JP2001131701A - Thermally sprayed steel sheet for tank - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a thermally sprayed steel sheet for a tank having excellent corrosion resistance and good applicability in an installation environment by being used for the bottom board of a petroleum tank or the like. SOLUTION: In this thermally sprayed steel sheet for a tank, the surface is provided with a sprayed coating layer containing, by weight, 0.05 to 20% Mg, and the balance Zn with an inevitable impurity layer at a thickness of >=10 μm. Preferably, the sprayed coating later moreover contains 0.05 to 55% Al.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thermal sprayed steel sheet for a tank, and more particularly, to a thermal sprayed steel sheet having excellent corrosion resistance and good workability when used for an oil tank bottom plate or the like.

[0002]

2. Description of the Related Art Large storage tanks and the like are generally installed and used in contact with the ground or buried in the soil. From the viewpoint of preventing leakage of stored items, countermeasures against porosity due to corrosion are important issues. . In particular, since the storage volume of oil tank bottom plates is enormous and the contents are classified as hazardous materials, it is required by the Fire Service Law to ensure a sufficient plate thickness during use. Requires some anti-corrosion measures. As a specific anti-corrosion measure, it is common that when the tank bottom plate foundation is made of sand, electric anti-corrosion is implemented. In addition, in order to remove water, which is an important substance that causes a corrosion reaction, from the environment of the oil tank bottom plate, and to suppress the corrosiveness of the environment, measures for using an asphalt structure as a base are often used.

[0003] Compared with the case where no anticorrosion measures are taken, these countermeasures have an anticorrosion effect, but may not exhibit the expected anticorrosion effect. The reason for this case is thought to be that various factors such as differences in individual environmental conditions, stability of construction, and changes in aging in individual application cases are intricately linked, but unfortunately it is not possible. The reasons for the stability are not always clear, and there is no technical solution for stably obtaining the expected effect of the anticorrosion state. That is, the conventional technique has a problem in terms of reliability.

Under such circumstances, a technique for solving the above-mentioned problem by using a zinc-sprayed steel sheet for an oil tank bottom plate has already been known. The inventors considered that an attempt to stably achieve the anticorrosion effect by spraying these sacrificial anticorrosion metals onto the surface of the oil tank bottom plate was an excellent technical solution, and considered its practical performance and use. For the purpose of grasping the characteristics, we examined from various angles including clarification of problems, including actual application to actual oil tanks,
Analysis has been performed.

[0005] As a result of these implementations and studies, in the actual operating environment of oil tanks, the use of zinc-sprayed steel sheets as the bottom plate of oil tanks has shown a marked improvement in the anti-corrosion performance. However, the manifestation effect of the anticorrosion effect still varies widely, and in order to balance the welding work, which is indispensable when constructing the bottom plate of the oil tank, and the anticorrosion performance of the zinc-sprayed steel plate, the welding heat input must be significantly reduced. Need to reduce.

As described above, even if a zinc-sprayed steel sheet is used, there are still many problems to be solved practically from the viewpoint of reliability and economy. In other words, the adoption of zinc-sprayed steel plates for oil tank bottom plates is recognized to be effective in improving corrosion resistance,
There are problems in the stability of the anticorrosion effect and the welding workability, and further improvement is necessary at present considering practicality.

[0007]

SUMMARY OF THE INVENTION In view of these circumstances, an object of the present invention is to provide a thermal sprayed steel sheet having excellent corrosion resistance and good workability for use as an oil tank bottom plate or the like.

[0008]

SUMMARY OF THE INVENTION The present invention solves the above problems and provides a thermal sprayed steel sheet having excellent corrosion resistance and good workability by using it for an oil tank bottom plate or the like. (1) A thermal spray coating layer containing 0.05 to 20% by weight of Mg and composed of a balance of Zn and unavoidable impurity layers,
A sprayed steel plate for a tank, having a thickness of 10 μm or more on the surface. (2) The thermal spray coating layer has a weight percentage of Al: 0.05 to 55
%. The sprayed steel sheet for a tank according to the above (1), further comprising:

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The inventors first studied from various viewpoints to develop a thermal sprayed steel sheet having excellent corrosion resistance in a corrosive environment such as an oil tank bottom plate. As a result, despite the fact that the actual condition of the corrosive environment and other soil corrosive environments and the like is a wet environment that contains an extremely low concentration of salt and is generally classified as a non-corrosive environment, it is extremely corrosive. Exists due to uneven contact or presence of sand or asphalt particles in the environment or corrosion products of the metal itself on the metal surface, or uneven condensation containing a low concentration of salt Was found to be due to the oxygen concentration cell used.

