ICS 25.220.20
Brünieren von Bauteilen aus Eisenwerkstoffen –Anforderungen und
Prüfverfahren
In keeping with current practice in standards published by the International Organization for Standardization (ISO), a comma has been used throughout as the decimal marker.
Foreword
This standard has been prepared by Technical Committee Chemische und elektrochemische Überzüge  of the Normenausschuss Materialprüfung  (Materials Testing Standards Committee).
Amendments
This standard differs from the November 1987 edition in that it has been harmonized with DIN EN 1403 and DIN 50960-1, and it has been editorially revised.
Previous editions
DIN 50938: 1973-12, 1987-11.
Ref.No.DIN 50938:2000-04English price group 06Sales No.0106
12.01
DEUTSCHE NORM April 2000
50938
{Continued on pages 2 to 5.
©No part of this translation may be reproduced without the prior permission of
DIN Deutsches Institut für Normung e.V., Berlin. Beuth Verlag GmbH , 10772Berlin, Germany,
has the exclusive right of sale for German Standards
(DIN-Normen).Black oxide treatment of ferrous products
Requirements and testing Translation by DIN-Sprachendienst.
In case of doubt, the German-language original should be consulted as the authoritative text.
Supersedes
November 1987 edition.1Scope
This standard specifies requirements for and methods of testing black oxide coatings produced on ferrous components by a chemical reaction during an immersion process. This process provides iron and steel (i.e.cast iron, unalloyed and low-alloy steel) components with uniform black coatings while substantially retain-ing their dimensional accuracy. The purpose of this treatment is to give the material a particular appearance and to increase its corrosion resistance even without aftertreatment. The degree of protection provided is,however, low.
The standard does not cover semi-finished products and fasteners. Threaded articles shall be coated as agreed.
2Normative references
This standard incorporates, by dated or undated reference, provisions from other publications. These normative references are cited at the appropriate places in the text, and the titles of the publications are listed below. For dated references, subsequent amendments to or revisions of any of these publications apply to this standard only when incorporated in it by amendment or revision. For undated references, the latest edition of the publication referred to applies.
DIN 50017Artificial climates in technical applications – Condensation water test atmospheres
DIN 50902Protective coatings on metals – Concepts, application methods and surface preparation DIN 50960-1Electroplated and chemically applied coatings – Designation and information in technical
documentation
DIN EN 1403Corrosion protection of metals – Electrodeposited coatings – Method of specifying general
requirements
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DIN50938:2000-04
DIN EN ISO2064Metallic and other non-organic coatings – Definitions and conventions concerning the meas-urement of thickness
ISO4519: 1980Electrodeposited metallic coatings and related finishes – Sampling procedures for inspec-tion by attributes
3Concepts
3.1Black oxide coating
A thermally produced dark-brown to black oxide conversion coating produced by immersion of a component in alkaline salt solution (cf. DIN50902) to which reducing agents or other reagents may have been added. The saturation of the colour obtained depends on the nature of the base metal (substrate), the heat treatment, the surface preparation and the composition and type of treatment bath.
3.2Significant surface
Unless otherwise agreed, significant surface is an area on a product that can be touched with a ball 20mm in diameter.*)
4Designation
4.1General
The designation shall include the following information:
a)The word ‘coating’;
b)the number of this standard;
c)  a hyphen;
d)the chemical symbol of the base metal or the main component of the alloy;
NOTE: It is recommended that the standard designation of the base material follow the symbol of the base metal.
e)  a solidus;
f)any additional symbols to denote further treatments to be carried out shall be separated by solidi and
give the sequence in which the coatings are applied (see subclauses 4.2, 4.3 and 4.4). If a specific treatment is not required and thus need not be referred to in the designation, this shall be indicated by a double solidus.
4.2Coating methods
There are three black oxide coating methods, each resulting in a different mean mass of coating per unit area (see table 1). These shall be designated as follows:
single-bath coating: A;
two-bath coating: B;
three-bath coating: C.
Table 1:Approximate mean mass per unit area of black oxide coatings
Designation of Approximate mean mass per
coating method unit area of coating, in g/m2
A4,5
B5,5
C6,5
4.3Heat treatment
The heat treatment shall be designated by the following:
a)the letter ‘H’;
b)the minimum heat treatment temperature, expressed in°C, in brackets;
c)the treatment time, in hours.
