JPH09290212A - Corrosion resistant coated steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the pitting corrosion of a scratch part even if a scratch is generated on a coating film of a steel surface by applying a coating film consisting essentially of a specific ratio of Zn and an organic resin or an inorganic resin and having a specific average film thickness on the steel containing a specific content of Cr, Cu, Mo and Ni. SOLUTION: The coating film consisting essentially of >=20wt.% Zn and the organic resin or the inorganic resin and having >=10μm average film thickness is applied on the steel containing 0.15-5wt.% total of one or more kinds of Cr, Cu, Mo and Ni. As a result, the under part of the coating film is prevented from being made cathodic and excellent pitting corrosion resistance at the scratch part is attained and the steel exhibits excellent performance particularly as a base material of civil engineering structure used in an environment containing many sea salt particles.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an inexpensive corrosion-resistant coated steel material which is expected to prevent corrosion when applied to civil engineering and building structures at sea and land, seawater or freshwater tanks.

[0002]

2. Description of the Related Art Steels containing Cr or Ni to improve corrosion resistance have been known for a long time and have been put to practical use as various stainless steels. Since stainless steel usually contains 13% by weight or more of Cr, which is an expensive element, and the material cost is very high, it is rarely used for civil engineering structures and structural members such as tanks.

On the other hand, as the corrosion resistant steel materials, in addition to stainless steel, low alloy steels which prevent corrosion by the protective rust generated by corrosion are known. These low alloy steels are C
Weather resistant steel containing r, Cu, P, etc. and Cr, Cu,
It is roughly classified into seawater resistant steel containing Mo and the like. Weather-resistant steel exhibits excellent anticorrosion effect in atmospheric environment, and seawater-resistant steel exhibits excellent anticorrosion effect in seawater.

These low alloy steels are cheaper than stainless steels and have better corrosion resistance than ordinary steels, and are therefore often used as structural members.

However, the above low-alloy steel tends to be limited in its application without coating for the following two reasons.

(A) Since a protective rust layer is formed to exert corrosion resistance, the initial corrosion rate until the formation of the rust layer is not much different from that of ordinary steel.

(B) When used in seawater, the effect of promoting corrosion by seawater is great.

Therefore, these low alloy steels are often used by being coated. However, when a flaw is generated in the coated film, there is a problem that the pitting corrosion at the flaw portion of the low alloy steel is remarkably promoted as compared with the ordinary steel, although the alloy content is high.

On the other hand, a coating for preventing corrosion of ordinary steel is
There are extremely many things from simple ones to full-scale heavy-duty anticorrosion coatings. However, as seen in heavy anticorrosion coatings,
Although there is an idea that "the coating will not be flawed" or "the steel will not appear on the surface even if the coating has some flaws", even if the coating is flawed and the steel is exposed to the external environment, there will be holes at the flaws. There has never been the idea that food should not be promoted. Needless to say, it is impossible to prevent the progress of pitting corrosion on the ordinary steel surface of the flaws when the flaws occur in simple coating such as various primer coatings.

[0010]

DISCLOSURE OF THE INVENTION The present invention provides a corrosion-resistant coated steel material in which pitting corrosion of a flaw portion is suppressed even if a flaw is generated in the coating film on the surface of steel by adjusting the composition of the coating film and the steel. For the purpose of provision.

[0011]

DISCLOSURE OF THE INVENTION The gist of the present invention is a corrosion resistant coated steel material comprising the following low alloy steel and coating film.

(1) A corrosion-resistant coated steel material comprising the following steel and the following coating coated on the surface of the steel.

Steel containing 0.15 to 5% by weight in total of one or more of Cr, Cu, Mo and Ni.

A coating containing Zn in an amount of 20% by weight or more and an organic resin or an inorganic resin as main components and having an average film thickness of 10 μm or more.

In the above, the composition of Zn in the coating is% by weight in dry%. Therefore, in the case of a coating material containing a solvent, which will be described later, it is the weight% in the coating in the used state after the steel is coated and dried.