When such an oxygen concentration cell is formed,
It has been found that a metal exhibits a local corrosion state at an extremely high corrosion rate, but also a large local corrosion progresses in a conventionally proposed zinc sprayed layer. That is, since the corrosion wear of the conventionally proposed general sprayed zinc layer in the corrosive environment of the oil tank bottom plate progresses as local corrosion, even though most of the zinc sprayed layer is left as a whole, the localized sprayed layer is localized. It was found that the corrosion of the zinc sprayed layer progressed and the surface of the base steel sheet was exposed to the corrosive environment at an early stage.

Furthermore, in an actual corrosive environment of a bottom plate of an oil tank, there are many cases where the period from the exposure of the surface of the base steel to the start of the corrosion of the base steel is extremely short. It was found by field survey and analysis. In general, when metallic zinc is present around the exposed part of the base steel in an environment containing a high concentration of salt such as seawater, if zinc develops a sacrificial anti-corrosion effect and prevents corrosion of the base steel for a long period of time, Although it can be considered, in a corrosive environment such as a petroleum tank bottom plate, as described above, there are many moist environments that are generally classified as non-corrosive environments containing extremely low concentration of salt, and therefore the salt content is low. Because there are many cases where the zinc sprayed layer has a very low conductivity in the wet environment and the sacrificial corrosion protection effect on the exposed portion of the base steel is difficult to occur, the zinc sprayed steel plate is used for the oil tank bottom plate. Also found that the anticorrosion effect exhibited a large variation.

Based on the above findings, the present inventors have studied various means for improving the stability of the anticorrosion performance of a zinc-sprayed steel sheet in the corrosive environment. As a result, by making the conventional zinc sprayed layer a zinc magnesium alloy sprayed layer containing 0.05 to 20% of Mg based on zinc,
Even in a corrosive environment such as a petroleum tank bottom plate where the conductivity of a humid environment is extremely low due to a low salt concentration, even when an oxygen concentration cell is formed, the local corrosion of the sprayed layer can be extremely reduced, that is, The uniformity of corrosion wear of the sprayed layer is greatly improved compared to zinc, and even if the base steel surface is exposed due to partial corrosion wear of the sprayed layer, the base steel exposed part It was found that the sacrificial anti-corrosion effect could be sufficiently ensured, and it was confirmed that the effect was surely provided not only in the laboratory but also in the actual corrosive environment of the oil tank bottom plate.

On the other hand, welding construction is indispensable for building an actual oil tank or the like. That is, it is necessary that a sprayed steel sheet having excellent corrosion protection performance in a corrosive environment such as the bottom plate of an actual oil tank described above can be used without reducing the corrosion protection performance even after welding. The inventors re-examined the corrosion resistance of the welded portion of the zinc-sprayed steel sheet, which was conventionally considered to be effective for the anti-corrosion performance, and in the case of the zinc-sprayed steel sheet, the effect of the welding heat when performing the welding work with normal heat input In the part, an alloyed layer is generated at the interface between the base steel and the zinc sprayed layer, and the surface metal zinc disappears at the same time as zinc oxide is generated by reacting with oxygen present in the environment on the free surface of the zinc sprayed surface It has been found that such a phenomenon occurs, and as a result, a significant decrease in the anticorrosion performance occurs. In order to secure the anticorrosion performance by suppressing the formation of such an alloyed layer or the formation of a surface oxide layer, a reduction in welding heat input has the most direct effect, but a reduction in welding efficiency is inevitable. However, this is not a preferable solution in view of practicality.

The suppression of the formation of an alloyed layer or the formation of a surface oxide layer in the heat-affected zone of the welding can be achieved by reducing the corrosion efficiency and the workability without lowering the work efficiency unlike the means such as reducing the heat input. The inventors conducted various studies and experiments on means for achieving compatibility from a completely different viewpoint from the corrosion resistance described above. As a result, in the sprayed steel sheet, by adding Mg to the zinc sprayed layer, the generation of the iron-zinc alloy layer generated at the ground iron interface is almost completely suppressed, and furthermore, the generation of the surface oxide layer is significantly suppressed. In addition, it was found that the porosity of the sprayed layer had a great influence on the suppression of formation of these harmful layers.

