JP2008292408A - Temporal evaluation method for crevice corrosion initiation - Google Patents
Temporal evaluation method for crevice corrosion initiation Download PDFInfo
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本発明は、フッ化物イオン、塩化物イオン、臭化物イオンおよびヨウ化物イオンなどのいわゆるハロゲン化物イオンまたは海水に含まれる生化学的成分もしくは水溶性塩類を含む食品・調味料などの水溶液環境を取り扱う機器である、海水淡水化プラントや製塩プラント・食品プラントなどに使用される貯蔵・輸送・製造タンク類、殺菌・滅菌装置類、輸送パイプ、配管類、パッキン類、バルブ類や、海洋鋼構造物防食ライニング材、船体材料などに使用されるステンレス鋼に発生するすきま腐食損傷を、未然に防止あるいは回避もしくは適正材料を選定するための方法であり、使用材料のメンテナンス管理分野で有用な、ステンレス鋼のすきま腐食発生の時間的評価方法に関わる。 The present invention relates to an apparatus for handling an aqueous solution environment such as foods and seasonings containing so-called halide ions such as fluoride ions, chloride ions, bromide ions and iodide ions, or biochemical components contained in seawater or water-soluble salts. Storage / transport / manufacturing tanks used in seawater desalination plants, salt production plants, food plants, etc., sterilization / sterilization equipment, transport pipes, piping, packings, valves, and marine steel structures It is a method to prevent or avoid crevice corrosion damage that occurs in stainless steel used for lining materials, hull materials, etc., or to select appropriate materials, and is useful in the field of maintenance management of materials used. It is related to the time evaluation method of crevice corrosion occurrence.
従来から、ハロゲン化物イオンを含有した海水、塩水に接する船体材料・ゲート材料・海洋鋼構造物防食ライニング材料・海水淡水化プラント・製塩プラントや醤油・みそ・食酢などを製造する食品プラント等の機器類に使用される材料は、塩濃度や温度、ｐＨなどによって、炭素鋼、低合金鋼、ステンレス鋼、Ｎｉ基合金、Ｔｉなどが使い分けられている。特にステンレス鋼を使用した機器類には、配管のつなぎ目や十字継ぎ手のすきま部、溶接欠陥・メニスカス部あるいは付着微生物など、少なからず回避しがたい潜在的なすきま構造を有しており、ハロゲン化物によるすきま腐食などの腐食損傷が懸念され、種々の電気化学的な手法により適正とされる材料の選定がなされているのが一般的である。 Conventionally, equipment such as seawater containing halide ions, hull materials that contact salt water, gate materials, anticorrosion lining materials for marine steel structures, seawater desalination plants, salt production plants, food plants that produce soy sauce, miso, vinegar, etc. As materials used for the above, carbon steel, low alloy steel, stainless steel, Ni-base alloy, Ti and the like are properly used depending on the salt concentration, temperature, pH and the like. In particular, equipment using stainless steel has a latent clearance structure that is difficult to avoid, such as pipe joints, gaps in cross joints, weld defects, meniscus parts, and adhering microorganisms. Corrosion damage such as crevice corrosion due to corrosion is a concern, and materials that are considered appropriate are generally selected by various electrochemical techniques.
すきま腐食を電気化学的に評価する方法として、一般的に知られている方法としては、非特許文献１がある。これは、当該環境で材料の腐食すきま再不動態化電位と自然ポテンシャルを比較対照し、すきま腐食が自発的に発生するかどうかを判定する方法である。しかし、この方法は、すきま腐食が発生しないか発生するかのいずれか一方を知る方法であり、時間的に「いつ頃発生する」のかの知見を得ることができない。したがって、実用的な適正材料選定には、深い経験や知識が必要であり、定量的な評価が難しいことは周知の事実である。 Non-Patent Document 1 is a generally known method for electrochemically evaluating crevice corrosion. This is a method for determining whether crevice corrosion occurs spontaneously by comparing and comparing the corrosion crevice repassivation potential and natural potential of the material in the environment. However, this method is a method for knowing whether crevice corrosion does not occur or not, and it is impossible to obtain knowledge about when it will occur. Therefore, it is a well-known fact that selection of practical appropriate materials requires deep experience and knowledge, and quantitative evaluation is difficult.
特許文献１には、ハロゲン化物イオンを含む液状環境中にすきまを人工的に付与した金属材料を浸漬し、外部の定電位電源により金属材料に一定の電位を印加した際に、材料に流れる電流密度の時間変化を測定することを特徴とするすきま腐食発生時間の評価方法が記載されている。例えば、外部の定電位電源により金属材料に一定の電位を印加した際に、材料に流れる電流密度の値が１０-5Ａ／ｃｍ2に達した時間をその電位におけるすきま腐食発生時間とすることにより、すきま腐食発生時間を評価することができる。 In Patent Document 1, when a metal material artificially imparted with a gap is immersed in a liquid environment containing halide ions, and a constant potential is applied to the metal material by an external constant potential power source, a current flowing through the material A method for evaluating the crevice corrosion occurrence time characterized by measuring a change in density with time is described. For example, when a constant potential is applied to a metal material by an external constant potential power source, the time when the value of the current density flowing through the material reaches 10 −5 A / cm 2 is the crevice corrosion occurrence time at that potential. Thus, the crevice corrosion occurrence time can be evaluated.