Hereinafter, in the case of a multi-layer coating, the coating which is in direct contact with the steel surface may be referred to as "undercoat layer" to distinguish it from the upper coating of the multi-layer coating. In the case of a multilayer coating, the above (1)
The coating in the above means the undercoat layer, and the dry weight% is the dry weight% in the undercoat layer. The technical background of the above invention will be described below.

1. Pitting corrosion on flaws of coated low alloy steel:
It is generally known that the corrosion of coated steel proceeds by the following electrochemical reaction.

Anode reaction: Fe → Fe ²⁺ + 2e ⁻ Cathode reaction: 1 / 2O ₂ + H ₂ O + 2e ⁻ → 2OH ^{− The} coating that covers the steel surface is generally permeable to water and oxygen, but is a negative ion such as OH ^−. Ions are hard to pass through. For this reason, the cathodic reaction under the coating (steel surface completely covered by the coating) causes the pH due to alkali (OH ⁻ ion) generation.
Is increased, the cathode reaction is suppressed, and the anodic reaction accompanied by elution of Fe atoms paired with the increase is suppressed, and the steel surface is passivated in the presence of corrosion products.
Therefore, this corrosion reaction stops when it has progressed to some extent. This state is the state in which the anticorrosion effect is exhibited by the coating. In order to continue this corrosion reaction, a larger potential difference is required as a driving force.

In the flaw portion of the coated low alloy steel, the electrochemical corrosion in which the undercoating is the cathode reaction and the flaw portion is the anode reaction is apt to be promoted, which promotes pitting corrosion in the flaw portion. In the flaw area, compared to the area under the film, the water content is sufficient and the ions can move easily,
Usually, the pH is constant around 7, and when the anodic reaction occurs selectively, the pH will be decreased opposite to that under the film due to the following reaction or concentration of chloride ion.

Fe ²⁺ + H ₂ O → Fe (OH) ⁺ + H ⁺ At this time, is there a potential difference as a driving force enough to continue the corrosion reaction, and is the potential difference different between ordinary steel and low alloy steel? There are two problems. Therefore, the potentials at pH 7 (assuming the anode site) and pH 11 (assuming the cathode site) were measured for the various steels used in the examples described below.

Table 1 shows the results of measuring the potential difference of these steels.

[0022]

[Table 1]

The electric potential of the low alloy steel containing a certain amount or more of corrosion resistance improving elements such as Cr, Cu, Ni and Mo is higher than that of ordinary steel at pH 11 corresponding to the pH under the coating and is equivalent to the pH of the flaw. It can be seen that at a pH of 7 which is the same as that of ordinary steel, the potential difference between under the coating and the flaw portion increases. As a result of performing this potential measurement, the present inventors have found that due to the expanded potential difference generated between the flaw portion and the undercoat of the low alloy steel, the low alloy steel has more pores in the flaw portion than the ordinary steel. It was the first time I confirmed that food would be accelerated.

2. Method of Preventing Pitting Corrosion at Defects The present inventors have inferred that it is effective to prevent cathodization under the coating as a countermeasure. Therefore, a test was carried out to prevent acceleration of pitting corrosion by including Zn in an amount of 20% by weight or more in the entire coating (single layer) or a multilayer coating, which is in direct contact with the steel surface.

As a result, it was confirmed that pitting corrosion on the flaws was suppressed very clearly. Further, it was confirmed that the effect of the coating film containing Zn was exhibited only by performing it on steel containing a certain range of alloying elements.

The present invention has been completed based on the results obtained by artificially flawing a coated steel material obtained by coating various low alloy steels containing Zn and performing a corrosion acceleration test for several months. .

[0027]

BEST MODE FOR CARRYING OUT THE INVENTION

1. Steel In the present invention, the steel is the total weight% of one or more alloying elements among Cr, Cu, Mo and Ni.
Therefore, 0.15 to 5% is included. In the following,% means% by weight.

