DINEN锌及锌合金涂层

GERMAN STANDARD July 2008 p t i o n - K N O R R -B R E M S E S y s t e m e f ür S c h i e n e n f a h r z e u g e G m b H - C u s t . n o . 4987428 - S u b s . n o . 00849501/002/001 - 2008-06-24 10:34:55

Metallic coatings –

Electroplated coatings of zinc and zinc alloys on iron or steel with supplementary Cr(VI)-free treatment

Document comsists of 13 pages

Standards Committee on Material Testing (NMP) within DIN

DIN 50979:2008-07

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Contents

Page

Foreword................................................................................................................................................3 1. Field of application.................................................................................................................3 2. Normative references.............................................................................................................3 3. Designation:............................................................................................................................4 3.1. Electroplated coatings...........................................................................................................4 3.2. Passivating .............................................................................................................................4 3.3. Sealing.....................................................................................................................................4 3.4. Essential areas (functional area)..........................................................................................5 3.5. Examples of designations.....................................................................................................5 4. Order data................................................................................................................................5 5. Base materials.........................................................................................................................6 6. Coating methods / Process technology...............................................................................6 6.1. Pretreatment and deposition of the zinc or zinc alloy coating..........................................6 6.2. Post-treatments......................................................................................................................6 6.2.1. Passivations ...........................................................................................................................6 6.2.2. Seals........................................................................................................................................7 6.3. Drum/Trestle (parts handling)...............................................................................................7 6.3.1. Drum parts..............................................................................................................................7 6.3.2. Trestle parts............................................................................................................................7 6.4. Hydrogen embrittlement........................................................................................................7 6.4.1. Basics......................................................................................................................................7 6.4.2. Method selection....................................................................................................................8 6.4.2.1. Materials with strengths < 1 000 N/mm 2...............................................................................8 6.4.2.2. Materials with strengths ≥ 1 000 N/mm 2.............................................................................8 7. Requirements for the coatings and test methods...............................................................9 7.1. Coat thickness........................................................................................................................9 7.2. Layer adhesion.......................................................................................................................9 7.3. Cr(VI) absence......................................................................................................................10 7.4. Resistance in short-term corrosion tests..........................................................................10 7.4.1. General..................................................................................................................................10 7.4.2. Minimum resistance of passivated zinc or zinc alloy coatings.......................................10 8. Test report .............................................................................................................................12 8.1. General information.............................................................................................................12 8.2. Special data for coating high-strength materials with

tensile strength ≥ 1 000 N/mm 2.........................................................................................12 8.3. Test results (12)

References (13)

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Foreword

This document has been formulated by the working committee NA 062-01-76 AA “Electroplated coatings” of the Standards Committee for Material Testing (NMP).

1. Field of application

This standard applies for electrodeposited and Cr(VI)-free passivated zinc coatings and zinc alloy coatings on iron materials. The zinc alloy coatings contain nickel or iron (zinc/nickel, zinc/iron) as alloy components.

The main purpose in the application of the coatings or coating systems is to protect components made from ferrous materials against corrosion.

This standard defines the designations for the coating systems indicated above and specifies minimum corrosion activities in the described test methods as well as the minimum layer thickness necessary for this.

2. Normative references

The documents cited below are necessary for the application of this document. In the case of dated references, only the edition referred to applies. In the case of undated references, the last edition of the document (including all amendments) referred to applies.

E DIN 50969-1:2008-02, Prevention of hydrogen-induced brittle fractures caused during production in high-strength steel components – Part 1: Preventative measures 1)

DIN EN 1403, Corrosion protection of metals – Electrodeposited coatings – Method of specifying general requirements DIN EN 15205, Determination of hexavalent chromium in corrosion protection layers – Qualitative analysis

DIN EN ISO 3497, Metallic coatings – Measurement of coating thickness – X-ray spectrometric methods DIN EN ISO 9227, Corrosion tests in artificial atmospheres – Salt spray tests

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3. Designation:

3.1. Electroplated coatings

The electroplated coatings consists of zinc or zinc alloys corresponding to Table 1.

