JPH05287555A - Surface-treated steel sheet excellent in coating suitability and its production - Google Patents

Abstract

PURPOSE:To produce a surface-treated steel sheet excellent in coating suitability over a long period by cathodizing a galvanized steel sheet in a soln. contg. chromic anhydride and an anion and then treating the steel sheet with a colloidal silica-contg. soln. CONSTITUTION:A galvanized steel sheet is cathodized in a soln. contg. chromic anhydride or a dichromic acid and an anion with the sheet as the cathode. Sulfate, chlorine, nitrate and fluorine ions are preferably used as the anion. Consequently, a metallic chromium layer is formed on the plating layer and a layer consisting of a chromium oxide contg. Cr<6+> and consisting essentially of Cr and the hydrous oxide thereon. The treated steel sheet is not dried but treated with a colloidal silica-contg. soln. A silica layer is formed on a chromate layer in this way. An electrolytically chromated galvanized steel sheet is obtained with the coating suitability not deteriorated even when worked under severe conditions or even if stored under various conditions for a long period.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface-treated steel sheet having excellent coatability and a method for producing the same, and more particularly, to a surface-treated steel sheet which has undergone severe processing and is stored for a long period of time under various conditions. TECHNICAL FIELD The present invention relates to an electrolytic chromate-treated zinc-based plated steel sheet which does not deteriorate in paintability and a manufacturing method thereof.

[0002]

2. Description of the Related Art It has been widely practiced in the past to subject a steel sheet to zinc plating or zinc-based alloy plating and then subject the surface thereof to a chromate treatment for the purpose of preventing corrosion of the steel sheet. Such chromate-treated zinc-based plated steel sheets are
It is used in large quantities in a wide range of fields such as home appliances, automobiles, and building materials.

However, in recent years, the demand for such chromate-treated zinc-based plated steel sheets is becoming more and more severe, and not only as a simple primary anticorrosion technique, but also capable of withstanding long-term use without painting. It is required to have high corrosion resistance, coatability with good coating adhesion even without pretreatment such as phosphate treatment, and further to have these two properties in combination.

Conventionally, the chromate treatment generally performed is classified into three types: coating type, reaction type and electrolytic type.
Among these, it is not so much affected by the difference in reactivity of the base steel sheet, it can be processed in a short time corresponding to the speeding up of the line, and the amount of coating is easy to control and uniform. Conventionally, electrolytic chromate treatment has been widely adopted because it can be treated. However, the coating obtained by this electrolytic chromate treatment is mainly composed of Cr ^{3+ and} has a small amount of Cr ⁶⁺ , so that the coating properties are better than those obtained by the coating type and reactive type chromate treatments. Although excellent, it is said to have poor corrosion resistance.

Therefore, in order to produce a chromate-treated zinc-plated steel sheet having both corrosion resistance and paintability, a conventional method is, for example, a method of subjecting a zinc-plated steel sheet to cathodic electrolytic treatment in a bath containing cationic colloidal silica. (Japanese Patent Publication 6
1-54800) or a method of subjecting a zinc-plated steel sheet to cathodic electrolytic treatment in a bath containing colloidal silica, alumina sol and fluoride (JP-A-62-278298).
Etc. have been proposed.

However, the surface-treated steel sheet obtained by these methods also has a coating film that peels off when subjected to severe processing, or is stored for a long period of time depending on storage conditions. There is still a problem with the paintability that is originally said to be excellent, such as deterioration in paintability later. Furthermore, when silica is mixed in the electrolytic bath, the pH of the bath
However, there still remains a manufacturing problem that it is difficult to control the bath, such as agglomeration of particles.

[0007]

DISCLOSURE OF THE INVENTION The present invention has been made to solve the above-mentioned problems in the conventional chromate-treated zinc-based plated steel sheet and its production. An object of the present invention is to provide a chromate-treated zinc-based plated steel sheet and a method for producing such a surface-treated steel sheet, in which paintability does not deteriorate even after being stored for a long period of time under various storage conditions. To do.

