JP5206386B2 - Corrosion promotion test method and corrosion amount prediction method for organic coated steel for civil engineering - Google Patents

Description

  The present invention evaluates the corrosion resistance of organic coated steel materials (particularly organic coated steel sheet piles and organic coated steel sheet piles) used under extremely severe corrosive conditions such as rivers and oceans, and the future corrosion amount of steel materials. The present invention relates to a corrosion acceleration test method and a corrosion amount prediction method for organic-coated steel materials for civil engineering.

  Currently, in Japan's harbor steel structures, organic coated steel materials obtained by coating a steel material with urethane elastomer, polyethylene, or an epoxy-based ultra-thick film resin are used to ensure the corrosion resistance of the steel material. These organically coated steel materials for civil engineering are generally used in combination with cathodic protection in order to suppress the corrosion protection cost. That is, the idea is that the corrosion protection of steel materials in the sea is secured by electrocorrosion, and the corrosion protection of steel materials in the tidal zone, splash zone, and marine atmosphere is secured by organic coating. In any case, the anticorrosion performance is expected to be maintained for about 20 to 50 years in the extremely severe corrosive environment such as the marine environment due to the nature of being used for harbor facilities.

  With regard to cathodic protection, many years of research have revealed the mass of the sacrificial anode required per unit area of the steel material to be protected, and the sacrificial anode required for anticorrosion for a predetermined period of time by using a predetermined formula. It is possible to design the mass of

  On the other hand, although various corrosion acceleration tests have been proposed for organic coatings that ensure corrosion protection of steel structures in the tidal zone, splash zone, and marine atmosphere, the superiority or inferiority between various materials has been proposed (for example, Patent Documents 1 and 2). However, there is no test method to predict the durable life of the coated steel material while reproducing and promoting the actual marine environment from the viewpoint of the corrosion degradation mechanism, so there is no choice but to rely on verification in long-term exposure tests in the ocean. It was.

  Incidentally, the technique described in Patent Document 1 is a process in which salt water is sprayed on a coated steel sheet, the process of spraying UV light and watering, and the process of wetting are periodically repeated in order, peeling, cracking and swelling of the paint, color difference, gloss remaining rate And a corrosion acceleration test method for measuring the peel strength of the coating film.

  The technique described in Patent Document 2 uses two kinds of metals and one kind of surface-treated steel, and defines various corrosive environments by the ratio of the corrosion rates of the two kinds of metals. This is a prediction method for extrapolating the corrosion rate ratio between the treated steel and one kind of metal to estimate the durable life in the actual use environment.

  However, since the technique described in Patent Document 1 uses a coated steel sheet used for a roof or outer wall of a building material or the like as a test target, the assumed corrosive environment is a general outdoor environment and does not simulate the ocean. . In addition, since the evaluation criteria after the test are centered on the deterioration of the coating film and the appearance, the structural steel (that is, the remaining plate thickness after corrosion) should be evaluated for its engineering steel material whose durability should be evaluated. Application is inappropriate. That is, the thickness of the organic coating layer of the coated steel sheet used for this kind of building material application is about 100 μm, and it easily cracks and swells even in an actual use environment. Although it can be a deterioration indicator for coated steel sheets, organic coated steel materials for civil engineering use organic coating layers with a thickness of 2 mm or more, so the durability of the coating layers is high, and peeling at the adhesion interface with the steel materials rather than the integrity of the coating film The progress of this and the amount of corrosion of the steel material at the peeled portion are degradation indicators.

On the other hand, the durable life prediction technology described in Patent Document 2 is a metal plate in which another metal (for example, zinc alloy) is plated on one type of metal (mainly steel), and further, the metal plate is organically coated. It is not applicable to steel for civil engineering that uses a steel material that is directly organically coated without being plated. In the technique described in Patent Document 2, it is assumed that the organic coating and the plated metal completely disappear with the progress of corrosion, and the durable life of the subsequent metal plate is determined by the corrosion rate of the single metal as the base material. However, in the corrosion process of the organic coating steel for civil engineering in the marine environment, the coating layer remains on the steel material even in the process of peeling of the coating layer and corrosion of the steel material under the coating layer, so the steel material under the coating layer There is a big difference that the corrosion rate of is slower than the corrosion rate of a single steel material.
JP 2008-185502 A JP 2006-234802 A