Table 1 – Designation of the electroplated coatings

Code Definition Zn Zinc coating without added alloy partner

ZnFe Zinc alloy coating with a percent by weight of 0.3 % to 1.0 % iron ZnNi

Zinc alloy coating with a percent by weight of 12 % to 16 % nickel

3.2. Passivating

Passivating designates the production of conversion layers by treatment with suitable Cr(VI)-free solutions in order to improve the corrosion resistance of the coatings. Colourations are possible.

As chromium (VI)-free passivations are new systems, a new nomenclature according to Table 2 has been adopted.

Table 2 – Passivations

3.3. Sealing

Seals increase the corrosion resistance and usually have a layer thickness up to 2 μm. Seals consist of Cr(VI)-free organic and/or inorganic compounds.

Products that can be removed with cold cleaner (e.g. based on oil, grease, wax) are not considered as a seal within the scope of this standard.

The effect of seals on the functional properties of the component, e.g. transition resistance, weldability, compatibility with working substance, bonded joints, are to be evaluated on a component-specific basis.

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If there are special requirements for the surface functionality, the use of the seal as well as the type of sealing agent must be agreed, as the range of potential surface modifications through sealing is large. REMARK The interference colours formed through passivation are usually also remedied through sealing.

Table 3 – Seals

Code Description T0 Without seal T2 With seal

3.4. Essential areas (functional area)

In the case of components with complex shapes, in particular components with hollow spaces, it is

possible that the requirements for the resistance in the short-term corrosion tests and for the minimum thickness cannot be complied with in all areas of the electroplated surface. In these cases, the areas essential for the surface protection must be marked with a dot-dash line on the drawing.

If no essential area is specified by the customer, the definition according to DIN EN 1403 applies.

3.5. Examples of designations

Designation for a zinc/nickel alloy coating on a component made from steel (Fe), a smallest local layer thickness of 8 μm (8) and iridescent passivated (Cn):

Eletroplated coating DIN 50979 – Fe//ZnNi8//Cn//T0

Designation for a zinc/iron alloy coating on a component made from steel (Fe), a smallest local layer thickness of 8 μm (8) and black passivated (Fn) and sealed:

Eletroplated coating DIN 50979 – Fe//ZnFe8//Fn//T2

For further information on the designation, see DIN EN 1403.

4. Order data

The customer must provide at least the following information to the coating company: a) Component strengths (with consideration of 6.4);

b) Data on the component: Basic material, component manufacturing process, heat treatments; c) Data on the essential areas in conjunction with 3.4; d) Designation of the coating to be applied (see 3.5).

If required, more detailed requirements for the coating properties and testing (e.g. appearance, sliding properties, media resistance) can be specified.

If necessary, additional information on requirements or restrictions for the coating process can be given.

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5. Base materials

The coating of components from low-alloy steels with coatings according to this standard is state-of-the-art. If other iron-based materials are present (e.g. cast or sintered materials, materials with distinct components of passivation-friendly alloy elements or also materials with special strength properties), it may be necessary to specially adapt the treatment processes (pretreatment, coating, post-treatment) and, if need be, implement additional measures in order to comply with the requirements of this standard. The coating company therefore needs to have detailed information on the composition, properties and production process of the components to be coated.

When coating high-strength steel parts with a tensile strength ≥ 1 000 N/mm2, the preproduction (e.g. material selection, hardening, joining) has to be carried out in such way that damage due to delayed hydrogen-induced brittle fractures is eliminated with a high degree of certainty.

The components to be coated must not exhibit any material, processing or surface faults which can affect the corrosion protection and/or the appearance of the coatings in an adverse or unexpected manner.

The impurities (corrosion products or scale, oil, grease, dirt etc.) occurring on the surfaces of the parts to be treated must be able to be removed in the cleaning and pretreatment processes normally utilised. An agreement concerning the surface quality might be necessary, if applicable.

