ASTM B841 Zinc Nickel Plated Fasteners
ASTM B841 specification covers the requirements for electrodeposited zinc nickel alloy coatings on metals & fasteners. ASTM B841 Zinc Nickel Plated / coated Fasteners are commonly used onshore and offshore oil and gas facilities including subsea applications. Ideally ASTM B841 Zinc Nickel Coated Fasteners & Stud Bolts are further coated with
PTFE coatings produced by whitford namely Xylan with options varying with respect to the applications such as Xylan 1014,
\Xylan 1070, Xylan 1424 to get better corrosion resistance . ASTM B841 Zinc Nickel Coated fasteners
generally perform better in service when the substrate over which they are applied is smooth and free of torn metal, inclusions, pores, and other defects.
There is one class of zinc nickel coating defined as follows:
- ASTM B841 Class 1 - Deposits having a minimum of 5 and maxi- mum 12 mass % nickel, the balance being zinc, There are five chromate conversion coating types that are defined as follows:
- ASTM B841 Type A - With colorless (blue bright) conversion coatings,
- ASTM B841 Type B - With yellow iridescent conversion coatings,
- ASTM B841 Type C - With bronze conversion coatings,
- ASTM B841 Type D - With black chromate conversion coatings, and
- ASTM B841 Type E - Any of the above plus organic topcoat.
There are three grades according to thickness and are defined as follows:
| Minimum Thickness, µm |
New ASTM Grade |
Old ASTM Grade |
| 5 |
5 |
1 |
| 8 |
8 |
2 |
| 10 |
10 |
3 |
- Substrate
-
The metal to be plated shall be free of flaws and defects that will be detrimental to the zinc alloy coating. It shall be subjected to such cleaning, pickling, and electroplating procedures as are necessary to yield deposits with the desired quality.
The electroplating shall be applied after all basis metal heat treatments have been completed.
- Nature of Coating
-
The coating shall consist of a zinc nickel alloy that has a minimum of 5 and maximum 12 mass percent nickel, the balance being zinc.
The coating shall be produced from an aqueous electroplating system that may be either an alkaline or acid formulation
as specified by the purchaser.
The coating shall have such supplementary conversion coatings as defined and specified in the purchase order.
- Appearance
-
The coating on all ready visible surfaces shall have an acceptable and characteristic appearance as agreed upon by the purchaser and seller. The coating shall be uniform insofar as the basis metal will permit. When the article is to be plated on
a rack, contact marks may be unavoidable. Location of such marks(s) shall be indicated on the article or its drawing.
Defects and variations in appearance that arise from surface conditions of the substrate (scratches, pores, roll marks, inclusions,
and the like) and that persist in the coating despite the observance of good metal finishing practices shall not be cause for rejection. The coating shall be adherent, free from blisters, pits, or discontinuities, and shall be free of cracks in
the as plated state. Flaking shall be cause for rejection in either the as plated state or after subsequent operations.
- Thickness
-
The thickness of the coating everywhere on the significant surfaces as defined and shall conform to the requirements of the specified grade as defined.
- Adhesion
-
The coating shall withstand normal handling and storage conditions without chipping, flaking, or other coating damage and shall conform to the minimum requirements set forth.
- Corrosion
-
The corrosion resistance of the coating may be evaluated using the method in Appendix X1 .
Pretreatment of Iron and Steel for Reducing the Risk of Hydrogen Embrittlement:
Parts that are made of steels with ultimate tensile strengths
of 1000 MPa (hardness of 31 HRC or greater) that have been machined, ground, cold formed, or cold straightened subsequent to heat treatment shall require stress relief heat treatment when specified by the purchaser, the tensile strength to be
supplied by the purchaser. Specification B849 may be consulted for a list of pre-treatments that are widely used.
