GB/T 5267.2-2021 PDF English (GB/T 5267.2-2017: Older version)
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GB/T 5267.2-2021: PDF in English (GBT 5267.2-2021) GB/T 5267.2-2021
GB
NATIONAL STANDARD OF THE
PEOPLE’S REPUBLIC OF CHINA
ICS 21.060.01; 25.220.40
CCS J 13
Replacing GB/T 5267.2-2017
Fasteners - Non-electrolytically applied zinc flake coatings
(ISO 10683:2018, Fasteners - Non-electrolytically applied zinc flake coating systems,
MOD)
ISSUED ON: DECEMBER 31, 2021
IMPLEMENTED ON: JULY 01, 2022
Issued by: State Administration for Market Regulation;
National Standardization Administration.
Table of Contents
Foreword ... 4
Introduction ... 7
1 Scope ... 8
2 Normative references ... 8
3 Terms and definitions ... 9
4 General characteristics of coatings ... 9
4.1 Zinc flake coating system ... 9
4.2 Composition of zinc flake coating systems ... 10
4.3 Mechanical properties, physical properties, curing ... 10
4.4 Avoiding internal hydrogen embrittlement ... 10
4.5 Coating systems and coating processes ... 11
5 Corrosion resistance and testing ... 11
5.1 General requirements ... 11
5.2 Neutral salt spray test ... 11
5.3 Sulfur dioxide test (Kesternich test) ... 12
5.4 Automated bulk handling and conveying and/or sorting, storage and transportation 13
6 Dimensional requirements and tests ... 13
6.1 General requirements ... 13
6.2 ISO metric thread fasteners ... 13
6.3 Other fasteners ... 15
7 Mechanical, physical properties and tests ... 15
7.1 Appearance ... 15
7.2 Corrosion resistance as a function of temperature ... 15
7.3 Test method for thickness or coating quality ... 15
7.4 Ductility ... 17
7.5 Adhesion/bonding ... 17
7.6 Sacrificial cathodic protection ... 17
7.7 Torque-clamping force relationship ... 17
7.8 Determination of hexavalent chromium ... 18
8 Adaptability test ... 18
8.1 General requirements ... 18
8.2 Mandatory tests for each batch of products ... 18
8.3 Process control tests ... 18
8.4 Tests specified by the user ... 19
9 Marking ... 19
9.1 Marking of zinc flake coating system ... 19
Fasteners - Non-electrolytically applied zinc flake coatings
1 Scope
This document specifies the technical requirements for non-electrolytically applied zinc
flake coatings on steel fasteners.
This document applies to the following coatings:
- With or without hexavalent chromium;
- With or without topcoat;
- With or without lubrication (self-lubricating and/or additional lubrication).
This document applies to ISO metric threaded bolts, screws, studs, nuts, non-ISO metric
threaded fasteners, unthreaded fasteners such as washers, pins, clamps, etc.
This document does not specify the weldability or coating performance of fasteners and
is not applicable to mechanical galvanizing.
Note: High-strength fasteners (tensile strength ≥ 1000 MPa) use coatings that comply with this
document to avoid the risk of internal hydrogen embrittlement (see 4.4).
For information on the design and installation of coated fasteners, see Appendix A.
2 Normative references
The contents of the following documents constitute essential provisions of this
document through normative references in the text. Among them, for referenced
documents with dates, only the version corresponding to that date applies to this
document; for referenced documents without dates, the latest version (including all
amendments) applies to this document.
GB/T 1237 Designation system for fasteners (GB/T 1237-2000, eqv ISO 8991:1986)
GB/T 3099.3 Terminology of fasteners - Coatings (GB/T 3099.3-2017, ISO 1891-
2:2014, MOD)
GB/T 3934 Specification of gauges for general purpose screw threads (GB/T 3934-
2003, ISO 1502:1996, MOD)
GB/T 6462 Metallic and oxide coatings - Measurement of coating thickness -
Microscopical method (GB/T 6462-2005, ISO 1463:2003, IDT)
GB/T 9789 Metallic and other non-organic coatings - Sulfur dioxide test with
general condensation of moisture (GB/T 9789-2008, ISO 6988:1985, IDT)
GB/T 10125 Corrosion tests in artificial atmospheres - Salt spray tests (GB/T 10125-
2021, ISO 9227:2017, MOD)
GB/T 16823.3 Fasteners - Torque/clamp force testing (GB/T 16823.3-2010, ISO
16047:2005, IDT)
ISO 3613:2010 Metallic and other inorganic coatings - Chromate conversion
coatings on zinc, cadmium, aluminium-zinc alloys and zinc-aluminium alloys - Test
methods
Note: GB/T 9791-2003 Chromate conversion coatings on zinc, cadmium,
aluminium-zinc alloys and zinc-aluminium alloys - Test methods (ISO 3613:2000,
MOD).
