current measurement.
[0033]
In this column 4A, in the anion exchange tower 7, the column is brought into contact with an anion exchanger supporting an anticorrosive anion, and immersed in cooling water after the corrosive anion is exchanged with an anticorrosive anion such as HCO ₃ ⁻ and OH ^−. Then, the potential noise measured by the potential measuring device 3 is reproduced by the reverse potential setting device 8 and applied to the test piece 1A that has been passivated. The anode response current of the test piece 1A when the potential noise was applied was measured, and this value was input to the control device 18 and integrated over time. From this result, the local corrosion sprouting radius was estimated. That is, from the integrated value S, the radius (r) of the local corrosion sprouting is calculated by the following formula [I], and when the estimated radius of the local corrosion sprouting is calculated to be 1 μm or more, the control device 18 sends the chemical injection pump 17 a The control signal of the water treatment chemical injection amount was output.
[0034]
(Equation 1)

[0035]
As shown in FIG. 4, at the beginning of monitoring of this cooling water system, since the injection amount of the water treatment chemical was not proper, potential noise indicating occurrence of local corrosion sprouting was confirmed. Also, as a result of applying the potential noise measured by the potential noise measuring device 3 to another passivated test piece 1A by the reverse potential setting device 8, the generated potential noise is all localized corrosion sprouting having a radius of 1 μm or more. Was found to correspond to the occurrence of Therefore, based on this result, chemical injection control was performed so that the chemical injection amount of the water treatment chemical was twice the current amount until the estimated radius of local corrosion sprouting obtained by the inverse setting method of the potential noise was less than 1 μm. As a result, after about 30 minutes, potential noise corresponding to a radius of 1 μm or more did not occur.
[0036]
After the test, the carbon steel tubes of the actual heat exchanger were examined in detail, and it was confirmed that no corrosion had occurred.
[0037]
Example 2
In the blow water of the boiler shown in FIG. 5, local corrosion monitoring of carbon steel was performed.
[0038]
In FIG. 5, as in FIG. 3, 1 is a test piece for measuring potential noise, 1A is a test piece for measuring anode response current, 2, 2A is a reference electrode (KCl saturated silver / silver chloride), 3 is a potential measuring device, , 4A is a column, 7 is an anion exchange column for removing corrosive anions from water and exchanging them with anticorrosive anions such as HCO ₃ ⁻ , OH ⁻ , and 8 is an anode passivated by potential noise measured by a potential measuring device. A reverse potential setting device to be applied to the response current measurement test piece 1A, 16 is a chemical solution tank, 17 is a chemical injection pump, and 18 is a control device. The configurations of the potential noise measurement system, the reverse potential setting system, and the chemical injection control system constituted by these devices are the same as those in FIG.
[0039]
In FIG. 5, softened water is supplied from a water supply tank 21 to a boiler can 20 through a pipe 22 as boiler water supply. Boiler water is fractionated through a blow-off valve 23 provided in the boiler can 20, and the boiler water is introduced into the columns 4, 4A. The boiler sent from the water supply tank 21 to the boiler can 20 in the same manner as in the first embodiment based on the result obtained by setting the reverse potential of the potential noise measured by the potential measuring device 3 by the reverse potential setting device 8. The supply amount of the water treatment chemical (in this embodiment, an aqueous solution of a deoxidizer) is controlled with respect to the water supply.
[0040]
As shown in FIG. 6, at the beginning of monitoring in this water system, since the amount of the water treatment chemical added was not appropriate, potential noise indicating occurrence of local corrosion sprouting was confirmed. Further, the estimated radius r of the local corrosion sprouting corresponding to the anode response current calculated from the integral value S of the anode response current by the above formula [I] by the reverse potential setting method was 1 μm or more. Therefore, as a result of changing to the chemical injection concentration control in which the water treatment chemical is injected into the boiler feedwater until the estimated radius of local corrosion sprouting corresponding to the anode response current is less than 1 μm, the generation of the potential noise itself disappears after about one hour. However, the occurrence of local corrosion sprouting was suppressed.
[0041]
After the test, the inside of the boiler can was examined in detail, but no occurrence of corrosion was observed.
[0042]
【The invention's effect】
As described in detail above, according to the method for monitoring local corrosion of carbon steel of the present invention, the potential noise superimposed on the spontaneous immersion potential of carbon steel in an aqueous system was measured, and this potential was applied to another passivated test piece. By setting the reverse potential of the noise, it is possible to monitor the scale of the local corrosion sprouting of carbon steel, and it is possible to quickly and effectively corrode the local corrosion sprouting of the scale that reaches the evolving local corrosion. Preventive measures can be taken.
[0043]
In addition, according to the method for preventing local corrosion of carbon steel of the present invention, it is possible to appropriately administer chemical injection by injecting a water treatment agent that suppresses pitting corrosion based on the monitoring result. . In addition, even in the case of performing a process of removing dissolved oxygen, it is possible to perform appropriate operation management of the deoxidizer.
[Brief description of the drawings]
FIG. 1 is a graph showing a potential noise of a natural immersion potential of carbon steel.
FIG. 2 is a graph showing an anode response current when a potential noise is reversely set on a test piece in which passivation is performed.
FIG. 3 is a system diagram showing a test apparatus used in Example 1.
FIG. 4 is a graph showing a change over time of a natural immersion potential in Example 1.
FIG. 5 is a system diagram showing a test apparatus used in Example 2.
FIG. 6 is a graph showing a temporal change of a natural immersion potential in Example 2.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Test piece for potential noise measurement 1A Test piece for anode response current measurement 2, 2A Reference electrode 3 Potential measuring device 4, 4A Column 5, 5A Constant flow valve 6, 6A Water pump 7 Anion exchange tower 8 Reverse potential setting device 10 Cooling Tower 11 Pit 13 Heat exchanger 16 Chemical tank 17 Chemical injection pump 18 Control device 20 Boiler can 21 Water supply tank

Claims

In a method for monitoring local corrosion of carbon steel in contact with water in an aqueous system,
By measuring a potential noise of a specific amplitude and a specific potential change rate superimposed on the natural immersion potential of the carbon steel, and applying the measured potential noise to another passivated test piece, A method for monitoring local corrosion of carbon steel, comprising estimating the scale of local corrosion sprouting of carbon steel.

The test piece according to claim 1, wherein the carbon steel is passivated by immersing the carbon steel in water after contacting the aqueous water with an anion exchanger supporting an anticorrosive anion. Characteristic local corrosion monitoring method for carbon steel.

A method for preventing local corrosion of carbon steel in contact with water-based water,
A carbon steel, wherein a chemical injection amount of a water treatment chemical for suppressing pitting corrosion of the carbon steel is controlled based on a monitoring result of the local corrosion monitoring method for carbon steel according to claim 1 or 2. Local corrosion prevention method.

A method for preventing local corrosion of carbon steel in contact with water-based water,
3. A method for controlling the injection of a water treatment chemical for removing dissolved oxygen in water, based on the monitoring result of the method for monitoring local corrosion of carbon steel according to claim 1 or 2. Local corrosion prevention method.

A method for preventing local corrosion of carbon steel in contact with water-based water,
A method for controlling the amount of deoxidation of a deoxidizing device for removing dissolved oxygen in the water, based on the monitoring result of the local corrosion monitoring method for carbon steel according to claim 1 or 2. Local corrosion prevention method.