JP4148132B2 - Method for evaluating corrosion resistance of steel - Google Patents

Description

  The present invention relates to a corrosion resistance evaluation method using Mg salt of steel material or steel material subjected to surface treatment containing zinc.

  Due to the social problem of car body corrosion loss, each automobile company is working to improve car body rust prevention as a response to social problems, and now car body rust prevention guarantee (perforation due to corrosion) has been extended to more than 10 years. . These rust prevention guarantees were achieved by optimizing rust preventive materials based on the investigation of car body corrosion in areas where snow melting agents were applied and the development of accelerated corrosion test methods for car body appearance that correlate with actual corrosion. .

  The following is disclosed and reported as an accelerated corrosion test method for vehicle body appearance.

  The method of continuously spraying 5% NaCl at 35 ° C., which is known in the salt spray test (hereinafter referred to as SST), is also applied as a method for evaluating the corrosion resistance of painted steel sheets for automobile bodies (see, for example, Non-Patent Document 1). . Since this method gives a better evaluation result for a cold-rolled steel plate than a galvanized steel plate, various combined cycle test methods that can obtain an evaluation result closer to the actual environment have been disclosed recently.

  The test conditions of the vehicle body outer surface corrosion test method standardized by the Japan Automobile Manufacturers Association are those in which 5% of saline is applied as chloride by spraying with salt water for 2 hours (see, for example, Non-Patent Document 2). . In this test method, setting the wetting time (the total time of the wetting time and the salt water spraying time) to be 50% of the total testing time is an important condition for achieving both the correlation with the actual environment and the acceleration rate. This is a test method that simulates the marine environment typified by exposure to Okinawa by testing at 60 ° C for the drying process and 50 ° C for the wet process.

Test conditions as a vehicle body outer surface corrosion test method standardized by SAE (Society of Automotive Engineers) assume corrosion by a snow melting agent and use a mixed solution of NaCl and CaCl ₂ (for example, Non-Patent Document 3). reference.). This test method is a test method that simulates car body appearance corrosion in a salt damage area where a snow melting agent is sprayed in the North American market, with the drying process conducted at 60 ° C. and the wet process conducted at 50 ° C.

By the way, the snow melting agents used in North America, Europe, and Japan are mainly composed of NaCl such as rock salt and salt, or CaCl ₂ as the main component. Agents are also used.

Corrosion resistance evaluation method assuming stainless steel structure used in coastal areas has been reported as an accelerated corrosion test method using MgCl ₂ although it is not intended for corrosion of the car body appearance or steel plate (for example, non-patent Reference 4). Pitting corrosion of stainless steel occurs under conditions where the relative humidity is less than 50%, but this method uses corrosion of stainless steel by using MgCl ₂ or CaCl _2, which has a lower saturated vapor pressure than NaCl, as spray salt. Has been found to be promoted and reproduced.

Corrosion resistance evaluation method for domestic acid rain areas has been reported as an accelerated corrosion test method using acidic solution and MgCl ₂ (for example, non-patent literature). 5). This method assumes an acid rain flying area on the Japan Sea side, which is known in heavy snowfall areas, and artificial seawater shown in ASTM D 1141 is used. MgCl _{2 is} 0.867 g / L against NaCl 4.088 g / L and CaCl ₂ 0.193 g / L. The ratio of the acid component is about 0.2 for nitrate ions / sulfate ions.
JIS Z 2371 JASO M610 Automotive Engineering Society: `` Automobile parts appearance corrosion test method '', JASO M610-92 (1992). HE Townsend: Corrosion Prevention, SP-1265, SAE, 53 (1997). Muto et al .: Materials and the environment, 42, 714 (1990). ISO TC156 document

  However, the above prior art has the following problems.

Recently, environmentally-friendly snow-melting agents based on Mg have emerged in China and started to be used in Japan and China due to the growing environmental awareness of ecosystem destruction caused by excessive application of snow-melting agents such as rock salt. It is well known that spraying snow melting agent has a significant effect on the corrosion of the exterior of the car body. However, these are knowledge of rock salt (NaCl-based) and CaCl _2- based snow melting agent spraying areas that have been used in the past. The effect of spraying the snow melting agent as a component on the corrosion of the car body rust-proof steel sheet is not clarified at all. For this reason, there has been a problem that there is no accelerated corrosion test necessary for selecting and developing an optimum car body rust-proof steel sheet for automobiles running in cold regions assuming a region where Mg-based snow melting agent is applied.