特許文献１に記載の方法では、確かに近似的なすきま腐食発生時間が測定される。ところで、特許文献１に記載の評価の途中で、試験片を取り出し、人工的に付与したすきま部におけるすきま腐食発生の有無を光学顕微鏡観察によって直接確認することにより、その時点ですきま腐食が発生しているか否かを直接把握することができる。このように試験途中で直接光学顕微鏡で観察する方法から求まるすきま腐食発生時間の値と、特許文献１に記載の評価方法で求まるすきま腐食発生時間を対比すると、両者は異なる場合があった。 In the method described in Patent Document 1, an approximate crevice corrosion occurrence time is surely measured. By the way, during the evaluation described in Patent Document 1, the test piece is taken out, and crevice corrosion occurs at that point by directly checking the occurrence of crevice corrosion in the artificially provided crevice portion by optical microscope observation. It is possible to grasp directly whether or not. Thus, when the value of the crevice corrosion occurrence time obtained from the method of directly observing with an optical microscope during the test is compared with the crevice corrosion occurrence time obtained by the evaluation method described in Patent Document 1, the two cases may be different.
一方、評価試験の途中で試験片を取り出し、直接光学顕微鏡ですきま腐食発生の有無を観察する方法では、試験片を取り出し、観察のためにすきま部を開いたこと等によって、すきま部の環境がリセットされてしまうので、この時点で試験を終了せざるを得ず、多数の試験を繰り返す必要があり、簡便な評価方法ということはできない。 On the other hand, in the method of taking out a test piece in the middle of an evaluation test and directly observing the presence or absence of crevice corrosion with an optical microscope, the environment of the crevice part is reduced by taking out the test piece and opening the crevice part for observation. Since it is reset, it is necessary to end the test at this point, and it is necessary to repeat a large number of tests, and it cannot be a simple evaluation method.
本発明は、上記のような従来の電気化学的すきま腐食評価法でなし得なかった、ステンレス鋼のすきま腐食の発生がいつ起きたのかを、より正確にかつ、前記光学顕微鏡観察によるすきま腐食発生の直接的な確認方法よりも、簡便にかつ、試験を中断せずに測定する方法に関するものであって、ハロゲン化物イオンまたは海水に含まれる生化学的成分もしくは水溶性塩類を含む水溶液環境中で使用される機器類に発生するすきま腐食を未然に防止もしくは、回避または使用環境に適正なステンレス鋼を選定するための、ステンレス鋼のすきま腐食発生の時間的評価方法を提供しようとするものである。 The present invention provides a more accurate and accurate observation of crevice corrosion when the occurrence of crevice corrosion of stainless steel, which could not be achieved by the conventional electrochemical crevice corrosion evaluation method as described above, by observation with the optical microscope. This method relates to a measurement method that is simpler than the direct confirmation method and without interrupting the test, and in an aqueous solution environment containing halide ions or biochemical components or water-soluble salts contained in seawater. It is intended to provide a time evaluation method of crevice corrosion occurrence in stainless steel to prevent or avoid crevice corrosion occurring in the equipment used, or to select stainless steel suitable for the usage environment. .
本発明者らは、典型的なハロゲン化物イオン環境として、自然界に多量に存在する河川水や海水を選び、この中でのステンレス鋼の自然電位測定を行った。また一方で、ステンレス鋼／ステンレス鋼すきまを有するすきま付き試験片およびすきま部がない自由表面のみの表面をもつすきまなし試験片を用いて、それぞれ個別に種々の電位に保持し、その時、両試験片に流れる電流密度の時間的変化を測定した。ここで、自由表面とは、ステンレス鋼表面から０．５ｍｍ以上の間隔を有する自由空間が存在している該ステンレス鋼表面を意味する。また、ここで、電流密度とは、試験片をある電位に置いた際に、試験片全体とその外部との間に生じるマクロ的な腐食電流を試験片の自由表面の面積で除した値をいう。そして、すきま付き試験片とすきまなし試験片の両者の電流密度差分値の時間的変化からステンレス鋼のすきま腐食が生起するまでの時間を検知することができることを見出し、本発明を完成したものであって、その要旨とするところは以下の通りである。 The present inventors selected river water and seawater that exist in a large amount in nature as a typical halide ion environment, and measured the natural potential of stainless steel in this. On the other hand, using a test piece with a clearance having a stainless steel / stainless steel clearance and a test piece without a clearance having only a free surface with no clearance, each test is held at various potentials. The temporal change of the current density flowing through the piece was measured. Here, the free surface means the stainless steel surface where a free space having a distance of 0.5 mm or more from the stainless steel surface exists. Here, the current density is a value obtained by dividing the macroscopic corrosion current generated between the whole test piece and the outside when the test piece is placed at a certain potential by the area of the free surface of the test piece. Say. And, it was found that it is possible to detect the time until the crevice corrosion of stainless steel occurs from the temporal change of the current density difference value of both the specimen with a gap and the specimen without a gap, and the present invention has been completed. The gist of this is as follows.