The reason for the effect of the addition of Mg to suppress the formation of the alloy layer and to prevent the oxidation of the surface layer is not necessarily clear, but the effect of the porosity is as follows. When a non-corrosive layer is formed, even when Mg is added, the alloy layer and the surface oxide layer clearly appear in the weld heat affected zone, whereas the porosity is 0.02% or more. As described above, the formation of the alloy layer and the suppression of the formation of the surface oxide layer were achieved in the thermal sprayed coating of No. 5, and it was confirmed that these effects were almost perfect when the porosity was 0.05% or more. Was. That is, the reason why the above-mentioned effect is brought about is that, in addition to the effect of Mg, the propagation of heat at the time of welding when pores are present in the sprayed layer at a certain ratio or more is greatly suppressed as compared with the case where there are no pores. It is considered that effects are brought about and the effects are achieved by combining these effects.

As described above, the addition of Mg to the sprayed zinc layer not only improves the corrosion resistance in the corrosive environment of the oil tank bottom plate, but also affects the welding heat when the oil tank bottom plate is constructed by welding. It has been newly discovered that the effect of remarkably improving the deterioration of the corrosion resistance of the part is obtained, and the present invention has been made based on these new findings.

The inventors conducted further studies in order to further improve the effects of the invention based on the above findings. As a result, by adding Mg to the sprayed zinc layer and then adding Al to the sprayed metal, the corrosion resistance of the sprayed layer is further improved, and the welding heat input without deteriorating the corrosion resistance of the weld heat affected zone. They have found that the range can be further expanded, and have completed the present invention.

The reasons for limiting the range of each component in the present invention are described below. Mg: As described above, the addition of Mg to the zinc sprayed layer is as follows.
For example, in an environment where the conductivity of a humid environment is extremely low due to a low salt concentration, for example, in a corrosive environment of an oil tank bottom plate, even if an oxygen concentration cell is formed, the local corrosion of the sprayed layer can be extremely reduced, that is, zinc The uniformity of corrosion has been greatly improved compared to the above, and the sacrificial anti-corrosion effect has been exerted on the exposed part of the base steel even when the base steel surface is exposed due to partial corrosion wear of the sprayed layer. This is effective in improving the corrosion resistance, which is sufficient. 0.05 added
If the amount is less than 20%, the effect of improving local corrosion is small. If the amount exceeds 20%, the effect is not only saturated but also impairs economic efficiency.

Further, the addition of Mg to the zinc sprayed layer is as follows:
For example, when the bottom plate of an oil tank is constructed by welding, an effect of remarkably improving the corrosion resistance deterioration of the heat affected zone is obtained.
%, The effect saturates. From the above results, the amount added to the zinc sprayed layer is limited to 0.05 to 20%. At the same time, the addition of Mg also has the effect of improving the corrosion resistance of the thermal sprayed layer itself, and from the viewpoint of simultaneously improving the corrosion resistance, the addition amount is more preferably 0.5% or more and less than 20%.

The sprayed steel sheet of the present invention has the above-mentioned composition range as a basic requirement, but may further contain Al according to the purpose. By adding Al to the zinc sprayed layer to which 0.05% to 20% of Mg is added, the corrosion resistance of the sprayed layer is further improved, and the welding heat input range can be reduced without deteriorating the corrosion resistance of the weld heat affected zone. Can be further expanded.

By adding 0.05% or more of Al,
The corrosion resistance of the thermal sprayed layer is improved without impairing the uniformity of the corrosion wear of the thermal sprayed layer. However, when the content exceeds 55%, the corrosion resistance is improved, but the corrosion and wear of the thermal sprayed layer is localized, which is not preferable. Further, the addition of Al increases the upper limit of welding heat input, that is, suppresses the formation of an alloy layer with the ground iron interface in the heat affected zone and the formation of an oxide layer on the free surface. %, The effect is not recognized, and even if it exceeds 55%, the effect is saturated. Therefore, from the viewpoint of achieving both corrosion resistance and weldability, the range of addition is limited to 0.05% or more and 55% or less.