Post-Coating Treatments of Iron and Steel for Reducing the Risk of Hydrogen Embrittlement:
Parts that are made of steels
with ultimate tensile strengths of 1000 MPa (hardness of 31 HRC or greater), as well as surface hardened parts, may require post-coating hydrogen embrittlement relief baking when specified by the purchaser, the tensile strength to be supplied
by the purchaser. Specification B850 may be consulted for a list of post-treatments that are widely used.
- Special Test Specimens
-
The permission or the requirement to use special test specimens, the number to be used, the material from which they are to be made, and their shape and size shall be stated by the purchaser.
NOTE 9-Test specimens often are used to represent
the coated articles in a test if the articles are of a size, shape, or material that is not suitable for the test or if it is preferred not to submit articles to a destructive test because, for example, the articles are expensive or few in
number. The specimen should duplicate the characteristics of the article that influence the property being tested.
Special test specimens used to represent articles in an adhesion, porosity, corrosion resistance, or appearance test shall
be made of the same material, in the same metallurgical condition, and have the same surface condition as the articles they represent, and be placed in the production lot of and be processed along with the articles they represent.
Special
test specimens used to represent articles in a coating thickness test may be made of a material that is suitable for the test method even if the represented article is not of the same material. For example, a low-carbon steel specimen may represent
a brass article when the magnetic thickness test is used (see Test Method B499 ). The thickness specimen need not be carried through the complete process with the represented article. If not, introduce it into the process at the point where
the coating is applied and carry it through all steps that have a bearing on the coating thickness. In rack plating, rack the specimen in the same way with the same distance from and orientation with the anodes and other items in the process
as the article it represents.
NOTE 10-When special test specimens are used to represent coated articles in a thickness test, the specimens will not necessarily have the same thickness and thickness distribution as the articles unless the
specimens and the articles are of the same general size and shape. Therefore, before finished articles can be accepted on the basis of a thickness test performed on special test specimens, the relationship between the thickness on the specimen
and the thickness on the part needs to be established. The criterion of acceptance is that thickness on the specimen that corresponds to the required thickness on the article.
- Composition of Coating
-
Verify the composition of the coating by atomic absorption spectrophotometry (AA), induc- tively coupled plasma (ICP), or directly coupled plasma (DCP) or other methods if found to be capable of results within 10 % of known standards.
- Appearance
-
Examine the coating at up to 10× magni- fication for conformance to the requirements of appearance.
- Thickness
-
Measure the coating thickness at locations on the signicant surface designated by the purchaser, and make the measurement with an accuracy of 10 % or better by use of one of the following test methods: Test Methods B487 , B499 , B504 ,
and B568 .
- Adhesion
-
Test the adhesion of the coating in accordance with Practice B571 . The choice of test method shall be one of those appropriate for zinc coatings.
- Corrosion Resistance
-
See Appendix X1 .
| Class |
Grade |
Type |
Base Metal Corrosion |
Alloy Corrosion Product |
| 1 |
5 |
A |
150 |
20 |
| 5 |
A/E |
300 |
150 |
| 8 |
A |
240 |
20 |
| 8 |
A/E |
400 |
150 |
| 10 |
A |
500 |
20 |
| 10 |
A/E |
620 |
150 |
| 1 |
5 |
B |
500 |
120 |
| 5 |
B/E |
620 |
240 |
| 8 |
B |
720 |
120 |
| 8 |
B/E |
840 |
240 |
| 10 |
B |
960 |
120 |
| 10 |
B/E |
1080 |
240 |
| 1 |
5 |
C |
750 |
180 |
| 5 |
C/E |
1000 |
300 |
| 8 |
C |
960 |
180 |
| 8 |
C/E |
1200 |
300 |
| 10 |
C |
1000 |
180 |
| 10 |
C/E |
1500 |
300 |
| 1 |
5 |
D |
500 |
100 |
| 5 |
D/E |
620 |
220 |
| 8 |
D |
720 |
100 |
| 8 |
D/E |
840 |
220 |
| 10 |
D |
960 |
100 |
| 10 |
D/E |
1080 |
220 |