（１）ハロゲン化物イオンまたは海水に含まれる生化学的成分もしくは水溶性塩類を含む水溶液環境中に、ステンレス鋼の表面どうしを向かい合わせて近づけ、互いに、その表面の一部を接触させた状態で形成される、間隔０．５ｍｍ未満のすきま部を設けた、すきま付き試験片と、前記ステンレス鋼の自由表面のみからなる、すきまなし試験片をそれぞれ同一の前記水溶液環境中に浸漬し、外部電源によって、それぞれの試験片に同一の電位を印加した時点から、それぞれの試験片に生じる電流値を、それぞれの試験片の前記すきま部に面する表面を除いた自由表面の面積で除して求まる電流密度の時間的変化を測定し、前記すきま付き試験片の電流密度から前記すきまなし試験片の電流密度を引いて求めた電流密度の差分値の時間的変化から、すきま腐食発生時間を決定することを特徴とする、ステンレス鋼のすきま腐食発生の時間的評価方法。ここで自由表面とは、ステンレス鋼表面から０．５ｍｍ以上の間隔を有する自由空間が存在している該ステンレス鋼表面をいう。
（２）さらに、外部電源により、前記すきま付き試験片と前記すきまなし試験片に同一の電位を印加した時点から、前記電流密度差分値の時間的変化を計測して、前記電圧を印加開始した時点から、前記電流密度差分値が連続的に０を超える状態になった時点までの時間を、前記ステンレス鋼の前記水溶液環境中におけるすきま腐食発生時間とすることを特徴とする、前記（１）に記載の、ステンレス鋼のすきま腐食発生の時間的評価方法。
（３）さらに、試験片に印加する前記電圧を一定に保つことを特徴とする前記（１）または（２）に記載のステンレス鋼のすきま腐食発生の時間的評価方法。
(1) In an aqueous solution environment containing halide ions or biochemical components or water-soluble salts contained in seawater, the surfaces of stainless steel are brought close to each other and part of the surfaces are in contact with each other. A formed test piece with a gap provided with a gap of less than 0.5 mm and a non-gap test piece made of only the free surface of the stainless steel are immersed in the same aqueous solution environment, From the point in time when the same potential is applied to each test piece, the current value generated in each test piece is divided by the area of the free surface excluding the surface facing the gap of each test piece. Measure the change in current density over time, and change the difference in current density over time by subtracting the current density of the test piece without gap from the current density of the test piece with gap. Et al., And determining the crevice corrosion occurrence time, time evaluation method of crevice corrosion generation of stainless steel. Here, the free surface refers to the stainless steel surface where a free space having a distance of 0.5 mm or more from the stainless steel surface exists.
(2) Further, from the time when the same potential was applied to the test piece with the gap and the test piece without the gap by an external power source, the temporal change of the current density difference value was measured, and the application of the voltage was started. The time from the time point to the time point when the current density difference value continuously exceeds 0 is defined as the crevice corrosion occurrence time in the aqueous solution environment of the stainless steel (1) The time evaluation method of crevice corrosion occurrence of stainless steel described in 1.
(3) The temporal evaluation method for crevice corrosion occurrence of stainless steel according to (1) or (2), wherein the voltage applied to the test piece is kept constant.
本発明のステンレス鋼のすきま腐食発生の時間的評価方法により、ステンレス鋼のすきま腐食が発生するまでの時間を従来の方法よりも、より真の値に近く決定でき、ハロゲン化物イオンまたは海水に含まれる生化学的成分もしくは水溶性塩類を含む水溶液環境中でのステンレス鋼の選定が適正に行える。 The time evaluation method for crevice corrosion of stainless steel of the present invention can determine the time until crevice corrosion of stainless steel occurs closer to the true value than the conventional method, and it is contained in halide ions or seawater. Stainless steel can be selected properly in an aqueous solution environment containing biochemical components or water-soluble salts.
まず発明者らは、ハロゲン化物イオン環境の代表である海水系環境で生じるステンレス鋼のすきま腐食現象の詳細を検討した。すなわち、種々の塩化物イオン濃度の希釈海水中に、すきま付き試験片ならびにすきまなし試験片を浸漬し、ポテンショスタットを用いて定電位電解を行い、電流密度／時間曲線の測定を行った。 First, the inventors examined the details of the crevice corrosion phenomenon of stainless steel that occurs in a seawater system environment that is representative of a halide ion environment. That is, a test piece with a gap and a test piece without a gap were immersed in diluted seawater with various chloride ion concentrations, and constant potential electrolysis was performed using a potentiostat, and a current density / time curve was measured.
評価装置の構成は、非特許文献１に記載の構成を基本とし、これに下記に記載する改良を加えた。 The configuration of the evaluation apparatus was based on the configuration described in Non-Patent Document 1, and the improvements described below were added thereto.