The thickness of the sprayed layer can be arbitrarily determined according to the anticorrosion period and the use environment. If the film thickness is less than 10 μm, it is difficult to ensure uniformity of the sprayed film thickness, and even if Mg is added in an amount of 0.05% or more to increase the uniformity of corrosion and abrasion of the sprayed layer, variation occurs in the anticorrosion effect. In some cases, the minimum film thickness is limited to 10 μm or more. Further, any thermal spraying method may be used as the thermal spraying method, including general flame thermal spraying.

When the steel of the present invention is manufactured and used, the composition of the base steel is not particularly limited. For example, after being manufactured as a steel ingot, the steel sheet or the bar is subjected to hot rolling, forging, cold rolling or wire drawing. Any shape such as wire, mold steel, sheet pile, etc. is formed into a predetermined shape by pressing or cutting, etc.
It may be manufactured as a product by applying thermal spraying after welding, or by applying a thermal spraying after performing secondary processing and welding etc. after first forming a steel sheet into a steel pipe shape such as an electric resistance welded steel pipe etc. It is also possible to select an optimum product manufacturing process depending on costs and restrictions on existing manufacturing facilities, including other processes, and it is only necessary that the steel of the present invention can be manufactured regardless of which manufacturing process is selected.

Further, the steel of the present invention does not deviate from the scope of the present invention even when any surface treatment is performed and used in combination before and after the process of spray coating and tank construction.
The steel of the present invention can be used in an environment where a tank can be installed, that is, an asphalt environment, a concrete environment, or the like, in addition to a soil corrosion environment including low-concentration chlorides such as an oil tank foundation adjacent to a beach having a concrete shield and soil near a river. It can be used in dew-corrosion environments containing low-concentration salts, and can be applied in the above-mentioned environments containing high-concentration salts where Zn spraying or the like is conventionally used, or in various corrosive environments where these corrosive environments are combined. it can.

[0025]

Embodiments of the present invention will be described below. (1) Preparation of Sample After making into a steel ingot by a usual smelting and steel ingot manufacturing process, hot rolling was performed to form a steel plate having a thickness of 9 mm, which was used as a sample base steel plate. After the surface of the steel sheet was subjected to the sandblast treatment, a thermal sprayed steel sheet was prepared by forming a film under the conditions shown in Tables 1 and 2 on one side of the base steel sheet by flame spraying and subjected to the following evaluation.

[0026]

[Table 1]

[0027]

[Table 2]

(2) Evaluation of Welded Area Fillet welding was performed under the conditions shown in Tables 1 and 2 by using a sample in which a sprayed coating layer shown in Tables 1 and 2 was formed to a thickness of 150 μm. Cut the heat-affected zone of the sprayed heat-affected zone to determine the thickness of the iron-zinc alloy layer at the interface between the sprayed layer and the ground iron, the thickness of the oxide layer on the free surface of the sprayed layer, and the extent of dissolution and disappearance of the sprayed layer. The cross section taken out was evaluated by observation with an optical microscope and surface analysis by EPMA.

The results are shown in Tables 1 and 2. In the table, ◎ indicates that at least one of the iron-zinc alloy layer at the interface between the thermal spray layer and the ground iron, the oxide layer on the free surface of the thermal spray layer, or the dissolution and disappearance of the thermal spray layer occurs, and the maximum thickness thereof is 2.5 μm or less. Thing, ○
Indicates that it was 5 μm or less, Δ indicates that it was 15 μm or less, and X indicates that it was 45 μm or more. No. of the present invention In the case of the thermal sprayed steel sheets 1 to 23, the formation of an iron-zinc alloy layer at the interface between the thermal spray layer and the ground iron, the formation of an oxide layer on the free surface of the thermal spray layer, and the occurrence of dissolution and disappearance of the thermal spray layer are hardly recognized. No. In the pure Zn sprayed steel sheets shown in Nos. 24 to 26, the generation of an iron-zinc alloy layer at the interface between the thermal spray layer and the ground iron, the generation of an oxide layer on the free surface of the thermal spray layer, and the occurrence of dissolution and disappearance of the thermal spray layer are remarkably observed. The effect of the present invention is clear.