  In view of such circumstances, the present invention clarifies the corrosion acceleration test conditions for reproducing the corrosion behavior of the organic coating steel for civil engineering in the actual marine environment, and corrodes the steel under the peeled coating layer that occurs as the peeling of the coating layer progresses. Corrosion acceleration test method and corrosion of organic coating steel for civil engineering, which clarifies the distribution of steel corrosion amount from the velocity distribution and predicts the remaining thickness distribution of steel necessary for estimating the residual strength of organic coating steel for civil engineering An object is to provide a quantity prediction method.

  In order to achieve the above object, the present invention has the following features.

  [1] Corrosion promotion test method simulating the marine environment of organically coated steel for civil engineering that is directly organically coated without subjecting the steel to plating, and the steel with a known plate thickness is partially subjected to plating without being plated Using organically coated partially organic coated steel, after multiple exposures with salt water spraying, drying and wetting processes as one cycle, remove the rust from the exposed steel that was not organically coated, and coat from the end of the coating layer When measuring the average rust penetration distance under the layer and the average thickness reduction amount of the exposed steel part, the average rust penetration distance under the coating layer from the edge of the coating layer with respect to the average thickness reduction amount of the exposed steel part The ratio is 5 or more and 80 or less, the corrosion accelerated test method for organic-coated steel materials for civil engineering.

[2] When the corrosion promotion test method described in [1] is performed, corrosion starts on the surface of the steel material at an arbitrary distance L from the edge of the coating layer due to the change over time of the rust penetration distance below the coating layer. The grace period b (L) is obtained, and at the same time, the corrosion rate v (L) is obtained from the change over time in the thickness reduction amount of the steel surface at an arbitrary distance L from the end portion of the coating layer. By obtaining the corrosion promotion magnification x from the change over time in the plate thickness reduction amount, the plate thickness reduction amount δt of the steel at an arbitrary distance L from the end of the coating layer after an arbitrary exposure period T is expressed by the following equation: A method for predicting the amount of corrosion of organic-coated steel for civil engineering, characterized by more predicting.
δt = v (L) × (T / x−b (L)) / 365

In addition, the most suitable combination of units in the above [2] is a combination of a) to e) below.
a) Arbitrary distance L from the edge of the coating layer: mm
b) Grace period in which corrosion starts b (L): day c) Corrosion rate v (L): mm / year d) Arbitrary exposure period T: day e) Reduction in plate thickness of steel material L apart from the edge of the coating layer Quantity δt: mm

  In addition, the “marine environment” in the present invention refers to an environment including a tidal zone and a splash zone generated by facing the sea from an underwater environment to a beach environment.

  According to the present invention, it is possible to simulate / promote the corrosion behavior of an organically-coated steel material for civil engineering in the marine environment, and further to predict the corrosion amount distribution of the steel material at an arbitrary time. It is possible to provide a residual thickness distribution after corrosion necessary for force calculation.

  Hereinafter, the present invention will be described in detail.

  First, the organic coating steel material for civil engineering, which is the subject of the present invention, is a steel material in which an organic coating layer is coated on a steel material without performing plating treatment. The organic coating layer to be coated is made of urethane elastomer, polyethylene or epoxy And an ultra-thick film type resin. In addition, in order to provide adhesion and other functions between the organic coating layer and the steel material, surface treatment other than plating treatment (for example, chromate treatment, iron phosphate treatment, silane coupling agent treatment, chromate substitute organic / inorganic composite surface) Including those that have been treated.

  Generally, in order to ensure long-term corrosion resistance in harsh corrosive environments such as the ocean, organic coated steel for civil engineering is applied as a coating layer with urethane elastomer, polyethylene, epoxy resin, etc. with a thickness of 2 mm or more that is excellent in weather resistance and durability. ing. Therefore, unlike a normal coated steel sheet, the coating layer itself deteriorates in an actual use environment, and it is rare that cracking or peeling occurs, rather, peeling progresses from the end of the coating layer unavoidable due to the shape of the steel material, Although the seemingly sound coating layer remains, the main deterioration mode is that the corrosion of the steel material proceeds under the coating layer.