3 Terms and definitions
The terms and definitions defined in GB/T 3099.3 apply to this document.
4 General characteristics of coatings
4.1 Zinc flake coating system
A non-electrolytically applied zinc flake coating system is formed, by applying zinc
flakes to the surface of steel fasteners by adding flake aluminum in a suitable medium.
Under the action of heat curing, the zinc flakes are bonded to each other and to the
substrate to form an inorganic topcoat, which has good conductivity and cathodic
protection. The coating may contain hexavalent chromium.
Measures shall be taken to avoid excessive or insufficient thickness of coating.
Measures shall be taken to avoid light and/or flat fasteners from being bonded together
(e.g. washers, clamps, fastener assemblies, flange nuts).
Additional topcoats can improve corrosion resistance and/or achieve specific functions
(e.g., torque-clamping force performance, chemical resistance, appearance, color,
electrical insulation/conductivity, see A.2).
Pretreatment processes using alkaline/solvent cleaning followed by mechanical
cleaning do not generate hydrogen, thus eliminating all risk of internal hydrogen
embrittlement (IHE).
When the functional properties are not suitable for mechanical cleaning (e.g., fastener
assemblies, internally threaded fasteners, oiled fasteners), chemical cleaning (pickling)
may be used, using acids with appropriate corrosion inhibitors and the shortest cleaning
cycles to minimize the risk of internal hydrogen embrittlement. Fasteners with hardness
above 390 HV or property class 12.9 or above shall not be pickled; the time interval
between cleaning and coating shall be as short as possible.
Phosphating can be an alternative to mechanical cleaning (hydrogen may be generated
during pretreatment, but hydrogen can diffuse outward during curing). The time interval
between phosphating and coating shall be as short as possible.
Electrolytic cleaning shall not be performed.
Note: Zinc flake coatings are highly permeable to hydrogen; hydrogen absorbed during
pretreatment can diffuse outward during curing.
4.5 Coating systems and coating processes
When selecting a coating system and the associated coating process, the type and
geometry of the fastener shall be considered, see A.2.
5 Corrosion resistance and testing
5.1 General requirements
The corrosion resistance measured by the accelerated corrosion test has no direct
correspondence with the corrosion resistance characteristics in a specific service
environment; however, the accelerated corrosion test is used to evaluate the corrosion
resistance of the coating.
Note: Appendix B gives a guide for selecting the thickness of the corrosion-resistant coating.
5.2 Neutral salt spray test
The neutral salt spray test (NSS) in accordance with GB/T 10125 is used to evaluate
the corrosion resistance of the coating system.
When the salt spray test chamber is required to be evaluated, it should refer to Appendix
C.
5.4 Automated bulk handling and conveying and/or sorting, storage
and transportation
Automated bulk handling and conveying and/or sorting, storage and transportation may
cause a significant reduction in the corrosion protection of the coating, depending on
the type and geometry of the coating system and the fastener. This may especially occur
in chromium-free (hexavalent) coating systems with poor self-healing properties and/or
topcoats that are sensitive to impact damage and/or abrasion.
If necessary, a supply and demand agreement shall be signed to determine the minimum
cycle of the neutral salt spray test and/or increase the thickness of the coating system.
6 Dimensional requirements and tests
6.1 General requirements
Before coating, the fastener dimensions shall be within the specified size range. For
special requirements for ISO metric threads, see 6.2.2, B.4, B.5.
6.2 ISO metric thread fasteners
6.2.1 Coating thickness
When considering the coating thickness required for the expected corrosion resistance,
the non-uniformity of the coating thickness distribution shall be taken into account, see
B.3.
Coating thickness has a significant effect on measurability and fitability; thread
tolerance and thread clearance shall be considered. The coating on external threads shall
not exceed the zero line (basic dimension); the internal threads shall not be below the
zero line, see B.4.
Note: Standard bolts, screws, studs, nuts are not specially processed to accommodate zinc flake
coatings, see B.4 and B.5.
6.2.2 Measurability and fitability
After coating, ISO metric threads shall be measured according to the through gauge for
external thread tolerance position h and internal thread tolerance position H specified
in GB/T 3934.
When measuring coated bolts, screws, studs with a ring gauge, the maximum torque
6.3 Other fasteners
This document does not specify the dimensional requirements for non-metric threaded
fasteners and unthreaded fasteners after coating. For more information, see A.3.
7 Mechanical, physical properties and tests
7.1 Appearance
The color of the zinc flake coating is initially silver-grey. Other colors can be obtained
by using a topcoat. Unless otherwise agreed, color changes shall not result in rejection,
see item h) in Chapter 10.
The coating shall not have blisters and uncoated areas that could adversely affect the
corrosion resistance of the fastener. Local over-thickness of the coating shall not affect
the functional properties (see Chapter 6 and A.2).