Any of Cr, Cu, Mo and Ni has a function of stabilizing the corrosion product film formed on the steel surface and facilitating the formation of protective rust. However, if its content is less than 0.15%, its effect is weak, and if it exceeds 5%, the workability of welding these steel materials is significantly deteriorated. Further, it does not meet the object of the present invention to provide an inexpensive steel material. Therefore, the total content of Cr, Cu, Mo or Ni is 0.15 to 5%.

Of these elements, Cr has a strong effect of forming a passivation film, so that by containing 0.3 to 4.5%, it is possible to more strongly prevent pitting corrosion in flaws. Further, when the total content of Cr, Cu, Mo or Ni is 0.3% or more, similarly, a strong effect of preventing pitting corrosion is exhibited. The total of the rates is preferably 0.3% or more.

There is P as an element having the same function as these. P improves the corrosion resistance, but it significantly deteriorates the weldability. For this reason, it is desirable to set it to 0.15% or less, more preferably 0.05% or less, which is an allowable range that does not significantly deteriorate the weldability.

In addition to the above alloy elements, C, Mn, Si, N
It goes without saying that elements such as b, V, W, Ti, Al, B, and Ca are positively added to improve characteristics other than corrosion resistance, and supplementarily to improve corrosion resistance. You can The desirable ranges of these elements are as follows.

C: C is 0.02% to secure the strength.
The above is desirable, and it is preferably 0.19% or less in order to secure weldability. A more desirable range is 0.04 to 0.
15%.

Mn: Mn is preferably 0.3% or more to secure the toughness and strength, and the weld heat affected zone (HAZ)
In order to secure the toughness of Al, it is desirable to set it to 2% or less. More preferably, it is 0.8 to 1.6%.

Si: Si is 0.0 to obtain a deoxidizing effect.
It is preferable to set the content to 2% or more. On the other hand, if the content exceeds 1%, the toughness of the base material and the HAZ is greatly deteriorated.

Nb: Nb need not be added, but the strength can be improved by adding it, so it is desirable to add it in the case of forming a high strength steel. However, even if it is contained, if it exceeds 0.05%, the surface properties of the continuously cast slab are greatly deteriorated, so the content is preferably 0.05% or less.

V: V need not be added, but V is also Nb
When added in the same manner as above, the strength is improved. Therefore, it is desirable to add it when the strength is high and the surface quality of the continuously cast slab is secured. However, V must be contained in a larger amount in order to obtain the same strength increase as compared with Nb, which causes a cost increase and deteriorates the toughness as compared with Nb. The following is desirable.

W: W may not be added, but it is desirable to add W in order to suppress pitting corrosion more strongly. However, when it is contained, if it exceeds 1%, the weldability is deteriorated, and the strength rises too much and the toughness decreases, so 1% or less is desirable.

Ti: Ti is desirable to be added because a small amount of Ti improves the HAZ toughness and the surface quality of the continuously cast slab. However, if it exceeds 0.05%, not only the base material but also the HAZ toughness deteriorates, so it is desirable to set it to 0.05% or less.

Al: Al remaining in the steel after deoxidation is effective for grain refining, and since solid solution nitrogen is fixed as AlN to improve toughness, it is preferable to add when improving toughness. . However, even in that case, if it exceeds 0.2%, the surface quality of the continuously cast slab is deteriorated, so it is desirable to set it to 0.2% or less.

B: B enhances hardenability and HAZ toughness, so that it may be added in a trace amount. However, if it exceeds 0.005%, the toughness of the base material deteriorates, so even if it is contained, it is preferable to make it 0.005% or less.

Ca: Ca lowers the plastic deformation of sulfide inclusions and prevents the formation of sulfide inclusions stretched in the rolling direction. Therefore, when the rolling anisotropy of mechanical properties is prevented, or It may be added to suppress hydrogen-induced cracking. However, if it exceeds 0.01%, the ductility and toughness of the base material deteriorate, so 0.01% is preferable.