Designation of heat treatment carried out for three hours at a minimum temperature of 190°C:
HT(190)3
*)Translator’s note. In DIN EN ISO2064, ‘significant surface’ is defined as ‘The part of an article covere
d or to be covered by the coating and for which the coating is essential for serviceability or appearance’.
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DIN50938:2000-04
4.4Supplementary treatments
Any supplementary treatments shall be designated as in table 2.
Table 2: Designation of supplementary treatments applied to black oxide coatings
Symbol Type of treatment
T1Application of paints, varnishes, pow-
der coatings or similar materials
T4Application of grease, oil or other lubri-
cants
T5Application of wax
T10Application of water-displacing anti-
corrosive agents
NOTE: The symbols conform to the standard designations for supplementary treatment for electrodeposited coatings as in DIN EN1403, except for T10.
Designation of a black oxide coating on a steel (Fe) component that has not been heat-treated, but has undergone the single-bath coating process (A) and then waxed (T5):
Coating DIN50938 – Fe//A/T5
5Order details
The customer shall provide the following information:
a)the designation of the coating (see clause 4);
b)whether heat treatment is required prior to coating (see subclause 4.3);
c)the significant surface (e.g. by providing a suitably marked specimen);
d)the quality, state and appearance of the base metal if they may affect the serviceability and appearance
of the coating;
e)the location of unavoidable defects (e.g. attachment points);
f)details of sampling, the acceptable quality level or any test requirement differing from those in ISO4519
(see clause 7).
The customer may provide the following supplementary information:
g)any special requirements or conditions relating to the surface preparation of the components;
h)any special requirements relating to the reprocessing of discarded components;
i)any other special requirements (e.g. regarding roughness).
6Procedure
6.1Preparation of surfaces
The surfaces of the components to be coated shall be carefully degreased and any scale, oxide and other impurities removed. To prevent the temperature of the bath dropping sharply, the components may be preheated in water at 60°C to 80°C.editorially
6.2Main treatment
One or more coating stages shall be carried out in boiling alkaline oxidizing solution at about 135°C to 145°C, optionally with reducing agents or other substances added to the bath. Depending on the composition, con-centration and temperature of the solution, the coatings will develop in immersion times of between five and twenty minutes. The coating time will also depend on the nature and quality of the components.
The suppliers’ operating and safety instructions shall be observed when making up and using the bath. The condition of the bath shall be continuously checked by monitoring the boiling point, which is dependent on concentration. If necessary, this point shall be adjusted to the required value by carefully
adding water or coating salt. No other bath checks are generally needed. In some cases, the quality of the coating may be improved by adding other salts to the bath.
In the case of the multi-bath methods, the second immersion and, where necessary, the third immersion shall be carried out in separate baths, the boiling point of each bath being about 5°C higher than that of the preceding bath, with a rinse in cold water between baths.
After coating, the components shall be rinsed, first in cold, stationary water, then thoroughly in cold, running water and finally in hot water at 60°C to 80°C.
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DIN50938:2000-04
6.3Drying and supplementary treatment
After rinsing, the components shall be dried immediately in an oven at about 120°C or in hot air. Drying is unnecessary if there is to be an aftertreatment with hot saponifiable oil at about 120°C, in oil emulsions at 80°C to 95°C or with hot, molten wax. If the components are to be aftertreated in water-displacing anti-corrosion agents, they shall be treated while cold to minimize the loss of solvent by eva
poration.
Supplementary treatments serve to increase the degree of protection. To meet special requirements, paints or similar coatings may be applied.
7Testing the untreated coating
The test to be used to assess the quality of the coating or of the bath shall be agreed on a case-by-case basis. Unless otherwise agreed, the samples shall be collected as specified in ISO4519 (see clause 5, item f)).
For accuracy experiments, the samples taken shall measure at least 50mm×50mm×1mm, be of the same material grade, have a surface texture as similar as possible to that of the component to be coated and have received the same treatment prior to coating.