  Accordingly, an object of the present invention is to provide a method for evaluating corrosion resistance of a steel plate for automobile bodies that runs in a cold region assuming a region where an Mg-based snow melting agent is dispersed.

The present inventors conducted a comparative investigation on the influence of MgCl ₂ , which is the main component of Mg-based snow melting agent, on the corrosion of NaCl, CaCl ₂ which is the main component of conventional snow melting agent.

For each chloride species, a corrosion test was performed with a single chloride species having a concentration of 0.5% by weight and a pH of 3.0. The corrosion test conditions other than the solution composition are basic conditions (A) for the accelerated corrosion test described later. The corrosion appearance is shown in FIG. The test material is a galvanized steel sheet GI50 for automobiles (amount of plating, 50 g / m ² per side). The test conditions using the MgCl ₂ solution were found to differ greatly in corrosion appearance compared to the test conditions using the CaCl ₂ and NaCl solutions that have been conventionally used as the main component in snow melting agents. That is, red rust was generated under the test conditions using CaCl ₂ and NaCl solutions, whereas red rust was not generated under the test conditions using MgCl ₂ solution, and it is expected that the corrosion behavior will be greatly different in the actual environment. I understood.

In the actual environment, in addition to MgCl ₂ , components from the environment are mixed to form a corrosive environment. Next, the present inventors conducted an investigation on the state of Mg-based snow melting agent application.

It was difficult to quantify the effects of Mg-based snow melting agent spraying since 2001, since the long-term data was missing from the actual corrosion survey of the car body. Spraying area is limited to urban Far East, NaCl from the NaCl-based and some CaCl ₂ based snow melting agent, such as rock salt in the suburbs is sprayed, as an environment in which the steel sheet is exposed, CaCl ₂ and MgCl ₂ It turned out that it became the mixed environment of a solution. These chlorides were also detected from chlorides adhering to the mating part of the steel plate, which is a severely corroded part of the vehicle body. In addition, as a result of investigating the pH of rainfall over the past 10 years, it was found that acidification of rainfall is progressing in urban areas where salt damage is assumed.

Next, the drying and wetting cycle conditions are narrowed down from the temperature and humidity changes in the area where the Mg-based snow melting agent is sprayed, and various solutions with different MgCl ₂ , NaCl, and CaCl ₂ mixing ratios and pH are attached to the test material for acceleration. A corrosion test was performed. Chloride concentration adhering to the actual vehicle was analyzed in the area where Mg-based snow melting agent was sprayed, and used as a standard for the concentration of various solutions. Various zinc-based plated steel sheets and cold-rolled steel sheets used as vehicle steel sheets were used as test materials. As a test piece subjected to the accelerated corrosion test, a test material having a shape provided with a gap simulating a joint portion having the most severe corrosion in a steel plate for vehicle bodies as shown in FIG.

As a result, it has been clarified that, with the MgCl ₂ concentration scattered in the actual vehicle in the actual environment, the corrosion aspects of the various steel sheets tend to be completely different from the results of the conventionally known knowledge and the results of the known accelerated corrosion test. Conventionally, MgCl ₂ has a higher saturated water vapor pressure than NaCl and wets at a low dew point temperature, and therefore has been supposed to promote corrosion. However, in the accelerated corrosion test simulating the vehicle body environment, the corrosion of the zinc-based plated steel sheet showed a tendency to be suppressed in the MgCl ₂ environment compared to the NaCl and CaCl ₂ environments. On the other hand, it was found that cold-rolled steel sheets are promoted by MgCl ₂ . The effect of this MgCl ₂ was found to be expressed when present in a concentration of MgCl ₂ is 1/3 or more with respect to the total sum of the NaCl concentration and CaCl ₂ concentrations.

Based on such investigation results, the present inventors established an accelerated corrosion test for vehicle body appearance corrosion that develops corrosion in the MgCl ₂ environment. The present invention has been made based on such findings, and the features thereof are as follows.