[0008]

A surface-treated steel sheet having excellent coatability according to the present invention has a metallic chromium layer on a zinc-based plating layer, and is composed mainly of Cr ³⁺ and has Cr ^{6 +.} It is characterized in that it has a layer made of chromium oxide containing ⁺ and a hydrated oxide, and further has a silica layer thereon. As shown in FIG. 1, a surface-treated steel sheet having excellent coatability according to the present invention has a metallic chromium layer 2 on a zinc-based plated steel sheet 1 which is an original plate, on which a Cr ³⁺ main component is mainly formed. , Cr ⁶⁺ containing chromium oxide and a hydrated oxide layer 3, and a silica layer 4 thereon.

The zinc-based plated steel sheet 1 has been widely used in the past for the purpose of improving the corrosion resistance of the steel sheet by subjecting the base steel sheet to zinc plating having a sacrificial anticorrosion ability. It is not particularly limited. Therefore, in the present invention, the zinc-plated steel sheet is not particularly limited, and in addition to pure zinc plating, various zinc alloy plated steel sheets such as zinc-nickel, zinc-iron, zinc-aluminum, etc. Including. Further, as a method for producing these zinc-based plated steel sheets, an electric plating method, a fusion plating method,
The vapor deposition method and the like can be mentioned, but the method is not particularly limited thereto.

The metal chromium layer 2 is mainly composed of a zinc-based plating layer as an underlying layer and an upper Cr ³⁺ layer, and is adhered to a chromium oxide and hydrated oxide layer containing Cr ⁶⁺ to form an underlying layer. It is important for maintaining the adhesion between the zinc plating layer and the chromate film. The amount of adhesion is not particularly limited, but if it is too large, the chromium oxide and hydrated oxide layers in the upper layer become thin, which is not preferable.
It is desirable to be / 5 or less.

The chromium oxide and hydrated oxide layer 3 has excellent adhesion to the upper layer and, at the same time, improves the corrosion resistance by the self-repairing action of the coating film by Cr ⁶⁺ . The Cr deposition amount is not particularly limited, but it is necessary to be at least 5 mg / m ² and preferably 10 mg / m ² or more in order to effectively exert the effect of improving the corrosion resistance and the coating property. The upper limit of the Cr adhesion amount is not limited at all, but there is also a problem of manufacturing cost, and even if the Cr adhesion amount is too large, the corrosion resistance is not improved in proportion to it, and also, Since the chromate film itself may undergo cohesive failure, the coatability may deteriorate.
Less than mg / m ² is good.

The amount of Cr ⁶⁺ in the chromium oxide and hydrated oxide coatings is not particularly limited either, but normally, the total Cr content is
It is preferably in the range of 0.002-0.2 based on the amount. When the lower limit of the Cr ⁶⁺ content in the chromium oxide and hydrated oxide coating is less than 0.002 with respect to the total Cr content, the self-repairing action is weak and improvement in corrosion resistance cannot be expected. When it exceeds 0.2, not only the coloring of the coating becomes remarkable but also the coating adhesion may be impaired.

In the surface-treated steel sheet according to the present invention, one important feature is to have the silica layer 4 on the surface of the chromate film. It has been conventionally known that chromate-treated zinc-based plated steel sheets in which silica is co-deposited in the coating have superior coatability and corrosion resistance compared to chromate-treated zinc-based plated steel sheets that do not contain silica. .. However, the present inventors have found that even a chromate-treated zinc-based plated steel sheet in which silica is codeposited in a coating film is stored for a long period of time under severe processing or under various environments. In some cases, it has been found that the paintability may be significantly deteriorated.

The inventors of the present invention have conducted extensive studies to solve such problems, and as a result, in a chromate-treated zinc-based plated steel sheet having a silica layer on the surface of the coating, chromate obtained by co-depositing silica in the coating. It has been found that it has the same corrosion resistance as the treated zinc-based plated steel sheet, and it can maintain excellent paintability even when subjected to severe processing or when it is stored for a long time in various environments. It was

When a chromate-treated zinc-based plated steel sheet having a silica layer on the surface of the coating is subjected to severe processing,
The reason why the excellent paintability is maintained even when stored for a long period of time under various environments is not always clear, but it is considered as follows. That is, as schematically shown in FIG. 2, in a chromate-treated zinc-based plated steel sheet in which silica is co-deposited in an electrolytic chromate coating, a large amount of silica is incorporated into the coating, and therefore, the outermost surface of the coating The state of silica dispersion is not always uniform, and the contact ratio between the polar group of silica on the surface of the coating and the coating becomes low. Even in such a state of the coating, the adhesion between the coating and the coating is maintained when it is used without being processed or stored, but when it is processed or it is used for a long time under various conditions. When stored for a while, the polar group density of silica on the surface of the chromate coating becomes insufficient to maintain good coating adhesion due to the deformation and modification of the coating.