When unavoidable impurities such as S and N are mixed, S:
It is allowed within the range of 0.03% or less and N: 0.01% or less.

2. Coating The coating in the present invention refers to a coating containing zinc (Zn) and an organic resin or an inorganic resin as a main component, and does not include a metal coating formed by plating or the like. The target of Zn is powder. The organic resin or the inorganic resin described later is a binder of Zn powder, and has the purpose of adhering to the steel surface.

Examples of the organic resin include epoxy resin, urethane resin, vinyl resin and polyester resin. Zn
In addition to the powder, a color pigment, an extender pigment, an additive and the like may be included. As the color pigment, red iron oxide, ocher, titanium white, titanium yellow, chromium oxide green, phthalocyanine blue, phthalocyanine green, etc., and body pigments such as talc, mica, silica, calcium carbonate, barium sulfate can be used. . As an additive,
A thixotropic agent, a dispersant, etc. may be added, and in order to improve coatability, it is naturally possible to include an appropriate amount of solvent before coating.

Examples of the inorganic resin coating film include those obtained by adding Zn powder to an alkyl silicate resin. Other additives and the like are the same as the organic resin.

Suitable examples of this coating include an organic zinc primer containing Zn powder in an epoxy resin and an inorganic zinc primer containing Zn powder in an ethyl silicate resin. These coatings can be formed by rust-preventing ordinary steel sheets during storage, or by directly using commercially available coatings which have been coated as an undercoating paint for anticorrosion coatings of ordinary steel sheets.

When these conventional coating materials containing Zn are applied to ordinary steel, the progress of pitting corrosion cannot be suppressed at the flaw portion as shown in the examples described later. As in the present invention, the pitting corrosion of the flaw portion can be suppressed to a practically acceptable level only by applying a coating film containing a certain amount or more of Zn to the steel containing the alloying element in a certain range. In the field of anticorrosion technology, there are so many means that can be used, such as electrocorrosion, so it has never been possible to invent a corrosion-resistant coated steel material as a material that can be expected to have a great ripple effect like the present invention.

The coating may be a single-layer coating or a multi-layer coating formed by repeated coating, but the coating in contact with the steel surface, that is, the undercoat layer, has a dry weight of 20% by weight or more (hereinafter referred to as "weight"). It is necessary to include Zn). This Zn-containing coating needs to have a film thickness of 10 μm or more.
If the Zn content is less than 20% or the film thickness is less than 10 μm, the suppression of cathodization under the coating becomes insufficient and the effect of suppressing pitting corrosion on the flaws is reduced.

Although the upper limit of the film thickness is not particularly set, it is desirable to set the film thickness to 100 μm, which is a sufficient value from the viewpoint of suppressing the coating man-hour. Furthermore, in order to more completely and easily suppress pitting corrosion on flaws, the thickness is preferably 15 to 50 μm.

Although the upper limit of Zn is not particularly set, it is preferably 96% by weight or less considering the film forming property. 96%
If it exceeds, the resin binder that binds the zinc powder becomes insufficient, and the film adhesion is significantly reduced. Further, in order to suppress pitting corrosion in a more severe environment and also to secure high film forming property, it is desirable to set the content to 40 to 92%.

The particle size of the Zn powder is preferably 5 μm or less as an average value of the longest length of each particle in order to secure sufficient film forming property even when the film thickness is about 10 μm. on the other hand,
If the average value of the longest length is less than 0.1 μm, not only the manufacturing cost of Zn powder is increased, but also the viscosity of the coating material is increased and the coating property is deteriorated. Needless to say, it is desirable that these Zn powders are uniformly dispersed in the coating.

In the case of a multi-layered coating, there is no particular limitation on the upper coating which adheres to the coating of the undercoat layer. Epoxy coating, tar epoxy coating, urethane coating, vinyl coating, phthalic acid resin coating, acrylic resin coating, etc. can be used. If weather resistance is required, acrylic urethane resin, acrylic silicone resin, fluorine resin, etc. can be used. It can be coated as a top layer coating.