すきま付き試験片は図１に示すように、２０w×２０l×２〜４tｍｍ及び２０w×５０l×２〜４tｍｍの寸法の試験片を１組として、これらの全面を湿式研磨 （４００番）し、５０℃の３０％硝酸溶液中に１時間浸漬し、不動態化処理を施した。次いで、２０w×５０lｍｍの試験片の上端にリード線を固定し、電流／時間曲線の測定直前に２０w×５０lｍｍ試験片の主面のうち片面のみ、２０w×２０lｍｍ試験片の主面の両面を再度、湿式研磨し、試験溶液を湿式研磨面に塗布した状態で図１のように、湿式研磨面どうしを向かい合わせて樹脂ボルト締めで組み立てたのち、電流密度／時間曲線の測定に供した。湿式研磨面どうしを向かい合わせて樹脂ボルト締めで組み立てることにより、表面の一部を接触させた状態で形成される、間隔０．５ｍｍ未満のすきま部を設けることができる。対極として白金板電極を用い、試験片の電位測定および電位制御の際の基準として照合電極を用いる。ここでは、便宜上、照合電極としてＡｇ／ＡｇＣｌ（ＫＣｌ ｓａｔ．）電極を用いた。 As shown in FIG. 1, the test pieces with a clearance are wetted on the entire surface of a set of test pieces having dimensions of 20 w × 20 l × 2 to 4 t mm and 20 w × 50 l × 2 to 4 t mm. Polishing (No. 400) and dipping in a 30% nitric acid solution at 50 ° C. for 1 hour were performed for passivation treatment. Then, 20 w × 50 l and an upper end fixed to the lead wire to the specimens mm, only one side of the main surface of 20 w × 50 l mm test piece immediately before the measurement of the current / time curve, 20 w × 20 l mm After wet-polishing both sides of the main surface of the test piece and applying the test solution to the wet-polished surface, as shown in FIG. 1, the wet-polished surfaces face each other and assembled by resin bolting, and then the current density / It used for the measurement of a time curve. By assembling the wet-polished surfaces facing each other by fastening with a resin bolt, it is possible to provide a gap portion with a distance of less than 0.5 mm formed with a part of the surface in contact. A platinum plate electrode is used as a counter electrode, and a reference electrode is used as a reference for potential measurement and potential control of the test piece. Here, for convenience, an Ag / AgCl (KCl sat.) Electrode was used as a reference electrode.
樹脂ボルト締めで組み立てたすきま付き試料片を対極とともに、試験溶液を満たした試験セルに浸漬し、照合電極を組み込んだ。その後直ちに、試験片の照合電極基準の自然浸漬電位を約１時間にわたり測定した。なお、前記湿式研摩から３０分以内に自然浸漬電位の測定を開始している。一般に表面を湿式研摩してからステンレス鋼を中性水溶液等に浸漬すると、時間と共に不働態皮膜が成長し、それにともなって試験片の自然浸漬電位がゆるやかに上昇してゆく。そこで、試験片の自然浸漬電位が一定の値に達した段階で、種々の電位にて定電位電解を行い電流密度／時間曲線を得ることが好ましい。試験開始時の試験片表面の不働態皮膜の状態を一定にそろえることで、再現性が高く、互いに有意な比較評価が行える結果が得られるからである。なお、便宜上、試験片の自然浸漬電位が−２００ｍＶ ｖｓ． Ａｇ／ＡｇＣｌ（ＫＣｌ ｓａｔ．）になった時点で、外部から試験片に所定の電位を印加して試験を開始すると良い。ステンレス鋼表面の不働態皮膜の状態が安定し、試験片間で試験開始条件をそろえ易く、実験作業の時間的制約への影響が小さいからである。 A sample piece with a gap assembled by resin bolting was immersed in a test cell filled with a test solution together with a counter electrode, and a reference electrode was incorporated. Immediately thereafter, the natural immersion potential of the test piece with reference electrode was measured for about 1 hour. The measurement of the natural immersion potential is started within 30 minutes after the wet polishing. In general, when stainless steel is immersed in a neutral aqueous solution after the surface is wet-polished, a passive film grows with time, and the natural immersion potential of the test piece gradually increases. Therefore, it is preferable to obtain a current density / time curve by performing constant potential electrolysis at various potentials when the natural immersion potential of the test piece reaches a certain value. This is because by aligning the state of the passive film on the surface of the test piece at the start of the test, a result can be obtained in which reproducibility is high and significant comparative evaluation can be performed. For convenience, the natural immersion potential of the test piece is −200 mV vs. When Ag / AgCl (KCl sat.) Is reached, the test may be started by applying a predetermined potential to the test piece from the outside. This is because the state of the passive film on the surface of the stainless steel is stable, the test start conditions are easily aligned between the test pieces, and the influence on the time constraint of the experimental work is small.