On the other hand, the chromate-treated zinc-based plated steel sheet having a silica layer on the surface of the coating has a uniform silica layer on the surface of the chromate coating, as shown in FIG. The polar group density is extremely high and the silica layer on the coating surface is uniform even if it is processed or stored under various conditions for a long time and the chromate coating is deformed or modified. And its polar group density is sufficient to maintain good coating adhesion.

In the present invention, the amount of silica deposited is not particularly limited, but at least 5 mg / m ² is required in order to deposit the silica uniformly on the chromate film and effectively exert the effect of improving the coatability. The above is preferable. On the other hand, the upper limit amount is not particularly limited, but even if it is too large, it is not possible to obtain an effect commensurate with it, and in consideration of manufacturing cost and manufacturing stability, Usually, it is desirable that the amount is 100 mg / m ² or less.

The surface-treated steel sheet according to the present invention is a solution containing at least one selected from chromic anhydride and dichromic acid and at least one anion selected from sulfate ion, chloride ion, nitrate ion and fluorine ion. It can be obtained by electrolytically treating the zinc-plated steel sheet with a zinc-based plated steel sheet as a cathode, and then treating the electrolytic chromate-treated zinc-plated steel sheet with a solution containing colloidal silica without drying.

That is, according to the present invention, first, at least one selected from chromic anhydride and dichromic acid and at least one anion selected from sulfate ion, chloride ion, nitrate ion and fluorine ion are contained. The solution is used as an electrolytic bath, and in this bath, a zinc-based plated steel sheet is used as the cathode for electrolytic treatment to form a metallic chromium layer on the zinc plated layer, and Cr ³⁺ is the main component on the layer. Form a chromate film having a chromium oxide layer containing Cr ⁶⁺ and a hydrated oxide layer.

In the present invention, the electrolytic chromate treatment temperature is not particularly limited, but in consideration of easiness of bath temperature control and economical efficiency, practically, it is usually in the range of normal temperature to 70 ° C. Is preferred. The current density is not limited either, but when it exceeds 20 A / dm ² , the current efficiency is lowered and it is economically disadvantageous.
It is preferably A / dm ² or less. The electrolysis time is also
Although not subject to any restriction, a range of 10 seconds or less is usually industrially suitable.

Next, according to the present invention, after the above electrolytic treatment is performed, the electrolytic chromate-treated zinc-based plated steel sheet is treated with a solution containing colloidal silica without being dried. It is possible to obtain a surface-treated steel sheet having excellent coatability. To treat an electrolytic chromate-treated zinc-based plated steel sheet with a solution containing colloidal silica, for example, dip the electrolytic chromate-treated zinc-based plated steel sheet in a solution containing colloidal silica, or electrolytic chromate-treated zinc-based A solution containing colloidal silica may be sprayed onto the plated steel sheet.

According to this method, the reaction between the active chromate layer on the surface of the electrolytically chromate-treated zinc-plated steel sheet and colloidal silica in the liquid results in electrolytic chromate treatment having a uniform silica layer on the surface of the coating film. A zinc-based plated steel sheet can be produced, which cannot be obtained by the method of cathodic electrolysis of a zinc-based plated steel sheet in an electrolytic bath containing colloidal silica, and has a uniform silica layer on the coating surface. An electrolytic chromate-treated zinc-based plated steel sheet can be obtained.

In the present invention, the concentration of the colloidal silica solution is not particularly limited, but usually 1 to 3
It is in the range of 00 g / l. When the concentration is less than 1 g / l, the obtained silica layer becomes non-uniform and the effect of improving the coatability is poor, while when it exceeds 300 g / l, the silica in the bath tends to gel and the solution management is difficult. Difficult and economically disadvantageous.

The colloidal silica solution is not particularly limited unless it has a function of forming a chromate film by bringing it into contact with a steel sheet, and the pH of the solution is not particularly limited, either. It can be used at the pH at which the colloidal silica in the liquid is most stable. Further, the temperature of the colloidal silica solution is not limited at all, but usually it is preferably in the range of room temperature to 70 ° C. for practical use. Further, in the present invention, industrially, the time for treating the electrolytic chromate-treated zinc-based plated steel sheet with the solution containing colloidal silica is usually 10 seconds or less.

