N02 207 Textile Systems
001-en Technical Design Manual Browning  DIN 50938 Page 1/1 Release: S. Liefert DTTNS  2003-10-21 Change:
The following standard is used at Rieter:
DIN 50938: April 2000, pages 1 - 5
Black Oxide Treatment of Steel Components
Requirements and Testing
Recommendations and Engineering Drawing Notation:
Designation for a black oxide coating following DIN 50 938 on a steel (Fe) part, process group (A) bath black oxide and post-treatment by greasing, oiling or other lubricants (T4).
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 covered 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);editorially
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.
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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