6. Coating methods / Process technology

6.1. Pretreatment and deposition of the zinc or zinc alloy coating

In order to ensure a reliable process sequence, the complete pretreatment and coating process, physical data (treatment times, temperatures) as well as all process chemicals must be recorded, documented and optimised if need be. The individual process intervention limits as well as the frequency of the monitoring and analysis processes must be defined. The resultant measures must be described and archived by the coating company.

A typical process sequence is shown below: a) Alkaline degreasing (coordinated in line with the existing oil/grease-based surface films); b) Pickling (usually HCl, inhibited); c) Alkaline electrolytic degreasing (preferably anodic); d) Metal deposition; e) Post-treatment through passivation and, if necessary, sealing; f) Drying.

6.2. Post-treatments

6.2.1. Passivations

Passivations are conversion coatings and are created by immersing or spraying the components with passivation solutions. At the same time, the deposited coating reacts with the passivation solution to form a thin film protecting the metallic coating. Part of the coating is usually dissolved by the reaction.

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6.2.2. Seals

Additional organic and/or inorganic substances are applied onto or embedded in the passivation during the sealing.

Layer accumulations can result, depending on the component geometry and process. These must be minimised, if possible, through suitable measures (e.g. blowing-off for trestle parts, movement of drum parts).

6.3. Drum/Trestle (parts handling)

6.3.1. Drum parts

Typical drum parts are bolts, nuts and other small components. The components are introduced into the coating drums as bulk material then pretreated and provided with the coating while the drum is rotating. The drum rotation ensures that all components are coated comparably. However, surface damage can result due to the movement of the parts. It is possible to minimise the damage e.g. through reduced drum rotation or lower drop heights when emptying the drum. Nevertheless, drum coatings usually yield a lower corrosion resistance than is the case with trestle coatings. 6.3.2. Trestle parts This involves parts which have to be coated on the trestle owing to their size, design or, possibly, special requirements. During this, the parts are coated while positioned on trestles. Depending on the position of the components on the trestle, different layer properties (mainly layer thickness of the metallic coating) can result Optimisation is possible, for example, by using component-specific trestles.

6.4. Hydrogen embrittlement

6.4.1. Basics

The steel parts to be coated can absorb hydrogen during the electroplating treatment for creating coatings according to this standard, e.g. during pickling, electrolytic cleaning and during the electroplating metal deposition. Active hydrogen diffused in the metal lattice preferably at energetically favourable areas (lattice structural faults, areas of high stress concentration).

Hydrogen-induced, delayed brittle fractures can arise from this, while the critical interaction of: – material and material state (strength, hardness); – hydrogen absorption during the pretreatment and coating process; – mechanical parts stress, also locally depending on the design of the parts. have to be taken into account in particular.

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Figure 1 – Interaction of material, mechanical stress and hydrogen absorption

The critical parameters for the material are the tensile strength and toughness. The hazard due to hydrogen increases as the strength increases.

All steel parts with a tensile strength of R m ≥ 1 000 N/mm 2 (also locally restricted, e.g. for case-hardened or cold-formed joints or in weld seam areas) are deemed to be high-strength and are classified as critical.

6.4.2. Method selection

6.4.2.1. Materials with strengths < 1 000 N/mm 2

The choice of treatment method is free insofar as the requirements comply with this standard and no damage to the usage properties occurs. 6.4.2.2. Materials with strengths ≥ 1 000 N/mm 2 Protection against delayed brittle fracture (hydrogen embrittlement) is paramount for the coating.

The surface treatment method must be realised so that damage due to delayed hydrogen-induced brittle fractures is eliminated with a high degree of certainty. The procedure for dealing with potential defective coatings (peeling-off of coatings and new coatings) must be examined and consequences resulting from this described.