  In addition, organic coated steel materials for civil engineering are rarely questioned about deterioration of appearance due to changes over time due to the characteristics of their use, but rather are required to maintain proof strength over a long period in a severe corrosive environment.

  Considering the above, the deterioration behavior of organic coated steel for civil engineering in the actual marine environment should be reproduced and promoted, and the durability of the organic coated steel for civil engineering should be evaluated and predicted from the viewpoint of residual strength, that is, residual sheet thickness. Is important.

  In addition, it is common that the organic coating steel materials for civil engineering are used together with electro-corrosion for the purpose of cost reduction, and in this case, the steel material exposed in the sea is protected by electro-corrosion. At that time, an oxygen reduction reaction occurs on the surface of the steel material polarized on the cathode, and a phenomenon called cathode peeling occurs in which the coating layer peels off due to the generated alkali. However, since the steel material surface exposed to the sea due to cathodic peeling is also protected by electrocorrosion, the steel material does not corrode, and the steel plate thickness does not decrease due to corrosion. Therefore, the marine environment targeted by the present invention is mainly a tidal zone, a splash zone, and a marine atmosphere portion where the effect of cathodic protection does not reach.

  In order to measure the thickness reduction amount after the corrosion acceleration test, the specimen used in the corrosion acceleration test of the present invention must measure in advance the thickness of a predetermined portion of the steel material exposed portion and the coating portion described later. The thickness measurement method and the number of measurement points are not limited in the present invention. For example, various micrometers, laser displacement meters, and the like can be used as the measurement method. In addition, it is important to record the position of the portion where the plate thickness is measured before the corrosion acceleration test, in order to obtain the thickness reduction amount by measuring the plate thickness of the same portion after the corrosion acceleration test.

  The specimen used for the corrosion acceleration test of the present invention must have a steel exposed part in order to reproduce the corrosion behavior in the actual use environment, that is, the progress of peeling from the end of the coating layer and the corrosion of the steel in the peeling part. Don't be. In the present invention, the area and shape of the steel exposed portion are not particularly limited. For example, when a 100 mm square test piece is used, a steel exposed portion having a width of about 20 mm is provided at the center, and the other portions are attached. What covered with the predetermined | prescribed organic coating layer is mentioned as an example.

  The corrosion acceleration test of the present invention has a salt spray process for supplying salt to the specimen, a drying process imitating the wet / drying of the steel surface in an actual marine environment, and a wetting process. After the salt spray process, the drying process A process called a wet process after the drying process is defined as one cycle. And it is the test method which accelerates | stimulates corrosion by repeating this 1 cycle according to a test period.

  The inventors have exposed test specimens that satisfy the above conditions to various marine environments and evaluated their corrosion behavior.The average rust penetration distance under the coating layer from the edge of the coating layer and the average plate of the exposed steel material It was clarified that the thickness reduction ratio was within the range of 5 to 80 (see FIG. 1 described later). That is, the peeling of the coating layer progresses from the edge of the coating layer, and corrosion occurs on the peeled steel material surface. The cause of the peeling of the coating layer is the anode ( This is an alkali produced by the cathode reaction under the coating layer, which is the counter reaction of the (metal dissolution) reaction. Therefore, the peeling phenomenon that triggers rust intrusion under the coating layer is closely related to the corrosion behavior in the exposed steel part, and the average rust penetration distance under the coating layer from the edge of the coating layer and the average plate of the exposed steel part The thickness reduction ratio of 5 or more and 80 or less is the basis for simulating the actual marine environment from the viewpoint of corrosion behavior.

  In the present invention, in a test specimen whose durability in an actual environment is unknown, a corrosion acceleration test satisfying the above conditions is carried out by changing the test period. Measure the aging change of the rust penetration distance and steel sheet thickness reduction at the rust penetration part. Since the corrosion rate of general steel materials in the actual marine environment is known (for example, about 0.1 mm / year in the tidal zone and about 0.3 mm / year in the splash zone), the corrosion rate of the exposed portion of the steel material is changed over time. By calculating the corrosion rate in the corrosion acceleration test and dividing it by the corrosion rate in the actual environment, the acceleration factor x of the corrosion acceleration test is obtained.