7.1Appearance
The black oxide coating shall cover the surface of the base metal evenly and completely, and its colour shall be uniform and as black as possible. There shall be no brownish or greenish spots due to an excessive concentration or contamination of the coating bath. If a coated component that has not been
aftertreated is wiped with a clean white cloth, no colour shall appear on the cloth, except in the case of cast materials and hardened components.
7.2Determining the degree of protection by exposure to a standard atmosphere
The anti-corrosive effect of the coating shall be determined by suspending components or specimens that have not been aftertreated, but dried as described in subclause 6.3, in the DIN50017–KK standard atmosphere. This test may be commenced immediately after the specimens have been dried and cooled to ambient tempera-ture.
Table 3:Minimum exposure times in DIN50017–KK test
Coating method Minimum time with no signs of corrosion, in hours
A24
B48
C72
The minimum exposure time shall be as specified in table 3, after which not more than 1% of the surface of the component tested shall be covered with rust, unless otherwise agreed. Slight changes in hue shall be ignored. When components having highly contoured surfaces are tested, the minimum exposure time shall be agreed on a case-by-case basis. To prevent edge corrosion, sharp edges shall be coated with acid-resistant paint after coating and prior to being placed in the test chamber. Any corrosion at such points shall be ignored.
7.3Determining the degree of protection with acetic acid at ambient temperature
7.3.1Test solution
20% (m/m) aqueous solution of analytical grade acetic acid, CH
3
COOH.
7.3.2Procedure
After coating, the components or specimens shall not be aftertreated, but dried as specified in subclau
se 6.3 and cooled to ambient temperature. About four drops of the test solution shall then be applied to a flat area of the coated specimen or component. The time taken for the surface of this area to be exposed shall not be less than that specified in table 4.
When components having highly contoured surfaces are to be tested, full immersion with amended times may be agreed.
Table 4:Minimum exposure times in acetic acid test
Coating method Minimum exposure time,
in minutes
A10 B20 C30
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DIN 50938:2000-04
Table 5:Assessment of oxalic acid test
7.4Determining the degree of protection with oxalic acid at ambient temperature
7.4.1Test solution
5% (m/m ) aqueous solution of analytical grade oxalic acid, C 2H 2O 4.2H 2O.
7.4.2Procedure
After coating, the components or specimens shall not be aftertreated, but only dried as specified in subclause
6.3 and cooled to ambient temperature. About three drops of the test solution shall then be applied to a flat area of the coated specimen or component.
After eight minutes’ exposure, the test solution shall be rinsed off with water and the affected area evaluated visually as in table 5.
7.5Determining the mass per unit area
7.5.1Test solution
Chemically pure 1mo l /l  hydrochloric acid with a commercial inhibitor added, the inhibitor being chosen so that the loss in mass, m 0, determined as specified in subclause 7.5.2, is less than 0,5g/m 2.
An adequate inhibitory action is achieved, for example, if a 4% (m/m ) solution of hexamethylenetetramine,C 6H 12N 14, in 1mo l /l  hydrochloric acid is used.
7.5.2Procedure
The coated, dried specimen or component shall be weighed (mass m 1) after the area of the surface, A , has been determined.
The specimen or component shall then be immersed in the test solution at ambient temperature until the coating has been dissolved. It shall then be removed, rinsed, dried and reweighed (mass m 2).
To determine the loss in mass of steel due to the action of the test solution containing inhibitor, m 0, uncoated test sheets or components made of the same material shall be weighed (mass m 3), treated for the same time as the coated component with test solution, dried and reweighed (mass m 4).
The loss in mass, m 0, is given by equation (1):
m 0=m 3–m 4(1)The value of m 0 shall be taken into account when calculating the loss in mass, D m , of the coated part, using equation (2):
D m =m 1–m 2+m 0
(2)
The mass per unit area, m A , in g/m 2, is given by equation (3):10×=A m m D A (3)where m  is in mg and A  is in cm².
8Test report
The test report shall refer to this standard and include the following details:
a)type and designation of coated component;
b)type and designation of coating;
c)type and results of testing as described in clause 7.Appearance of exposed
Quality of coating coating area
Grey with light-grey halo Inadequate Black to brownish black
Good with exposed halo 1)
1)Even good quality coatings may have very
bright halos due to metal being exposed at the
boundary of the area to which the solution was
applied. However, assessment shall be based
solely on the area covered by the drops.

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