(1) Use of Mg salt of steel material or surface-treated steel material containing zinc characterized in that corrosion resistance is evaluated by repeating the process consisting of the following process (A) and the following process (B) multiple times. Corrosion resistance evaluation method.
(A) A solution containing at least NaCl, sulfate ions, MgCl ₂ , and the pH of the solution is pH 3.0 to 5.0, and the total concentration of chloride in the solution is 0.1 to 2.0% by weight, and step, characterized in that the NaCl concentration of the solution is less than 3 times the MgCl ₂ concentration steel material surface, the processing time is deposited in the range of 1 to 15 minutes.
(B) It consists of a drying step of 3 to 12 hours at a relative humidity of 10 to 50% and a wetting step of 2 to 8 hours at a relative humidity of 80 to 98%, and the temperature range is 10 to 60 ° C. and the drying step temperature ≧ wetting step A step in which the drying step and the wetting step in which the transition time from the wetting step to the drying step included in the drying step time is set to 20 to 60 minutes is set as one cycle, and this cycle is performed one or more times.

(2) Use of Mg salt of steel material or zinc-treated steel material characterized by evaluating corrosion resistance by repeating the process consisting of the following process (A) and the following process (B) multiple times Corrosion resistance evaluation method.
(A) A solution containing at least CaCl ₂ , NaCl, sulfate ions, MgCl ₂ , the pH of the solution is pH 3.0 to 5.0, and the total concentration of chloride in the solution is 0.1 to 2.0. A process characterized by adhering a solution having a total weight of NaCl concentration and CaCl ₂ concentration of not more than 3 times the MgCl ₂ concentration to the steel surface within a range of 1 to 15 minutes.
(B) It consists of a drying step of 3 to 12 hours at a relative humidity of 10 to 50% and a wetting step of 2 to 8 hours at a relative humidity of 80 to 98%, and the temperature range is 10 to 60 ° C. and the drying step temperature ≧ wetting step A step in which the drying step and the wetting step in which the transition time from the wetting step to the drying step included in the drying step time is set to 20 to 60 minutes is set as one cycle, and this cycle is performed one or more times.

According to the present invention, it is possible to establish a method for evaluating the corrosion resistance of a steel material that exhibits corrosion in the MgCl ₂ environment, and it becomes possible to develop and select a rust-proof steel material having an optimum cost.

  In the following description, among steel materials, a steel plate or a steel plate subjected to surface treatment is particularly targeted, but the application of the present invention is not limited to this. A corrosion resistance evaluation method for a steel sheet assuming a Mg-based snow melting agent application area or a steel sheet subjected to a surface treatment containing zinc includes a solution composition, a solution adhesion, a drying process, and a wetting process. Here, the solution composition and solution adhesion will be described first.

  The solution attached to the surface of the steel sheet is made acidic to simulate acid rain. The pH at which rainfall equilibrates with carbon dioxide in the atmosphere is pH 5.6. Since the corrosion promotion effect by acidification is not clear in a solution exceeding pH 5.0, the upper limit of the pH of the test solution is set to pH 5.0. Moreover, since the acidity of the rainfall in the Far East region where the Mg-based snow melting agent is used is hardly lower than pH 3.0, the lower limit of the solution pH was set to pH 3.0. That is, the pH of the test solution is 3.0 to 5.0.

  In the Far East region, where Mg-based snow melting agents are used, SOx from coal fuel is used, and the precipitation contains sulfate ions. Further, in an actual environment in which an automobile travels, nitrate ions are included in rainfall due to nitrogen oxides contained in exhaust gas. In the investigation by the present inventors, coal-based fuels were used particularly frequently in the far east automobile traveling area where the Mg-based snow melting agent was sprayed, and the concentration ratio of nitrate ion / sulfate ion was 1/8 or less. Therefore, the concentration when nitric acid is added to the test solution is 1/8 or less of the concentration of sulfate ions. Sulfate ions can be supplied as sulfuric acid and nitrate ions as nitric acid. These acids are mixed with a chloride solution to prepare a predetermined solution having a pH of 3.0 to 5.0.

The chloride species essential for the solution are MgCl ₂ , which is the main component of the environmental snow melting agent, and NaCl mixed as rock salt and flying sea salt. Further, the MgCl ₂ snow melting agent sometimes contained CaCl _2, but the concentration was 50% by weight or less in the range of the investigation. CaCl ₂ may be mixed into the test solution, but the MgCl ₂ concentration should not be exceeded. The chloride concentration of the solution collected from the road surface sprayed with the snow melting agent was about 0.08 to 2% by weight. It is known that the corrosion of zinc plating deviates from the actual environment when the concentration of NaCl in the test solution increases. On the other hand, if it is less than 0.1% by weight, the corrosion acceleration rate is remarkably small and it is not practical as an accelerated test method. Therefore, the range of the total chloride concentration in the test solution is 0.1 to 2% by weight.