7.2 Corrosion resistance as a function of temperature
High temperatures affect the corrosion resistance of coated fasteners. This test is for
process control and not for checking the condition of coated fasteners and mating parts.
After the coated fastener is heated to 150 °C (fastener temperature) and kept at this
temperature for 3 h, the corrosion resistance shall still comply with the provisions of
Chapter 5.
Other technical requirements can be agreed upon by the supplier and buyer when
placing an order.
7.3 Test method for thickness or coating quality
The coating thickness or coating quality shall be determined by one of the following
test methods:
- Magnetic method (determine the local total thickness in the measurement area);
- X-ray method (can only measure the local thickness of the base coating in the
measurement area);
- Weighing method (remove the coating by chemical or mechanical methods and
determine the average total coating mass of the fastener, that is, the average coating
amount per unit area);
7.4 Ductility
Zinc flake coating systems generally do not have good ductility, i.e., deformation after
coating may affect corrosion resistance. Ductility shall be matched to the elastic
deformation that occurs during fastener assembly, e.g., tightening threaded fasteners,
flattening tapered washers, bending clamps during installation.
The ability of the zinc flake coating system to deform shall not affect the performance
of the fastener, e.g., corrosion resistance, torque-clamping force relationship, when
specified. Therefore, the suitability test for a specific application shall be determined
by agreement between the supplier and the buyer.
Note: Insufficient ductility can cause cracking/flaking of the coating that affects corrosion
resistance.
7.5 Adhesion/bonding
This test can be performed at any stage of the application process.
A 25 mm wide tape with an adhesion of (7 ± 1) N is pressed by hand onto the surface
of the coated part; then quickly pulled away perpendicular to the surface. The zinc layer
shall not fall off the base metal; however, a small amount of coating material is allowed
to adhere to the tape.
Note: Coating material visible on the fastener surface and on the tape is usually due to
insufficient bonding; coating material visible on the base metal and on the tape is usually due
to insufficient adhesion.
7.6 Sacrificial cathodic protection
The sacrificial (anodic) cathodic protection capability of the coating can be tested as
follows:
- Use a tool with a nominal width (edge width) of 0.5 mm to scratch the fastener
coating to the base metal;
- Perform a neutral salt spray test as specified in Chapter 5. After 72 hours of testing,
there shall be no red rust on the scratched area.
7.7 Torque-clamping force relationship
When required, metric threaded bolts and nuts with self-lubricating and/or additional
lubricating coatings may specify a torque-clamping force relationship.
The test method shall be in accordance with GB/T 16823.3 or other relevant technical
specifications, as agreed by the supplier and the buyer.
The requirements for the torque-clamping force relationship shall be agreed by the
supplier and the buyer, see A.2 for information.
Storage conditions shall not affect the torque-clamping force performance of coated
fasteners (see A.4).
7.8 Determination of hexavalent chromium
The presence of hexavalent chromium Cr(VI) can be determined. If determination is
required, it shall be carried out in accordance with 5.5.2 of ISO 3613:2010.
8 Adaptability test
8.1 General requirements
All requirements specified in Chapters 5 ~ 7 apply to the general characteristics of the
coating or the characteristics specified by the user.
8.2 Mandatory tests for each batch of products
The following tests shall be carried out on each batch of fasteners (see GB/T 90.1):
- Thread measurement (see 6.2.2);
- Appearance (see 7.1).
8.3 Process control tests
The following tests do not need to be performed on every batch of fasteners and shall
be used for process control when relevant (see GB/T 90.3):
- Corrosion resistance: Neutral salt spray test (see 5.2), or sulfur dioxide test only
when explicitly required (see 5.3);
- Temperature resistance (see 7.2);
- Coating thickness or coating quality (see 7.3);
- Adhesion/bonding force (see 7.5).
Examples 1 ~ 4 provide examples of coating marking for orders.
Example 1: Non-electrolytically applied zinc flake coated fasteners (flZn), requiring a
minimum 240 h neutral salt spray test, the marking is:
[Fastener Marking] -flZn/240 h
Example 2: Non-electrolytically applied zinc flake coated fasteners, self-lubricating (flZnL),
without hexavalent chromium (nc), without topcoat, requiring a minimum 480 h neutral salt
spray test, lubricated but without special torque-clamping force requirements, the marking is:
[Fastener Marking] -flZnL/nc/480 h
Example 3: Non-electrolytically applied zinc flake coated fasteners (flZn), with hexavalent
chromium (yc), with topcoat and self-lubricating (TL), requiring a minimum 720 h neutral salt
spray test, friction coefficient μ within the range [0.10 ~ 0.20] (C), the marking is:
[Fastener marking]-flZn/yc/TL/720 h/C
Example 4: Non-electrolytically applied zinc flake coated fasteners (flZn), without hexavalent
chromium (nc), no self-lubrication, topcoat but no self-lubrication (Tn), additional lubrication
(L), minimum 960 h neutral salt spray test required, friction coefficient μ = 0.17 ± 0.03 (C), the
marking is:
[Fastener Marking]-flZn/nc/Tn/L/960 h/C
9.2 Label marking of zinc flake coating system
The label shall contain at least the following information, separated by slashes (/):
- flZn indicates zinc flake coating (primer);
- yc indicates hexavalent chromium coating or nc indicates no hexavalent chromium
coating;
- Indicate the minimum corrosion resistance cycle (neutral salt spray test) in hours.