The measures for minimising the risk of delayed hydrogen-induced brittle fractures and the processes necessary for this must be agreed between the customer/manufacturer and the coating company.

The required process inspection and process testing accompanying production can typically be carried via stress tests on a sufficient number of suitable hydrogen-sensitive samples. The information given in E DIN 50969-1 must be observed.

Material

Mechanical stress (also internal or local

stress)

Hydrogen in the

material

Fracture, at critical interaction

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Heat treatments are necessary to avoid brittle fractures. These must be carried out after the electroplating for the hydrogen effusion and, if necessary, also before the electroplating to relieve component internal stresses. For this, see also E DIN 50969-1:2008-02, 3.4 and 4.4.

It is particularly important to ensure that the applied metallic coating as a diffusion barrier does not prevent the success of the heat treatment for the hydrogen effusion. The properties of the parts must not be altered detrimentally. REMARK Reference values for a heat treatment for the hydrogen effusion are given in Table 4:

Table 4 – Reference values for heat treatment for hydrogen effusion after an electroplating

treatment corresponding to this standard Tensile strength

R m N/mm 2 Heat treatment conditions in the air circulation furnace minimum retention

time at parts temperature (215 ± 15) °C

h

1,000 to 1,250 6 1,251 to 1,450 12 1,451 to 1,600 20 1,601 to 2,000

24

In conjunction with the coating of components at potential risk from hydrogen, tests as per E DIN 50969-2 must be carried out in addition to the above measures.

7. Requirements for the coatings and test methods

7.1. Coat thickness

The methods according to Table 5 give minimum material thicknesses (d min ) and apply for the essential areas.

The thicknesses of the zinc and zinc alloy coatings should preferably be determined with the X-ray spectrometric method as per DIN EN ISO 3497.

Other applicable layer thickness measuring methods are e.g.: – Microscopic measurement as per DIN EN ISO 1463; – Coulometric method as per DIN EN ISO 2177; – Magnetic method as per DIN EN ISO 2178

The layer thickness of passivations and seals is not included in the consideration.

7.2. Layer adhesion

The test parts are stored for 30 minutes at (220 ± 10) °C and then immediately quenched in water with a temperature of 15 °C to 25 °C. Flaking and blister formation must not occur in the coating (thermal shock test as per DIN EN ISO 2819). REMARK A bending or surface grinding of the components, if feasible, is recommended as a further test for the

adhesive strength.

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7.3. Cr(VI) absence

The applied layer must be chromium-(VI)-free in agreement with DIN EN 15205.

7.4. Resistance in short-term corrosion tests

7.4.1. General

Salt spray tests as per DIN EN ISO 9227-NSS must be carried out. REMARK The corrosion performance of the coated components in use cannot be concluded from the results of the

short-term corrosion tests without more information. When using the components, very different load profiles

(temperature, moisture, flow …) can occur, while only the specified, limited test climates occur in the short-term corrosion tests.

7.4.2. Minimum resistance of passivated zinc or zinc alloy coatings

Depending on the coating system and test, minimum test times are specified concerning the test duration by which no corrosion products of the coating (white rust) or basic material (red rust) may occur. The essential areas of the tested component are to be evaluated.

The requirements for the minimum resistance apply in the “coated state” as well as after a heat storage at 120 °C/24 h before the corrosion test. The heat storage is not necessary for the coating system Zn//An//T0.

Non-agreed treatments (e.g. waxing, greasing) which can improve the resistance in the corrosion tests are not permissible.

Influences (e.g. through sorting processes, transport, assembly or aggressive media), which can damage the corrosion protection properties of the coatings must be avoided before the corrosion test. An evaluation or limitation of the extent of the damage is not an object of this standard.

The attainable corrosion resistance of coatings can also depend greatly on the components to be coated (material, geometry) in addition to the coating system and coating quality. In the case of components where an optimum coating quality cannot be achieved without further measures (e.g. due to material defects or complex component geometry), an agreement concerning lower corrosion resistance might be necessary.