また、同様な試験をすきまなし試験片についても、主面のうち片面のみ再度、湿式研磨行なったもので実施した。このようにして試験片を準備した理由は、着目している試験面に腐食が優先的に生じる試験となるように、試験面を試験直前に湿式研磨してその表面の不働態皮膜を一旦除去し、試験時には、再生した薄い不働態皮膜が試験面に存在する状態にすることができるからである。すきまなし試験片においても、すきま付き試験片の試験開始の際と同じ自然浸漬電位に達してから、試験片に所定の電位を印加して試験開始した。これにより、試験開始時における表面の不働態皮膜の状態をすきま付き試験片と、すきまなし試験片間でほぼ同じにすることができるため、再現性が高く、互いに有意な比較評価ができる結果が得られる。このようにして準備したすきま付き試験片とすきまなし試験片の初期電流密度差分値は、ほぼ０になるので、その差分値の試験初期値を０にキャリブレーションし、その差分値の時間的変化を評価した。 In addition, the same test was performed on a non-gap test piece with only one of the main surfaces subjected to wet polishing again. The reason for preparing the test piece in this way is that the test surface is wet-polished immediately before the test and the passive film on the surface is temporarily removed so that the test surface where corrosion is preferentially corroded. In the test, the regenerated thin passive film can be present on the test surface. In the test piece without a gap, the test was started by applying a predetermined potential to the test piece after reaching the same natural immersion potential as that at the start of the test of the test piece with a gap. As a result, the state of the passive film on the surface at the start of the test can be made substantially the same between the test piece with the gap and the test piece without the gap, so that the results are highly reproducible and can be compared with each other significantly. can get. Since the initial current density difference value between the test piece with clearance and the test piece without clearance prepared in this way is almost 0, the initial test value of the difference value is calibrated to 0, and the time-dependent change of the difference value Evaluated.
次に、図２および図３に一例として１／１００海水（塩化物イオン濃度：１９０ｐｐｍ）中でＳＵＳ３０４ステンレス鋼を用いて電流密度／時間曲線を測定した結果をすきまなし試験片及びすきま付き試験片について示す。ここで、試験片にすきま部がない場合（すきまなし試験片）は、ステンレス鋼の不働態皮膜の安定化にともない、時間の対数値に対して電流密度の対数値は直線的に低下するのに対し、すきま付き試験片の場合には、ある時間経過すると電流密度が時間とともに増大するようになる。この電流密度の時間的変化の差が、すきま部があることによって引き起こされる腐食損傷、すなわち、すきま部における前記ステンレス鋼の腐食発生によるもので、ステンレス鋼の連続的な溶解にともなう電流増加である。この場合、すきま腐食が実際に起きる時間（すなわち、すきま腐食発生時間）は、図４に示すように、すきま付き試験片に流れる電流密度／時間曲線（ａ）から、すきまなし試験片に流れる電流密度／時間曲線（ｂ）を差し引くことにより得ることができる。ここで、定電位試験初期からすきま腐食発生直前までの電流密度の差分が図４に示されていないが、これは、すきま付き試験片に流れる電流密度と、すきまなし試験片に流れる電流密度とが、すきま腐食発生前では近似的に等しいため、その差分値が０以下の負の値を示した数値は全て０とみなしていることによる。すなわち、（ａ）−（ｂ）において、継続的に正の値を示す初期値をすきま腐食発生時間：ｔINCUとした。このようにして例えば、４４０ｍＶ ｖｓ． Ａｇ／ＡｇＣｌ（ＫＣｌ ｓａｔ．）の電位における種々の塩化物イオン濃度環境中での各種ステンレス鋼のすきま腐食発生時間を測定した結果を図５に示した。海水中の塩化物イオン濃度が高いほど各種ステンレス鋼のすきま腐食発生時間が短縮することや、耐すきま腐食性指標値ＣＩが高いステンレス鋼ほどすきま腐食発生時間が長くなることを具体的に示すことができた。 Next, as an example in FIG. 2 and FIG. 3, the results of measuring the current density / time curve using SUS304 stainless steel in 1/100 seawater (chloride ion concentration: 190 ppm) are shown as a non-gap test piece and a test piece with a gap. Show about. Here, when there is no gap in the specimen (no gap specimen), the logarithmic value of the current density decreases linearly with the logarithmic value of time as the passive film of stainless steel stabilizes. On the other hand, in the case of a test piece with a gap, the current density increases with time after a certain period of time. This difference in the temporal change in current density is due to the corrosion damage caused by the presence of the gap, that is, due to the occurrence of corrosion of the stainless steel in the gap, which is an increase in current due to continuous melting of the stainless steel. . In this case, the time when crevice corrosion actually occurs (that is, crevice corrosion occurrence time) is the current flowing through the creviceless specimen from the current density / time curve (a) flowing through the crevice specimen as shown in FIG. It can be obtained by subtracting the density / time curve (b). Here, the difference in the current density from the beginning of the constant potential test to immediately before the occurrence of crevice corrosion is not shown in FIG. 4, but this is the difference between the current density flowing in the test piece with the gap and the current density flowing in the test piece without the gap. However, since the crevice corrosion is approximately equal before the occurrence of crevice corrosion, all the numerical values showing a negative value of 0 or less are regarded as 0. That is, in (a)-(b), the initial value continuously showing a positive value was defined as crevice corrosion occurrence time: tINCU . Thus, for example, 440 mV vs.. The results of measurement of crevice corrosion occurrence time of various stainless steels in various chloride ion concentration environments at the potential of Ag / AgCl (KCl sat.) Are shown in FIG. Specifically, the higher the chloride ion concentration in sea water, the shorter the crevice corrosion occurrence time of various stainless steels, and the longer the crevice corrosion occurrence time the higher the crevice corrosion resistance index value CI stainless steel. I was able to.