CaCl ₂ , NaCl and MgCl ₂ are the ratios of chloride concentrations. CaCl ₂ and NaCl are surface treatments that include cold-rolled steel and zinc in a specimen that simulates a steel plate joint that creates a severe corrosive environment in the car body. The corrosion behavior of the same tendency was shown for both steel sheets. Based on these findings, MgCl ₂ should treat CaCl ₂ and NaCl equivalently as the concentration at which the corrosion inhibition effect of galvanizing is expressed, and prepare a solution in the range of NaCl concentration + CaCl ₂ concentration ≤ 3 x MgCl ₂ concentration. is required.

  When the solution treatment time on the steel sheet is shorter than 1 minute, the gap in the test piece is not sufficiently filled with the solution, and when it exceeds 15 minutes, dissolution of the plating layer is controlled by the electrochemical cell in the solution. Become. The solution treatment time for the steel sheet is 1 to 15 minutes. As an adhesion method, an immersion or shower method is preferable instead of a spray method with a large adhesion error. The process after the solution adhesion is a drying process in order to fix the adhered chloride or the like on the surface of the test material. The time required for solution attachment is included in the time of the drying process in the test cycle.

  Next, the drying process and the wetting process will be described.

  The temperature range of the wet and dry repetitive process is 10 ° C or higher on the low temperature side in consideration of avoidance and acceleration of solution freezing. Furthermore, the upper temperature limit in the real environment in summer, safety when handling test materials, and resin-based jigs From the viewpoint of shape stability, the upper limit of the temperature is set to 60 ° C. or less. Regarding the temperature of the drying and wetting process, the actual environment is taken into consideration, and the drying process temperature ≧ the wetting process temperature.

  In the transition from the wet process to the dry process, the corrosion corresponding to the repeated dry and wet conditions in the actual environment proceeds with the concentration of the solution by drying. When the transition time from the wetting to the drying process is less than 20 minutes, the time for contributing to the corrosion due to the concentration of the solution is short and the acceleration rate of corrosion in accordance with the actual environment is inferior. Further, even if the transition time is increased beyond 60 minutes, it does not contribute to the promotion of corrosion. The transition time from the wet process to the dry process is preferably set to 20 to 60 minutes. The transition time from the wet process to the dry process is included in the time of the dry process.

  After the solution adhesion, the drying step and the wetting step are set as one cycle, and the corrosion reaction by the adhesion solution can be efficiently advanced by performing this cycle one or more times. Even if the drying / wetting process is repeated more than necessary for one solution adhesion, the acceleration of the corrosion reaction due to the solution adhesion is not detected. It is desirable that the number of repeated wet and dry cycles per solution adhesion is 12 or less. The solution deposition time shall be incorporated into the drying time when building the test cycle.

  The relative humidity in the drying process is set to a range of 10 to 50% from the correspondence with the actual environment. If the relative humidity is less than 10%, the load applied to the test apparatus in order to perform the conditions of the present invention is extremely large and unstable. When the relative humidity exceeds 50%, the gap is not dried at all and is kept wet, and corrosion in an aqueous solution different from the actual environment proceeds rapidly. It takes 3 hours or more in the drying process for the moisture content inside the gap of the test material having the gap shape to be in a steady state. Further, since the corrosion hardly proceeds even if the moisture is kept in a dry state in a steady state for a long time, the drying process is set to 12 hours or less. That is, the drying step is within a range of 3 to 12 hours. The transition time from the drying process to the wetting process is included in the time of the wetting process.

  The relative humidity in the wetting process is in the range of 80 to 98% as an environment where wetting can be reliably obtained. In addition, it takes two hours or more for the moisture content inside the gap of the test material having the gap shape to become wet. In addition, since the corrosion that is retained for a long time while the inside of the gap is wet proceeds as a galvanic cell in the same manner as aqueous solution corrosion, the wetting step is set to 8 hours or less. That is, the wetting step is in the range of 2 to 8 hours.

  When the accelerated corrosion test according to the present invention is all performed manually without using an automatic control device or the like, the drying process may be performed throughout the day when work such as weekends and holidays is difficult.

  A gap portion corresponding to a matching portion was imparted to various test materials and subjected to an accelerated corrosion test including a salt adhesion step, a wetting step, and a drying step. In addition, some of the test materials were subjected to accelerated corrosion tests after painting and initial damage such as crosscutting was introduced. The accelerated corrosion test conditions were changed in the kind, concentration, and repeated wet and dry conditions of the chloride solution used. After the test, the corrosion degree of each test material was measured and compared with the existing accelerated corrosion test method corresponding to the actual environment.