Examples 1 ~ 3 are label marking examples.
Example 1: Hexagonal bolt GB/T 5782-M12×80-10.9-flZn/nc/720 h
Example 2: Hexagonal nut GB/T 6170-M12-10-flZn/yc/480 h
Example 3: Flat washer GB/T 97.1-12-300HV-flZn/nc/240 h
- Organic topcoats: Electrical insulation, high chemical resistance or color selection,
etc.;
- Inorganic topcoats: Impact/wear resistance or thermal impedance, etc.
Additional lubrication may be selected to adjust the torque-clamping force relationship.
A.1.3 Coating process
Zinc flake coating systems can be applied in bulk or on a rack using a dip-spin or spray
process.
Zinc flake coating is usually applied in large batches. When small batches need to be
coated, it may be necessary to use an appropriate coating line and/or process to achieve
the required performance and function of the coated fasteners. For large-sized or high-
mass fasteners or to reduce the risk of thread damage, consider using a rack coating
instead of a large batch process.
The curing process (especially at higher temperatures and/or longer cycles) may affect
fastener performance/function:
- When the curing temperature exceeds the tempering temperature, the reduction in
hardness may affect the performance of case-hardened or carbonitrided fasteners
(such as self-extruding screws or self-drilling and tapping screws), or elastic
deformation and plastic deformation (such as clamps);
- For cold-worked fasteners or fasteners with rolled threads after heat treatment,
residual stress may be reduced.
A.2 Functional characteristics
A.2.1 Fitability and installability
The clearance between the connecting parts (such as holes), the dimensional tolerances
of the functional parts of the fastener, positioning (such as retaining rings), insertion
points (such as cross slots and internal wrenches) and installation should not be affected.
The dimensional requirements of threaded fasteners after coating are shown in 6.2 and
Appendix B.
The adaptability of the coating system to the tightening process should be considered,
especially when tightening at high speeds, the risks of overheating, sticking/slipping
should be considered.
It should consider the compatibility of coated fasteners with clamping parts such as
threaded holes, aluminum, magnesium, stainless steel clamping parts, electroplated
coated parts, hot-dip galvanized parts, plastics, and wood.
To achieve the clamping force and stable torque/clamping force relationship specified
for ISO metric threaded fasteners, one side of the meshing threaded fastener should be
lubricated (at least). Zinc flake coating systems provide lubrication solutions (see A.1.2).
The torque/clamping force relationship can be determined in accordance with GB/T
16823.3 and expressed as a friction coefficient μ (or K factor).
A.2.2 Other properties of coated fastener joints
A.2.2.1 Chemical resistance
Organic topcoats over zinc flake coating primer are more resistant to acidic and alkaline
chemicals than inorganic topcoats.
A.2.2.2 Electrical conductivity
The electrical conductivity of zinc flake coating primer with inorganic topcoats is
generally suitable for electroplating and antistatic. Zinc flake coatings are not suitable
for electrical grounding.
A.2.2.3 Galvanic corrosion
To reduce the risk of contact corrosion, it should consider all parts of the joint (coated
fasteners and clamping parts). Direct metal contact of uncoated clamping parts should
be avoided, especially stainless steel, magnesium, copper or copper alloys. Organic
topcoats improve resistance to galvanic corrosion due to the insulating effect.
The items listed in A.2.2 are not exhaustive. When selecting the coating system, all
special service conditions should be considered.
A.2.2.4 Cleanliness
The suitability of the zinc flake coating system should be checked for cleanliness
requirements (e.g., dust, particle size, particle type, particle count).
A.3 Specific issues for fasteners and coating processes
A.3.1 General
When selecting the coating system and the associated coating process, the type of
fastener should be considered. A.3.2 ~ A.3.9 list the main issues for each type of fastener.
When the specified characteristics require 100% sorting, the supplier and the buyer
shall reach an agreement when ordering. Appropriate measures should be taken for the
following potential problems.
A.3.2 ISO Metric thread fasteners
......
Source: Above contents are excerpted from the PDF -- translated/reviewed by: www.chinesestandard.net / Wayne Zheng et al.
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