(3) Evaluation of Corrosion Resistance Corrosion tests in a laboratory and an actual oil tank installation environment were performed on the sprayed steel sheet including the welding heat effect shown in Table 1 and the sprayed steel sheet not including the welding heat effect shown in Table 2. Carried out. The thermal sprayed steel sheets shown in Tables 1 and 2 were cut out to a size of 50 × 70 mm, and the entire surface other than the thermal spray layer surface or the thermal spray layer surface including the thermal effect was coated with an epoxy resin and subjected to a corrosion test. In the laboratory evaluation, a burial test and a spray test were performed assuming an environment containing low and high concentrations of salt.
An environment containing a low concentration of salt was created based on a solution containing a chloride ion concentration of 30 ppm by weight. On the other hand, in an environment containing a high concentration of salt, the chloride ion concentration is 2% by weight.
It was prepared based on a solution containing 000 ppm.

In the burial test, these solutions were dried in masago to cause the most severe local corrosion by the oxygen concentration cell.
wt. 2% by embedding the sample in sand mixed at
The test was carried out by storing in a constant temperature room at 5 ° C. The present test simulates an actual oil tank installed on a masago foundation, and the present inventors consider that one year under low-concentration salt test conditions in this test is equivalent to about 2.5 years of actual conditions. It has been clarified. The burial test period in the laboratory was 765 days. The results are shown in Tables 1 and 2.

On the other hand, the spray test was a test in which a solution having the above-mentioned salt concentration was sprayed while being kept at 30 ° C. in a salt spray tester specified by JIS. In this test, it has been clarified by the inventors that one year in the lab corresponds to about 3.7 years of the actual condition when a low concentration salt is used. The spray test period in the lab was 720 days. The results are shown in Tables 1 and 2.
Are also shown.

Further, in addition to the above-described laboratory test, a similar sample was embedded in an actual oil tank bottom plate installation environment, and the corrosion resistance in an actual environment was evaluated. When burying,
Assuming an oil tank bottom plate, two types were used: masago foundation soil and asphalt foundation. Further, at the time of embedding the sample, a waterproof seal was applied to portions other than the soil surface so as to prevent moisture from entering from the surface other than the soil surface under the same conditions as the actual bottom plate. The actual burial test period was 10 years. The results are shown in Tables 1 and 2.

In the above corrosion test, evaluation was made based on the presence or absence of appearance rust caused by corrosion of the base steel after the corrosion test. O: No occurrence of red rust due to corrosion of the base steel was observed. O: Red rust occurred in a portion having an area ratio of more than 20% with respect to the test area. Are indicated by x. As is clear from the results of Tables 1 and 2, the sprayed steel No. of the present invention. Nos. 1 to 23 and Nos. Nos. 27 to 49 show no corrosion and excellent corrosion resistance in any of the tests. 24 to 26 and No. 2; 50 ~
In the comparative sprayed steel of No. 52, the generation of red rust was observed partially or entirely, and the effect of the present invention is clear.

[0035]

As described above, by using the sprayed steel sheet of the present invention as a material for a tank, excellent corrosion resistance and good workability in an installation environment can be obtained. Therefore, it can be said that the industrial value of the present invention is extremely high.

 ──────────────────────────────────────────────────の Continued on the front page (72) Inventor Kenji Kato 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corporation Technology Development Division (72) Inventor Keiichi Hattori 2-6-3 Otemachi, Chiyoda-ku, Tokyo Nippon Steel Corporation (72) Inventor Takashi Yashiki 11 Minamihama-cho, Chita City, Aichi Prefecture (72) Inventor Yuho Ishimoto 37-24 Nittacho, Chuo-ku, Chiba City, Chiba Prefecture F-term (reference) 3E070 AA03 AB03 BA02 BB02 DA01

Claims (2)

[Claims] 1. A tank containing, by weight, 0.05 to 20% of Mg and having a thermal spray coating layer composed of a balance of Zn and an unavoidable impurity layer with a thickness of 10 μm or more on the surface. Sprayed steel sheet. 2. The thermal spray coating according to claim 1, wherein the weight percentage of Al: 0.0
The sprayed steel sheet for a tank according to claim 1, further comprising 5 to 55%.