The plate thickness of the test material used for the test is 0.8 mm. Electroplated galvanized steel sheet for automobiles EG20 (plating coverage, 20 g / m ² per side), EG50 (plating coverage, 50 g / m ² per side), electroplated zinc / nickel alloy steel sheet for automobiles Zn-Ni (Amount of plating, 20 g / m ² per side), galvanized steel sheet GI50 for automobiles (amount of plating, 50 g / m ² per side) and cold-rolled steel (CRS) were used.

  FIG. 1 is a configuration diagram of a test piece, (a) is a front view, and (b) is a plan view. The configuration of the test piece is that the test material 1 is cut into 150 mm × 70 mm, the end surface and the back surface of the test material 1 are sealed with a sealing tape 2, and the test plate 1 is formed using bolts and nuts 6 with an acrylic plate 3 having a thickness of 5 mm. The gap 4 is formed with the gap in between. The gap 4 is formed between the test material surface and the acrylic plate 3 with a gap of 50 μm using a polytetrafluoroethylene spacer 5 having a thickness of 50 μm. The surface to be tested was subjected to a phosphate chemical conversion treatment and a 20 μm-thick cationic electrodeposition coating as necessary.

  The prepared specimens were subjected to an accelerated corrosion test under various conditions to evaluate the corrosion state. Evaluation of the corrosion state of the test material was performed by evaluating the red rust occurrence time by visual observation through the acrylic plate, and removing the corrosion product after the completion of the test period, and actually measuring and evaluating the maximum corrosion depth of the steel plate with a micrometer.

  Tables 1 and 2 show the basic conditions of the accelerated corrosion test. Here, the basic conditions described in Table 1 are referred to as basic acceleration test conditions A, and the basic conditions described in Table 2 are referred to as basic acceleration test conditions B.

  Below, the Example which performed the accelerated corrosion test on various conditions and evaluated the corrosion state using the said test piece is described in detail.

  Table 3 shows the amount of corrosion when the chloride species are changed under the basic condition B in which various conditions such as the solution composition are configured based on the investigation of the actual environment.

The test materials are electroplated galvanized steel sheets EG20 and EG50 and cold rolled steel sheets (CRS). The chloride concentration is changed to 0.8% by weight based on the solution composition of the basic condition B. In test condition 1 in which MgCl ₂ is 0.8% by weight according to the present invention, compared to comparative conditions 1 and 2 in which CaCl ₂ and NaCl were conventionally used as the main component in a snow melting agent at 0.8% by weight. Thus, it was found that the amount of corrosion decreased with EG20 and EG50, and the amount of corrosion increased with cold-rolled steel sheets (CRS), and the corrosion tendency varied greatly depending on the test material.

The right column of Table 3 shows a comparison of the correlations with the corrosion test results of various materials using SAEJ2334, which is an accelerated corrosion test assuming a conventional snow melting agent application area where the main components are CaCl ₂ and NaCl.

R ² represents the correlation with the corrosion of various materials in SAEJ2334 by the above formula. In the formula, n is the number of test material levels to be compared, and x and y are the corrosion amounts of the test materials according to SAE J2334 and the test materials obtained under the present invention conditions and comparative conditions in Table 3. Indicates the amount of corrosion.

  The Zn / CRS corrosion ratio is as shown in Table 3 for the corrosion ratio of galvanized steel sheet (EG20) to the corrosion amount of the cold-rolled steel sheet that corrodes most in the actual environment, where the ratio obtained by SAE J2334 is 100. The ratio obtained under the conditions is shown.

Comparative conditions 1 and 2 using CaCl ₂ and NaCl solutions that have been used as main components in snow melting agents in the past have a high R ² of 0.95 and Zn / CRS corrosion ratio of 70% or more. The trend is consistent. On the other hand, in test condition 1 using the MgCl ₂ solution according to the present invention, R ² is 0.6 or less and the Zn / CRS corrosion ratio is 10% or less and the corrosion tendency is different from SAEJ2334.

Therefore, it has been found that the accelerated corrosion test for the corrosive environment using the MgCl ₂ snow melting agent requires the test conditions using the MgCl ₂ solution according to the present invention.