[0025]

The present invention will be described below with reference to examples.
The present invention is not limited to these examples. In addition, in the following Examples and Comparative Examples, a zinc-based plated steel sheet was treated, dried at a temperature of 60 ° C. for 10 minutes, and subjected to a paintability test and a corrosion resistance test described below.

Example 1 A galvanized steel sheet (zinc coverage: 20 g / m ² ) cleaned by a known method was subjected to cathodic electrolysis treatment under the following conditions. Electric bath composition CrO ₃ : 50 g / l SO ₄ ²⁻ / CrO ₃ : 0.004 Bath temperature 45 ° C. Electrolysis conditions Current density: 5 A / dm ² Electrolysis time: 2 seconds After that, the steel sheet treated in this way had a silica concentration. 30 g /
It was dipped in a liquid having a temperature of 45 ° C. for 2 seconds.

Example 2 A galvanized steel sheet (zinc coverage: 20 g / m ² ) cleaned by a known method was subjected to cathodic electrolytic treatment under the following conditions. Electric bath composition CrO ₃ : 50 g / l SO ₄ ²⁻ / CrO ₃ : 0.004 Bath temperature 45 ° C. Electrolysis conditions Current density: 5 A / dm ² Electrolysis time: 2 seconds After that, the steel sheet treated in this way had a silica concentration. 100g
/ L and the temperature of 45 ° C. for 2 seconds.

Example 3 A galvanized steel sheet (zinc coverage: 20 g / m ² ) cleaned by a known method was subjected to cathodic electrolysis treatment under the following conditions. Electric bath composition CrO ₃ : 50 g / l SO ₄ ²⁻ / CrO ₃ : 0.004 Bath temperature 45 ° C. Electrolysis conditions Current density: 5 A / dm ² Electrolysis time: 2 seconds After that, the steel sheet treated in this way had a silica concentration. 200 g
/ L and the temperature of 45 ° C. for 2 seconds.

Example 4 A galvanized steel sheet (zinc coverage: 20 g / m ² ) cleaned by a known method was subjected to cathodic electrolysis treatment under the following conditions. Electrolytic bath composition CrO ₃ : 30 g / l SO ₄ ^2- / CrO ₃ : 0.006 Bath temperature: 45 ° C Electrolysis conditions Current density: 5 A / dm ² Electrolysis time: 2 seconds After that, the steel plate treated in this way had a silica concentration. 30 g /
It was dipped in a liquid having a temperature of 45 ° C. for 2 seconds.

Example 5 A galvanized steel sheet (zinc coverage: 90 g / m ² ) cleaned by a known method was subjected to cathodic electrolysis treatment under the following conditions. Electrolytic bath composition CrO ₃ : 50 g / l SO ₄ ^2- / CrO ₃ : 0.004 Bath temperature 45 ° C Electrolysis conditions Current density: 5 A / dm ² Electrolysis time: 6 seconds After this, the steel plate treated in this way was treated with silica. 30 g /
It was dipped in a liquid having a temperature of 45 ° C. for 2 seconds.

Example 6 A galvanized steel sheet (zinc coverage: 20 g / m ² ) cleaned by a known method was subjected to cathodic electrolytic treatment under the following conditions. Electrolytic bath composition CrO ₃ : 50 g / l SO ₄ ²⁻ / CrO ₃ : 0.004 Bath temperature 45 ° C. Electrolysis conditions Current density: 10 A / dm ² Electrolysis time: 2 seconds After that, the steel plate treated in this way had a silica concentration. 30 g /
It was dipped in a liquid having a temperature of 45 ° C. for 2 seconds.

Example 7 A galvanized steel sheet (zinc coverage: 20 g / m ² ) cleaned by a known method was subjected to cathodic electrolytic treatment under the following conditions. Electric bath composition CrO ₃ : 50 g / l SO ₄ ²⁻ / CrO ₃ : 0.004 Bath temperature 45 ° C. Electrolysis conditions Current density: 5 A / dm ² Electrolysis time: 2 seconds After that, the steel sheet treated in this way had a silica concentration. 30 g /
It was dipped in a liquid having a temperature of 45 ° C. for 2 seconds.

