BS 6682:1986 pdf free download.British Standard Method for Determination of bimetallic corrosion in outdoor exposure corrosion tests.
1 Scope and field of application
BS 6682 specifies methods for the deterrrwnation of bimetallic corrosion of metals and alloys, and metals and alloys with metallic and non-metallic non-organic coatings, in outdoor exposure corrosion tests.
NOTE — In the text of BS 6682, the term “metal” is used for both metals and aHoys, and the term “coated metal” for metals and afloys with metallic and non-metalhc non-organ.c coatings.
The methods are intended for the determination of the amount and type of corrosion effect, arising in natural atmospheres, caused by contact with different metals (coated metals), and for the evaluation of the effectiveness of bimetallic corrosion protection treatments.
2 References
ISO 1456. Metallic coatings — Electroplated coatings of nickel plus chromium.
ISO 2081, Metallic coatings — Elec troplated coatings of zinc on iron or steel1)
ISO 2082, Metallic coatings — Electroplated coatings of cadmium on iron or 5t/2
ISO 4540, Metallic coatings — Coatings cathodic to the substrate — Rating of electroplated test specimens subjected to corrosion tests.
ISO 4542. Metallic and other non-organic coatings — General rules for ste tionafy outdoor exposure corrosion tests.
ISO 92, Metallic materials — Tensile testing.
3 General principles
3.1 The test consists of simultaneous exposure of test specimens and reference specimens at atmospheric field stations and subsequent comparative evaluation of their corrosion resistance.
4.1 The test specimens shall be as shown in figure 1 or figure 2.
Anodic plates may be up to 6 mm thick and their length should be suitable for tensile testing in accordance with ISO 62.
Cathodic plates shall have a thickness of 1 to 6 mm. In the case of precious metals, cathodic plates may comprise a foil covering a plate of inert material, such as a plastic material; the con tact between two plates of metal should be achieved by pressure. The thickness should not be less than 1 mm.
If it is not known which of the metals (coated metals) constituting the test specimen is nobler, each metal (coated metal) should be tested in one complete set of specimens as the anode, and in the other, as the cathode.
The presence of gaps and crevices between anodic and cathodic plates should be minimized and such clearances should not exceed 0,02 mm. When assembling the test specimens, the maximum value of the torque applied while tightening the bolts should be more than 5.0 N.m.
4.2 The surfaces of specimens shall be tree from visible defects, such as non-uniformity of rolling, scale, exfoliation, cracks, pores, blisters, scratches, dents.
If there are no surface defects, specimens shall be tested with the surface in the condition as delivered or after treatment, as recommended for the material (articles) concerned.
If defects are removed by mechanical means, the surface roughness (Re) of test specimens, reference specimens and control specimens. including cut edges, shall be within the range 0,75 to 2,5 pm.
4.3 The surface finish (inckiding cut edges) of metallic plates (coated metals) shall be in accordance with appropriate International Standards, for example ISO 1456, ISO )81, ISO 2O.
If a metal is tested in contact with a coated metal, damage to, or absence of, the coating is permissible only on the cut edges of anodic plates.
4.4 The evaluation of bimetallic corrosion resistance accord ing to all criteria except loss in mass shall be carried Out using specimens as shown in figure 1.
The evaluation of bimetallic corrosion resistance according to all criteria except changes in mechanical properties shall be carried out usinq specimens as shown in figure 2.
4.5 The material, dimensions, direction of cutting, method of surface treatment and other parameters of anodic plates of test specimens, reference specimens and control specimens shall be the same.
4.6 Specimens shall be identified by marking. Such marking shall include the characteristics and agreed designation of the tested ariodic and cathodic metal (coated metal), in accordance with an inventory of specimens (see annex A) and serial numbers assigned to the metallic plates. The marking may be made with a stamp or using indelible paint or by drilling notches on the face of each metallic plate of the specimen.
The designation of a metal (coated metal) shall be marked in the top left-hand cornet and the serial number in the top right- hand corner of anodic and cathodic plates.
Areas beanng markings made by mechanical means shall be protected by water-resistant Varnishes. The markings should be legible and durable throughout the period of test
4.7 Washers and sleeves shall be used to insulate bolts from the metallic plates and to avo4d pressing metallic plates together during the period of test. Ceramics or other insulating materials which are not susceptible to creep or degradations over extended periods of time are recommended as washer materials. Sleeves of polyethylene or polypropylene are recom mended
Metallic bolts and washers shall be made of 188 or higher chromium grades of stainless steel
4.8 The number of test specimens, reference specimens and control specimens removed for inspection at any one time shall be not less than three
If mechanical tests are not required by the test programme. the number of control specimens may be decreased to three per
5 Preparation of specimens for test
5.1 Test specimens, reference specimens and control speci mens shall be examined for conformity to the requirements of clause 4 by visual exanwnation and appropriate measurements.
5.2 Immediately before testing, the surfaces of specimens shall be degroased using organic solvents, for example ethanol, white spirit
After degreasing. the specimens shall be handled only by the cut edges and when wearing cotton (or rubber) gloves
5.3 After degreasing. test specimens and reference speci mens, the corrosion behaviour of which is to be evaluated by loss in mass, shall be kept in desiccators containing a desiccant (for example silica gel) for not less than 24 h. They shall then be weighed to the nearest 0,000 1 g.
5.4 Control specimens shall be stored throughout the test period under conditions which prevent corrosion, for example in desiccators or in polyethylene bags containing a desiccant (silica gel) at a relative humidity of less than 50 %.
Rmc is the ultimate tensile strength, in megapascals, of a
control specimen;
Rmr is the ultimate tensile strength, in megapascals, of the reference specimen.
NOTE — Changes in yield strength (R91 and e’ongation (A) are calculated sinlarIy.
7.5.5 Bimetallic corrosion res.stance is determined by the corrosion increase index, that is, the difference between the relative changes in mechanical properties of test specimens and reference specimens. The test results shall be recorded on a form as shown in annex D.
7.6 Evaluation based on loss in mass
7.6.1 For evaluations based on loss in mass, the test speci mens and reference specimens shall be examined, and dust. dirt and corrosion products shall be removed,
Anodic plates with evidence of corrosion effect under the cathodic plates extending for a distance of more than 3 mm from the contact edge shall not be examined.
It is necessary to remove glue or varnish used to prevent crevice corrosion of specimens, before removal of corrosion products.
7.6.2 After removal of the corrosion products, the metallic plates (coated metals) shall be washed, drsed, degreased, stored for 24 h in a desiccator, and then weighed.
7.6.3 The rate of corrosion of test specimens, expressed in grams per square metre per year, is given by the formula.