  Next, from the change over time of the rust penetration distance under the coating layer, the peeling tip reaches the steel material under the coating layer that is L mm away from the end of the coating layer, and from the start of the test to the actual start of corrosion at that site. The grace period b (L) is obtained. As a matter of course, the grace period b is a function of the distance L from the end portion of the coating layer, and the grace period b in which corrosion starts increases as the distance L increases.

  Furthermore, the time-dependent change of the corrosion amount (thickness reduction amount) on the steel material surface infiltrated with rust that is L mm away from the edge of the coating layer is measured, and the horizontal axis represents the time obtained by removing the grace period b (L) from the test time. The corrosion rate v (L) is calculated from the slope of the graph, taking the steel corrosion amount (sheet thickness reduction amount) at that time on the vertical axis. The corrosion rate v of the steel material surface Lmm away from the end portion after the start of corrosion is a function of the distance L. The smaller the distance L, the closer the value is to the corrosion rate of the exposed steel material part, and the larger the distance L, the more The value becomes smaller.

In the present invention, the plate thickness reduction δt at an arbitrary time T on the surface of the steel material that is L mm away from the end portion of the coating layer is predicted using the following formula, using the results of the corrosion acceleration test described above.
δt = v (L) × (T / x−b (L)) / 365

  The shape of steel sheet piles and steel pipe sheet piles for civil engineering, the range of initial sheet thickness distribution, and the coating range are specified as product specifications.Thus, based on these values, the sheet thickness reduction obtained by the above prediction formula By subtracting the expected amount δt, the remaining plate thickness and the remaining cross-sectional shape at an arbitrary time T can be predicted, so that it is possible to contribute to the remaining load resistance performance and durability life calculation of the steel structure using a known technique.

  Examples of the present invention will be described below.

(Measurement of plate thickness)
A steel plate having a thickness of 6 mm and a square of 100 mm was prepared and removed using black skin steel blast on both sides. The roughness of the steel sheet surface after blasting was RZ 50 μm. The plate thickness of this steel plate was measured with a laser displacement meter with the coated surface facing up and the entire surface at 1 mm intervals. During measurement, a special jig is attached to the stage of the laser displacement meter so that the position of each specimen (each measurement point) does not shift before and after the corrosion acceleration test, so that the plate thickness can always be measured at the same position in the horizontal direction. I made it.

(Organic coating)
After the plate thickness measurement is completed, the surface of the central part of the steel plate is masked with a width of 20 mm, and then a urethane primer (Permguard 331 Primer / Daiichi Kogyo Seiyaku Co., Ltd.) and urethane elastomer (Permguard 137 / Daiichi Kogyo Seiyaku Co., Ltd.) )) Were coated at 50 μm and 3 mm, respectively, and cured for 1 week. A cut was made in the coating layer along the previously masked portion, and the coating layer was partially peeled off to produce a steel exposed portion. The side surface and the back surface of the test piece were protected against corrosion using tape and sealant.

(Implementation of corrosion acceleration test)
The produced test piece was put into a wet and dry repeated tester, and a corrosion acceleration test was conducted. The test conditions were: a salt spray process for 2.5 hours, then a drying process for 3 hours, then a wetting process for 2.5 hours, with a total of 8 hours as one cycle, this being a plurality of cycles for the test period described later. Repeated. In the salt spray process, a 3% NaCl aqueous solution was used, and the test was performed at an ambient temperature of 35 ° C. In the drying process, the inside of the test tank was maintained at an atmospheric temperature of 60 ° C. and a relative humidity of 40% or lower, and similarly in the wet process, the atmospheric temperature was maintained at 50 ° C. and the relative humidity of 65% or higher. The test period was 30 days, 60 days, 80 days, 120 days and 180 days, respectively.