Example 8 A galvanized steel sheet (zinc coverage: 90 g / m ² ) cleaned by a known method was subjected to cathodic electrolytic treatment under the following conditions. Electric bath composition CrO ₃ : 50 g / l SO ₄ ²⁻ / CrO ₃ : 0.004 Bath temperature 45 ° C. Electrolysis conditions Current density: 5 A / dm ² Electrolysis time: 2 seconds After that, the steel sheet treated in this way had a silica concentration. 30 g /
It was dipped in a liquid having a temperature of 45 ° C. for 2 seconds.

Example 9 Zinc-nickel plated steel sheet cleaned by a known method (zinc coverage: 20 g / m ² , nickel coverage: 2.2 g / m ² ).
Was subjected to cathodic electrolysis treatment under the following conditions. Electric bath composition CrO ₃ : 50 g / l SO ₄ ²⁻ / CrO ₃ : 0.004 Bath temperature 45 ° C. Electrolysis conditions Current density: 5 A / dm ² Electrolysis time: 2 seconds After that, the steel sheet treated in this way had a silica concentration. 30 g /
It was dipped in a liquid having a temperature of 45 ° C. for 2 seconds.

Comparative Example 1 A galvanized steel sheet (zinc coverage: 20 g / m ² ) cleaned by a known method was subjected to cathodic electrolysis treatment under the following conditions. Electric bath composition CrO ₃ : 50 g / l SO ₄ ^2- / CrO ₃ : 0.004 Bath temperature 45 ° C Electrolysis conditions Current density: 5 A / dm ² Electrolysis time: 2 seconds

Comparative Example 2 A galvanized steel sheet (zinc coverage: 20 g / m ² ) cleaned by a known method was subjected to cathodic electrolytic treatment under the following conditions. Electric bath composition CrO ₃ : 50 g / l SO ₄ ^2- / CrO ₃ : 0.004 SiO ₂ sol: 30 g / l Bath temperature 45 ° C. Electrolysis conditions Current density: 5 A / dm ² Electrolysis time: 2 seconds

Comparative Example 3 A galvanized steel sheet (zinc coverage: 20 g / m ² ) cleaned by a known method was subjected to cathodic electrolysis treatment under the following conditions. Electric bath composition CrO ₃ : 50 g / l SO ₄ ^2- / CrO ₃ : 0.004 SiO ₂ sol: 100 g / l Bath temperature 45 ° C. Electrolysis conditions Current density: 5 A / dm ² Electrolysis time: 2 seconds

Comparative Example 4 A molten zinc plated steel sheet (zinc coverage: 90 g / m ² ) cleaned by a known method was subjected to cathodic electrolysis treatment under the following conditions. Electrolyte bath composition CrO ₃ : 50 g / l SO ₄ ^2- / CrO ₃ : 0.004 SiO ₂ sol: 30 g / l Bath temperature 45 ° C. Electrolysis conditions Current density: 5 A / dm ² Electrolysis time: 2 seconds

Comparative Example 5 A galvanized steel sheet (zinc coverage: 20 g / m ² ) cleaned by a known method was subjected to cathodic electrolysis treatment under the following conditions. Electrolyte bath composition CrO ₃ : 30 g / l SO ₄ ^2- / CrO ₃ : 0.006 SiO ₂ sol: 30 g / l Bath temperature 45 ° C. Electrolysis conditions Current density: 5 A / dm ² Electrolysis time: 2 seconds

Comparative Example 6 A galvanized steel sheet (zinc coverage: 20 g / m ² ) cleaned by a known method was subjected to cathodic electrolysis treatment under the following conditions. Electrolytic bath composition CrO ₃ : 50 g / l SO ₄ ^2- / CrO ₃ : 0.004 SiO ₂ sol: 30 g / l Bath temperature 45 ° C. Electrolysis conditions Current density: 5 A / dm ² Electrolysis time: 6 seconds

Comparative Example 7 A galvanized steel sheet (zinc coverage: 20 g / m ² ) cleaned by a known method was subjected to cathodic electrolysis treatment under the following conditions. Electrolyte bath composition CrO ₃ : 50 g / l SO ₄ ^2- / CrO ₃ : 0.004 SiO ₂ sol: 30 g / l Bath temperature 45 ° C. Electrolysis conditions Current density: 10 A / dm ² Electrolysis time: 2 seconds

Comparative Example 8 A galvanized steel sheet (zinc coverage: 20 g / m ² ) cleaned by a known method was treated with a commercially available phosphate solution.