Prior to coating, it is preferable that the steel surface is thoroughly cleaned by skeletal or blast treatment.
When coating the steel surface contaminated with rust or oil,
It is not preferable because the coating film easily peels off.

[0054]

EXAMPLES The effects of the present invention will be described below with reference to examples.

Table 1 is a list showing the chemical compositions of the low alloy steels of the present invention and comparative examples as described above. These low alloy steels were melted in the laboratory and made into steel plates by forging and hot rolling. From this steel plate, 150 mm length x 70
A test piece material having a width of 3 mm and a thickness of 3 mm was cut out.

Table 2 shows the base resin of the coating material used for coating. Reference symbols A to D are base resins, and reference symbols E and F are base resins for top coating.

[0057]

[Table 2]

Table 3 shows the types of pigments added to the coating material.

[0059]

[Table 3]

The above low alloy steel was coated with a combination of a base resin and a pigment to obtain a coated or uncoated test body,
The coated test piece was cross-cut with a cutter knife so that the flaw could reach the steel surface, and the uncoated one was used for the test as it was.

The test was based on a 90-day exposure test in a salt spray. For uncoated specimens of each low alloy steel,
After the corrosion product was immersed in an aqueous solution of diammonium hydrogen citrate to remove rust, the weight was measured and the weight loss before and after the test was used to measure the corrosion weight loss. The corrosion weight loss was expressed in g / m ² by dividing by the area of the test material. This uncoated specimen (bare material)
Corrosion weight loss is shown in Table 4 and Table 5 below as reference values.
Shown in

The coating film test body was prepared by removing the coating film around the cut with a remover (concentrated alkaline solution), followed by rust removal by the above-mentioned method, and measuring the depth of the deepest corroded portion with a laser microscope. Further, the measured value shows a value obtained by subtracting this value as a blank because the initial flaw depth measured by a laser microscope after the film removal on the sample before the test by the same method was 20 μm.

Tables 4 and 5 are a list showing the results of the above tests together with the conditions of the coating.

[0064]

[Table 4]

[0065]

[Table 5]

In the example of the present invention, in the coated state,
Due to the effect of the Zn-containing coating or undercoat layer, pitting corrosion at the scratched portion is suppressed to a practical level without any problem.

On the other hand, in the comparative example, when the film containing Zn is thin (test numbers: 29, 30, 33, 3).
4, 35, 44 to 57) or a low Zn concentration (test number: 31, 32, 36, 37), cathodization under the coating promotes pitting corrosion in the flaws of the coating. .

The total content of the corrosion resistance improving elements is 0.15.
In test numbers 38 to 43, which is less than%, the pitting corrosion promoting effect by cathodization under the coating is small, but since the corrosion resistance of the steel material itself is poor, the depth of pitting corrosion in the flaw portion is The value reaches three times or more as large as that of the coated steel material of the invention example. Therefore, only by the combination of the steel and the coating of the present invention, the pitting corrosion on the scratched portion can be suppressed to such an extent that there is no practical problem.

[0069]

INDUSTRIAL APPLICABILITY The present invention applies Z to a steel surface having a certain range of composition.
By applying a coating containing 20% or more of n, as a result of suppressing cathodization under the coating, excellent pitting corrosion resistance of flaws can be realized, especially for civil engineering structures in environments with many sea salt particles. It is expected to be used in earnest as a material.

Claims (1)

[Claims] 1. A corrosion-resistant coated steel material comprising the following steel and the following coating coated on the surface of the steel. : One or two of Cr, Cu, Mo and Ni
Steel containing 0.15 to 5% by weight in total of at least one kind. : A film containing 20% by weight or more of Zn and an organic resin or an inorganic resin as main components and having an average film thickness of 10 μm or more.