  After performing the corrosion acceleration test for a predetermined period, the test piece was taken out, washed with water and dried, and then the seal on the back surface and the end surface was removed. After the test piece was pickled to completely remove rust on the exposed steel part, the organic coating layer on the steel material surface was removed, and the rust penetration distance under the coating layer was measured using calipers. The specimen was pickled again to remove rust that had entered under the coating layer, and the thickness after the corrosion acceleration test was measured using a laser displacement system. The steel plate thickness value of the test piece measured before the test was subtracted from that after the corrosion acceleration test to obtain the distribution of the corrosion amount (plate thickness reduction amount) of the test piece.

  As a result, as shown in FIG. 1, the ratio of the average thickness reduction amount of the exposed steel portion to the average rust penetration distance under the coating layer after the corrosion acceleration test is within the range of 5 to 80. It can be judged that the corrosion behavior in the marine environment is satisfied by the above test method.

(Prediction of corrosion amount)
As shown in FIG. 2, the average value of the rust penetration distance when the corrosion promotion test was performed on the 30th, 60th, 80th, 120th and 180th days was plotted against the test period. For each plot, a straight line was drawn using a least square approximation method, and a grace period b (L) at which corrosion started on the surface of the steel material located at an arbitrary distance Lmm from the coating end was obtained.

  Next, as shown in FIG. 3, the average thickness reduction at a plurality of arbitrary distances L (here, 2 mm, 5 mm, and 10 mm) is taken on the vertical axis, and each horizontal axis is obtained from FIG. 1 from the test period. Data obtained by subtracting the grace period b (L) at position L was plotted. A straight line was drawn for each distance from the coating end using the least square method, and the inclination was defined as the corrosion rate v (L) of the steel material separated by L mm from the coating layer end.

On the other hand, in FIG. 3, the corrosion rate c of the steel exposed portion in the corrosion acceleration test was obtained from the change over time in the thickness reduction amount of the steel exposed portion. Since the corrosion rate of steel in the tidal zone is 0.1 mm / y, the acceleration magnification x is
x = c / 0.1
It becomes.

Using the values of the acceleration magnification x, the corrosion rate v (L) and the grace period b (L) obtained by the above operation, the thickness reduction amount of the steel material at an arbitrary time T from the coating layer end at an arbitrary time T. δt was determined from the following equation.
δt = v (L) × (T / x−b (L)) / 365

  Table 1 shows the prediction formula for the thickness reduction amount of steel at positions 2 mm, 5 mm, and 10 mm away from the coating edge, and the thickness reduction amount δt expected after 50 years at each part derived from the formula. Indicated.

In the Example of this invention, it is a figure which shows the relationship between the average board thickness reduction | decrease amount of a steel material exposure part, and the average rust penetration distance under a coating layer. In the Example of this invention, it is a figure which shows the relationship between a test period and an average rust penetration distance. In the Example of this invention, it is a figure which shows the relationship between a test period-grace period, and board thickness reduction | decrease amount.

Claims (2)

  This is an accelerated corrosion test method that simulates the marine environment of organically-coated steel for civil engineering that has been organically coated without steel plating, and is a part of a steel plate with a known plate thickness that is partially organically coated without being plated. After using organic coating steel material and exposing multiple cycles of salt spray process, drying process and wet process as one cycle, remove rust on exposed steel parts not coated with organic coating, average under coating layer from edge of coating layer When measuring the rust penetration distance and the average thickness reduction amount of the steel exposed portion, the ratio of the average rust penetration distance under the coating layer from the end portion of the coating layer to the average thickness reduction amount of the steel exposed portion is 5 or more A corrosion acceleration test method for organically-coated steel materials for civil engineering, characterized by being 80 or less. When the corrosion promotion test method according to claim 1 is performed, a grace period b (where corrosion starts on the surface of the steel material at an arbitrary distance L from the edge of the coating layer due to the change over time of the rust penetration distance under the coating layer. L) is obtained, and at the same time, the corrosion rate v (L) is obtained from the change over time in the thickness reduction amount of the steel surface at an arbitrary distance L from the end of the coating layer. Predicting the thickness reduction amount δt of the steel material at an arbitrary distance L from the end of the coating layer after an arbitrary exposure period T by obtaining the corrosion acceleration magnification x from the change over time of A method for predicting the amount of corrosion of organically coated steel for civil engineering.
δt = v (L) × (T / x−b (L)) / 365

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