The electrolytic chromate-treated zinc-based plated steel sheets obtained as described above were tested for corrosion resistance and paintability by the following methods. Paintability test Immediately after preparation of the sample and after storage in an atmosphere at a temperature of 60 ° C. and a relative humidity of 30% for 80 days, an aminoalkyd-based paint was applied by a bar coater and baked to a thickness of 2
A coating film having a thickness of 0 μm was formed and an adhesion test of the coating film was performed.
The adhesion of the coating film is determined by marking the surface of the coating film with a 1 mm x 1 mm grid with a cutter knife, extruding 6 mm with an Erichsen tester, applying an adhesive tape, and then peeling off the tape. The residual rate of the coating film at that time was evaluated. O indicates that the coating film was not peeled off, Δ indicates that the coating film was partially peeled off, and X indicates that most of the coating film was peeled off. Corrosion resistance test According to the method specified in JIS Z-2371,
A salt spray test was conducted to measure the time until the white rust occurrence area ratio reached 1%, and the time was evaluated. That is, 1% white rust occurrence time is 120 hours or more, and Δ is 70
~ 120 hours, x indicates 70 hours or less.

Table 1 shows the results of the paintability test and the corrosion resistance test of the above Examples and Comparative Examples. According to the examples of the present invention, all are excellent in corrosion resistance, coatability immediately after sample preparation and after storage. On the other hand, Comparative Example 1 in which the coating does not contain silica, Comparative Examples 2 to 7 in which silica is co-deposited in the coating, and Comparative Example 8 in which the phosphate treatment is performed.
All of them are inferior in at least one of corrosion resistance, coatability immediately after preparation of the sample and after storage, particularly, coatability after storage.

[0045]

[Table 1]

[0046]

As described above, according to the method of the present invention,
It has a metallic chromium layer on the zinc-based plating layer, and Cr on it.
^{It is} mainly composed of ³⁺ and has a layer consisting of chromium oxide containing Cr ⁶⁺ and hydrated oxide, and further has a silica layer on top of it, so even if it is subjected to severe processing, it can be used under various conditions. It has stable and excellent coatability even after being stored for a long period of time underneath.

[Brief description of drawings]

FIG. 1 is a diagram in which a composition change in a depth direction of a surface-treated steel sheet according to the present invention is analyzed by ESCA.

FIG. 2 is a schematic view showing a chromate-treated zinc-based plated steel sheet in which silica is co-deposited in the coating.

FIG. 3 is a schematic diagram showing a chromate-treated zinc-based plated steel sheet having a uniform silica layer on a chromate film according to the present invention.

[Explanation of Codes] 1 ... Coating film, 2 ... Polar group (hydroxyl group) of silica, 3 ...
Colloidal silica, 4 ... A layer mainly composed of Cr ³⁺ and made of chromium oxide and hydrated oxide containing Cr ⁶⁺ , 5 ... Metal chromium layer, 6 ... Zinc-based plating layer.

Front Page Continuation (72) Inventor Tomio Kajita 1-3-18 Wakihamacho, Chuo-ku, Kobe City Kobe Steel, Ltd. Kobe Head Office

Claims (3)

[Claims] 1. A metallic chromium layer on a zinc-based plating layer,
Cr ³⁺ is the main constituent on top of it, and it has a layer consisting of chromium oxide containing Cr ⁶⁺ and hydrated oxide, and has a silica layer on top of it, which is excellent in coatability. Surface treated steel sheet. 2. An electrolytic chromate-treated zinc is electrolyzed in a solution containing at least one selected from chromic anhydride and dichromic acid and an anion using a zinc-plated steel sheet as a cathode, and then without drying. A method for producing a surface-treated steel sheet having excellent coatability, which comprises treating a steel plate with a base material with a solution containing colloidal silica. 3. The method for producing a surface-treated steel sheet having excellent coatability according to claim 2, wherein the anion is a sulfate ion, a chlorine ion, a nitrate ion or a fluorine ion.