発明者らは、上記のような電気化学的手法を用いてすきま腐食発生時間：ｔINCUの評価が、海水環境の他に醤油環境や食酢環境、含ハロゲン化物イオン環境にも広く適用できることを見出し、鋭意努力の結果、本発明を完成させた。 The inventors have found that the evaluation of crevice corrosion occurrence time: t INCU using the electrochemical method as described above can be widely applied to soy sauce environment, vinegar environment, and halide ion environment in addition to seawater environment. As a result of diligent efforts, the present invention has been completed.
以下に本発明の構成要件の限定理由を述べる。 The reasons for limiting the constituent requirements of the present invention will be described below.
本発明において、「ステンレス鋼の表面どうしを向かい合わせて近づけ、互いに、その表面の一部を接触させた状態で形成される、間隔０．５ｍｍ未満のすきま部を設けた」理由は、間隔０．５ｍｍ以上のすきまでは、すきま腐食特有のすきま部における各種イオンの濃縮が生じにくく、すきま腐食現象を再現性良く発生させることが難しいからである。 In the present invention, the reason why “the gaps with a distance of less than 0.5 mm formed with the surfaces of the stainless steel facing each other and with parts of the surfaces in contact with each other is provided” is that the distance is 0 When the clearance is 5 mm or more, it is difficult for various ions to concentrate in the clearance part peculiar to crevice corrosion, and it is difficult to generate the crevice corrosion phenomenon with good reproducibility.
また、「すきま付き試験片と、すきまなし試験片のそれぞれの試験片に生じる電流密度の時間的変化を測定し、それらの電流密度の差分値の時間的変化から、すきま腐食発生時間を決定する」としたのは、以下の理由による。 Also, “Measure the temporal change in the current density generated in each of the specimens with and without the gap, and determine the crevice corrosion occurrence time from the temporal change in the difference between the current densities. The reason is as follows.
定電位法では基本的に材料に流れる電流密度の経時変化を測定する。この場合、すきま腐食がステンレス鋼に生じたか、生じていないかは、電流密度が時間的に増加したか減少したかで判断される。すなわち、一般に、ステンレス鋼の使用環境においては、その自由表面に孔食が発生しない限りは、不働態皮膜が成長し、電流密度が低下してゆく。一方、すきま部には各種イオンが濃縮してゆき、腐食性の強い環境へと変化する。やがて腐食がすきま部に発生し、その腐食による溶解反応にともない、電流密度が上昇し始める。我々は、種々の電位において定電位試験を膨大な数繰り返し、すきま付き試験片において、すきま腐食が発生する前までの電流密度／時間曲線は、すきまなし試験片で得られる電流密度／時間曲線にほぼ等しいとの見解に達した。これより、これら２つの電流／時間曲線の差分値をとると、より精度の高いステンレス鋼のすきま腐食発生時間：ｔINCUが決定できることを見出し、本発明にいたったものである。 In the constant potential method, basically, the change with time of the current density flowing in the material is measured. In this case, whether or not crevice corrosion has occurred in stainless steel is determined by whether the current density has increased or decreased over time. That is, in general, in the environment where stainless steel is used, the passive film grows and the current density decreases unless pitting corrosion occurs on the free surface. On the other hand, various ions are concentrated in the gap and change to a highly corrosive environment. Eventually, corrosion occurs in the crevice, and the current density starts to increase with the dissolution reaction due to the corrosion. We have repeated a number of constant potential tests at various potentials, and the current density / time curve before the occurrence of crevice corrosion in the specimen with crevice is the current density / time curve obtained with the specimen without crevice. Reached the view that they were almost equal. From this, it has been found that taking the difference value between these two current / time curves, it is possible to determine the crevice corrosion occurrence time: t INCU of stainless steel with higher accuracy, and the present invention has been achieved .