  FIG. 3 shows an appearance photograph after the corrosion test of the corrosion test performed by applying the present invention condition 1 and comparative conditions 1 and 2 shown in Table 3 of Example 1 to two types of galvanized steel sheets. The test materials are electrogalvanized steel sheet EG20 and electroplated zinc / nickel alloy steel sheet Zn-Ni. The content of Ni with respect to zinc is about 10%.

As shown in FIG. 3, EG20 tends to be excellent in the present invention condition 1 using the MgCl ₂ solution according to the present invention shown in Table 3, but the CaCl ₂ and NaCl solutions that have been conventionally used as the main component in the snow melting agent. In Comparative Conditions 1 and 2 using No. 1, Zn-Ni tended to be excellent, and it was found that different results were obtained between the present invention and the conventional evaluation method depending on the difference in the composition of the zinc-based surface-treated steel sheet.

FIG. 4 shows the change in corrosion amount when the chloride concentration ratio of MgCl ₂ and NaCl + CaCl ₂ is changed at a basic concentration B of 0.5% by weight. An electroplated galvanized steel sheet EG20 for automobiles was used as a test material. It was found that when the sum of the NaCl concentration and the CaCl ₂ concentration is 3 times or less of the MgCl ₂ concentration which is the condition of the present invention, the tendency of corrosion changes and the influence of MgCl ₂ appears.

  Table 4 shows the penetration rate of the test solution into the test material gap and the zinc elution amount from the test material when the immersion time of the test material was changed. The penetration rate evaluated the wet area of the gap | interval part visually observed through an acrylic board when a sample was pulled up after immersion. The zinc elution amount was determined by measuring the concentration of zinc ions eluted in 100 cc of the test solution by the ICP method. When the immersion time of the test material in the solution is less than 1 minute, the solution does not sufficiently penetrate into the test material gap. Further, when the immersion time of the test material exceeds 15 minutes, elution of zinc from the gap between the test materials is accelerated by galvanic corrosion of zinc and iron in the aqueous solution, and deterioration by a mechanism different from the actual environment proceeds. It was found that the dissolution of the plating layer is dominant due to the electrochemical cell in the solution, and the immersion time is preferably 1 to 15 minutes which is the condition of the present invention.

  Table 5 shows the results of collecting the solution staying on the snow melting agent spraying surface and measuring the Cl ion concentration, the Mg ion concentration, and the Na ion concentration. It was found that the total chloride concentration in accordance with the actual environment was 2% by weight or less.

  Table 6 shows changes in the corrosion amount when the total chloride concentration of the immersion solution is changed under the basic condition B. It was found that when the total chloride concentration is less than 0.1% by weight, the progress of corrosion is extremely slow and it is difficult to apply as an accelerated corrosion test.

  Table 7 shows test results obtained by changing various test conditions in the basic condition B.

  Regarding the correlation, a Zn / CRS ratio of 40% or less was determined to be acceptable. The condition of the test object was visually observed through the acrylic surface exceeding 40%, and the conditions having difficulties in carrying out the test were determined as x.

  As test materials, electroplated galvanized steel sheets EG20 and EG50 for automobiles and cold rolled steel sheets (CRS) were prepared into test pieces simulating mating portions using an acrylic plate shown in FIG. The gap is 50 μm.

  Test conditions 1 to 3 in which the test conditions are within the scope of the present invention have obtained good results.

  In comparative condition 1, the drying time is shorter than 3 hours, and the relative humidity during drying is higher than 50%, so the gap is not dried in the drying process. Further, since the temperature range when wet was less than 10 ° C., the solution such as the gap portion was partially frozen during the test period due to the temperature fluctuation of the apparatus.

  In comparative condition 2, since the relative humidity during drying was less than 10%, the test conditions could not be controlled within the test time of the drying process. Further, since the relative humidity in the wetting process was less than 80% and the wetting time was less than 2 hours, a wet state could not be obtained in the gap portion of the test material in the wetting process.

  Comparative condition 3 is that the drying time that does not contribute to the corrosion reaction is longer than 12 hours, and the transition time from the wet state where the corrosion proceeds due to solution concentration to the drying process is as short as less than 20 minutes. Does not make sense as an accelerated test. Also, if the test temperature is higher than 60 ° C, troubles of the testing machine occur at high temperatures, making it difficult to work with the sample, and the deformation of the resin product at the stress-applying part is remarkably reliable. descend. The test temperature is desirably 60 ° C. or lower.