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Table 5 – Minimum layer thicknesses and test duration for passivated (transparent, iridescent)

zinc and zinc alloy coatings for testing as per DIN EN ISO 9227-NSS

Minimum test duration

h

Type of surface protection layer

Version type Method

without coating corrosion

without basic material

corrosion

(depending on the Zn or Zn alloy layer thickness)

5 μm 8 μm 12 μm Drum 8 48 72 9

6 Electroplated zinc coating,

transparent passivated

Zn//An//T0

Trestle 16 72 96 120 Drum 72 144 216 288 Electroplated zinc coating,

iridescent passivated

Zn//Cn//T0

Trestle 120 192 264 336 Drum 120 192 264 360 Electroplated zinc coating,

iridescent passivated and sealed

Zn//Cn//T2 Trestle 168 264 360 480 Drum 96 168 240 312 Electroplated zinc-iron alloy coating, iridescent passivated

ZnFe//Cn//T0

Trestle 168 240 312 384 Drum

144 216 288 384 Electroplated zinc-iron alloy coating, iridescent passivated and sealed ZnFe//Cn//T2

Trestle 216 312 408 528 Drum 120 480 720 720a Electroplated zinc-nickel alloy coating, iridescent passivated

ZnNi//Cn//T0

Trestle 192 600 720 720a Drum

168 600 720 720a Electroplated zinc-nickel alloy coating, iridescent passivated and sealed

ZnNi//Cn//T2

Trestle

360

720

720a

720a

a

To limit the expense of the tests, the requirements are restricted to 720 h.

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Table 6 – Minimum layer thicknesses and test duration for passivated (black) zinc and zinc alloy

coatings for testing as per DIN EN ISO 9227-NSS

Minimum test duration

h

Type of surface protection layer

Version type Method

without coating corrosion

without basic material

corrosion

(depending on the Zn or Zn alloy layer thickness)

5 μm 8 μm 12 μm Drum 120 192 264 360 Electroplated zinc-iron alloy coating, black passivated and sealed ZnFe//Fn//T2

Trestle

168 264 360 480 Drum 168 480 720 720a Electroplated zinc-nickel alloy coating, black passivated and sealed ZnNi//Fn//T2

Trestle

240 600 720 720a Drum 48 480 720 720a Electroplated zinc-nickel alloy coating, black passivated

ZnNi//Fn//T0

Trestle

72

600

720

720a

a

To limit the expense of the tests, the requirements are restricted to 720 h.

The occurrence of mild visual changes (grey fog) without voluminous character is permissible and does not represent any impairment to the corrosion protection.

There are still no universally recognised values for the corrosion resistance at present for electroplated zinc coatings, black passivated and sealed (Zn//Fn//T2).

8. Test report

8.1. General information

The following must be tested by the coating company: a) Reference to this standard (i.e. DIN 50979);

b) Conformity of the coatings with the requirements of this standard; c) Coating company for the surface protection;

d) Applied technology (e.g. trestle or drum coating and applied coating system).

8.2. Special data for coating high-strength materials with tensile strength ≥ 1 000

N/mm2

Designation and confirmation of the measures implemented for minimising the risk of delayed hydrogen-induced brittle fractures.

8.3. Test results

Results of the technological tests according to this standard (see section 7). The tests as per 7.2, 7.3, 7.4 must be carried out accompanying the process. The test bodies (supplier and/or subcontractor and/or independent test institute) are to be appointed.

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References

DIN EN ISO 1463, Metallic and oxide coatings – Measurement of coating thickness – Microscopical method

DIN EN ISO 2177, Metallic coatings – Measurement of coating thickness – Coulometric method by anodic dissolution

DIN EN ISO 2178, Non-magnetic coatings on magnetic substrates – Measurement of coating thickness – Magnetic method

DIN EN ISO 2819, Metallic coatings on metallic substrates – Electrodeposited and chemically deposited coatings – Review of methods available for testing adhesion