本発明はまた、上記発明において、外部電源により、すきま付き試験片とすきまなし試験片に同一の電位を印加し、電流密度差分値の時間的変化を計測するに際し、電圧を印加開始した時点から、電流密度差分値が連続的に０を超える状態になった時点までの時間を、ステンレス鋼の前記水溶液環境中におけるすきま腐食発生時間とすると好ましい。すきま付き試験片にすきま腐食が発生する以前においては、すきま付き試験片とすきまなし試験片の電流密度差分値は、試験初期においてほぼ０になるので、電圧印加直後の試験開始時の電流密度差分値を０にキャリブレーションしておく。試験を開始してから時間が経過すると、すきま部の試験片表面の一部において、不働態皮膜が破壊されてはすぐに、再不働態化が生じる現象が起き始め、瞬間的に腐食電流が生じては直ちに収まる状態が断続的に現れ、電流密度差分値が断続的に０を超える傾向を示す場合が多い。一方、すきま付き試験片にすきま腐食が発生し成長に移った段階以降は、すきま付き試験片の電流密度が確実に増大し、電流密度差分値が連続的に０を超える状態となるので、その状態になった時間をすきま腐食発生時間として決定することができる。 In the above invention, the present invention is the above invention, wherein the same potential is applied to the test piece with a gap and the test piece without a gap by an external power source, and the time change of the current density difference value is measured from the time when voltage application is started. The time until the time when the current density difference value continuously exceeds 0 is preferably the crevice corrosion occurrence time in the aqueous solution environment of stainless steel. Before crevice corrosion occurs on the specimen with crevice, the difference in current density between the specimen with crevice and the specimen without crevice is almost zero at the beginning of the test. Calibrate the value to 0. As time elapses from the start of the test, as soon as the passive film is destroyed on a part of the surface of the test piece in the gap, a phenomenon that causes repassivation starts and instantaneous corrosion current is generated. In many cases, the state immediately falls within, and the current density difference value tends to intermittently exceed 0. On the other hand, after the stage where crevice corrosion occurs in the crevice test piece and it starts to grow, the current density of the crevice test piece surely increases and the current density difference value continuously exceeds 0. The time when the condition is reached can be determined as the crevice corrosion occurrence time.
なお、電流密度差分値が連続的に０を超える状態となったと判断した直後に、試験片を試験液から引き上げ、すきま付き試験片のすきま部を開いて、すきま部に面していた表面を光学顕微鏡で観察すると、必ず直径約５μｍ以上の孔食が１個以上発生していることが確認でき、実際のすきま腐食発生時間を精度良くとらえていると考えられる。 Immediately after it was determined that the current density difference value continuously exceeded 0, the test piece was pulled up from the test solution, and the clearance portion of the test piece with the clearance was opened. When observed with an optical microscope, it can be confirmed that at least one pitting corrosion having a diameter of about 5 μm or more has occurred, and the actual crevice corrosion occurrence time is considered to be accurately captured.
本発明では、すきま付き試験片とすきまなし試験片の両者の照合電極基準の電位が同じである限り、試験中に試験片の電位を適宜変化させる試験を含み、その際も両者の電流密度差分値ですきま腐食発生を検知できるが、さらに、試験片に印加する照合電極基準の電圧を一定に保つこととすると、その結果の解釈や試験どうしの比較がし易くなるため、好ましい。 In the present invention, as long as the reference electrode reference potential of both the test piece with the gap and the test piece without the gap are the same, the test includes changing the potential of the test piece as appropriate during the test. It is possible to detect the occurrence of crevice corrosion by the value, but it is also preferable to keep the reference electrode reference voltage applied to the test piece constant because it is easy to interpret the results and compare the tests.
以下に、実施例に基づいて本発明を説明する。 Hereinafter, the present invention will be described based on examples.
表１は供試材に用いたステンレス鋼の主な化学組成（質量％）である。また、表２は海水を１／１００に薄めた溶液を５０℃に保って試験溶液とし、この試験溶液に試験片を浸漬してステンレス鋼の耐すきま腐食性の評価を行った例であるが、従来法（特許文献１記載の方法）で測定を行ったすきま腐食発生時間と本発明の方法で測定したすきま腐食発生時間の比較結果である。印加電位については、表２に示す１５０ｍＶ〜７００ｍＶの一定電位とした。従来法による評価値は本発明法の評価値に比べて非常に長い。 Table 1 shows the main chemical composition (mass%) of the stainless steel used for the test material. Table 2 shows an example in which a solution obtained by diluting seawater to 1/100 was kept at 50 ° C. to make a test solution, and a test piece was immersed in this test solution to evaluate crevice corrosion resistance of stainless steel. 3 is a comparison result of the crevice corrosion occurrence time measured by the conventional method (the method described in Patent Document 1) and the crevice corrosion occurrence time measured by the method of the present invention. The applied potential was a constant potential of 150 mV to 700 mV shown in Table 2. The evaluation value according to the conventional method is much longer than the evaluation value according to the method of the present invention.