  Comparative condition 4 was a case where the transition time from the wet process to the dry process was longer than 60 minutes, and the effect of accelerating the corrosion was saturated, and the variation of the test results tended to increase. The transition time from the wet process to the dry process is desirably 60 minutes or less.

  Comparative condition 5 is a case where the number of repeated wet and dry cycles per solution adhesion is longer than 12 times, but the effect of promoting corrosion is saturated.

Table 8 shows the test results obtained by changing various test conditions for the test material electrodeposited with respect to the basic condition B. The test material is an unprocessed flat plate-shaped surface-treated steel plate to which 20 g / m ² of zinc is adhered. After a phosphate chemical conversion treatment (3 g / m ² ), electrodeposition coating with a thickness of 20 μm was applied. A cut part (initial damage) that reaches the ground iron by a cutter was introduced on the electrodeposited surface. Evaluation measured the maximum swelling width of the coating film from a cut part through an acrylic board. As the suitability of the test method, the case where the swollen width of the galvanized steel sheet was smaller than the coated cold-rolled steel sheet was accepted: ◯, and the case where the swollen width of the galvanized steel sheet was larger than the painted cold-rolled steel sheet was rejected: x.

  Test conditions 4 and 5 in which the test conditions are within the scope of the present invention have obtained good results.

  In comparative condition 6, the relative humidity in the wetting process is higher than 98%, and the wetting time is longer than 8 hours. Therefore, corrosion from the damaged part of the coating to the bottom of the coating is accelerated by the galvanic corrosion reaction in the aqueous solution. Is done. Corrosion of galvanized steel sheet and cold-rolled steel sheet is not suitable.

In the block diagram of the test piece, (a) is a front view and (b) is a plan view. Appearance photograph after corrosion test when the concentration of chloride species of MgCl ₂ , NaCl, and CaCl ₂ is 0.5 wt% (corrosion test conditions other than solution composition are basic condition A) Appearance photograph after corrosion test of corrosion test applied to two types of galvanized steel sheets (Test conditions shown in Table 3) A graph showing the relationship between the chloride concentration ratio of MgCl ₂ and NaCl + CaCl ₂ and the amount of corrosion

Explanation of symbols

1 Test Material 2 Sealing Tape 3 Acrylic Plate 4 Gap 5 Polytetrafluoroethylene Spacer

Claims (2)

Corrosion resistance evaluation using Mg salt of steel or surface treated steel containing zinc characterized by evaluating the corrosion resistance by repeating the process consisting of the following step (A) and the following step (B) multiple times Method.
(A) A solution containing at least NaCl, sulfate ions, MgCl ₂ , and the pH of the solution is pH 3.0 to 5.0, and the total concentration of chloride in the solution is 0.1 to 2.0% by weight, and step, characterized in that the NaCl concentration of the solution is less than 3 times the MgCl ₂ concentration steel material surface, the processing time is deposited in the range of 1 to 15 minutes.
(B) It consists of a drying step of 3 to 12 hours at a relative humidity of 10 to 50% and a wetting step of 2 to 8 hours at a relative humidity of 80 to 98%, and the temperature range is 10 to 60 ° C. and the drying step temperature ≧ wetting step A step in which the drying step and the wetting step in which the transition time from the wetting step to the drying step included in the drying step time is set to 20 to 60 minutes is set as one cycle, and this cycle is performed one or more times. Corrosion resistance evaluation using Mg salt of steel or surface treated steel containing zinc characterized by evaluating the corrosion resistance by repeating the process consisting of the following step (A) and the following step (B) multiple times Method.
(A) A solution containing at least CaCl ₂ , NaCl, sulfate ions, MgCl ₂ , the pH of the solution is pH 3.0 to 5.0, and the total concentration of chloride in the solution is 0.1 to 2.0. A process characterized by adhering a solution having a total weight of NaCl concentration and CaCl ₂ concentration of not more than 3 times the MgCl ₂ concentration to the steel surface within a range of 1 to 15 minutes.
(B) It consists of a drying step of 3 to 12 hours at a relative humidity of 10 to 50% and a wetting step of 2 to 8 hours at a relative humidity of 80 to 98%, and the temperature range is 10 to 60 ° C. and the drying step temperature ≧ wetting step A step in which the drying step and the wetting step in which the transition time from the wetting step to the drying step included in the drying step time is set to 20 to 60 minutes is set as one cycle, and this cycle is performed one or more times.