表２において、本発明法と従来法のいずれの試験においても、それぞれの試験方法ですきま腐食発生を検知後、直ちにすきま付き試験片を引き上げ、すきま部を開いて、すきま部表面を光学顕微鏡観察した。本発明方法によってすきま腐食発生を検知した本発明例では、いずれも、直径約５〜２０μｍの孔食が１〜３個発生していることが確認できた。すなわち、本発明方法では、すきま発生を早期に確実に検知することができた。一方、従来法では、すきま腐食発生を検知した後、直径１０〜２００μｍの孔食が３個以上数十個発生していることが確認できたが、試験片によって孔食の大きさまたは個数に差が生じ、本発明方法におおよそ匹敵する感度で検知できる場合は、ＳＵＳ３０４で電圧５００ｍＶで試験した１例のみあった。それ以外の従来法では、検知までに時間がかかり、その間に実際の腐食が進行していた。したがって、本発明法の方がすきま発生時間が真の値に近く検知でき、信頼性高く、評価できることがわかった。 In Table 2, in both the test of the present invention method and the conventional method, after detecting the occurrence of crevice corrosion in each test method, immediately pull up the test piece with a clearance, open the clearance, and observe the surface of the clearance with an optical microscope. did. In the examples of the present invention in which crevice corrosion was detected by the method of the present invention, it was confirmed that 1 to 3 pitting corrosion having a diameter of about 5 to 20 μm was generated. That is, according to the method of the present invention, it was possible to reliably detect the occurrence of a gap at an early stage. On the other hand, in the conventional method, after detecting the occurrence of crevice corrosion, it was confirmed that 3 to several tens of pitting corrosion having a diameter of 10 to 200 μm was generated. There was only one example tested with SUS304 at a voltage of 500 mV if a difference occurred and could be detected with a sensitivity roughly comparable to the method of the present invention. In other conventional methods, it took time until detection, and actual corrosion progressed during that time. Therefore, it was found that the method according to the present invention can detect the gap occurrence time closer to a true value, and can be evaluated with high reliability.
このようにして、従来法よりも、さらに確度高く、すきま腐食が発生するまでの時間を決定できるようになった。 In this way, the time until crevice corrosion can be determined with higher accuracy than the conventional method.
Claims (3)
ここで自由表面とは、ステンレス鋼表面から０．５ｍｍ以上の間隔を有する自由空間が存在している該ステンレス鋼表面をいう。 It is formed with the surface of stainless steel facing each other in an aqueous solution environment containing halide ions or biochemical components or water-soluble salts contained in seawater, and a part of the surface is in contact with each other. In addition, a test piece with a gap provided with a gap of less than 0.5 mm and a test piece without a gap consisting of only the free surface of the stainless steel were immersed in the aqueous solution environment, and each test was performed by an external power source. The time of the current density obtained by dividing the current value generated in each test piece from the time when the same potential is applied to the piece by the area of the free surface excluding the surface facing the gap of each test piece. From the change over time in the difference value of the current density obtained by measuring the change and subtracting the current density of the test piece without the gap from the current density of the test piece with the gap. And determining the eating time of occurrence, the time evaluation method of crevice corrosion generation of stainless steel.
Here, the free surface refers to the stainless steel surface where a free space having a distance of 0.5 mm or more from the stainless steel surface exists.
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| JP2011102790A (en) * | 2009-10-16 | 2011-05-26 | Jfe Steel Corp | Method for speedily evaluating corrosion resistance to contents of can molding |
| CN104321636A (en) * | 2012-05-14 | 2015-01-28 | 梅西耶-布加蒂-道提公司 | Analysis method for quantifying a level of cleanliness of a surface of a part |
| JP2015072250A (en) * | 2013-09-06 | 2015-04-16 | 国立大学法人九州大学 | Corrosion sensor and method of manufacturing the same |
| US9030204B2 (en) | 2011-07-07 | 2015-05-12 | Seiko Epson Corporation | Sensor device |
| JP2015225037A (en) * | 2014-05-29 | 2015-12-14 | 新日鐵住金ステンレス株式会社 | Gap corrosion test method |
| CN110346274A (en) * | 2019-07-05 | 2019-10-18 | 南京钢铁股份有限公司 | A kind of corrosion proof electrochemistry experiment method of test polar region marine steel |
| CN115876681A (en) * | 2023-03-01 | 2023-03-31 | 中南大学 | Safety degree evaluation method and test device for sealing gasket |
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Cited By (7)
| Publication number | Priority date | Publication date | Assignee | Title |
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| JP2011102790A (en) * | 2009-10-16 | 2011-05-26 | Jfe Steel Corp | Method for speedily evaluating corrosion resistance to contents of can molding |
| US9030204B2 (en) | 2011-07-07 | 2015-05-12 | Seiko Epson Corporation | Sensor device |
| CN104321636A (en) * | 2012-05-14 | 2015-01-28 | 梅西耶-布加蒂-道提公司 | Analysis method for quantifying a level of cleanliness of a surface of a part |
| JP2015072250A (en) * | 2013-09-06 | 2015-04-16 | 国立大学法人九州大学 | Corrosion sensor and method of manufacturing the same |
| JP2015225037A (en) * | 2014-05-29 | 2015-12-14 | 新日鐵住金ステンレス株式会社 | Gap corrosion test method |
| CN110346274A (en) * | 2019-07-05 | 2019-10-18 | 南京钢铁股份有限公司 | A kind of corrosion proof electrochemistry experiment method of test polar region marine steel |
| CN115876681A (en) * | 2023-03-01 | 2023-03-31 | 中南大学 | Safety degree evaluation method and test device for sealing gasket |
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