ASTM B879-97(2003)e1
(Practice)Standard Practice for Applying Non-Electrolytic Conversion Coatings on Magnesium and Magnesium Alloys
Standard Practice for Applying Non-Electrolytic Conversion Coatings on Magnesium and Magnesium Alloys
SIGNIFICANCE AND USE
The processes described in this practice clean and provide a paint base for the finishing of magnesium and magnesium alloys. Service conditions will determine, to some degree, the specific process to be applied.
SCOPE
1.1 this practice covers a guide for metal finishers to clean and then provide a paint base for the finishing of magnesium and magnesium alloys using chemical conversion coatings. Where applicable (for example, aerospace) secondary supplementary coatings (for example, surface sealing) can be used (see Appendix X1).
1.2 Although primarily intended as a base for paint, chemical conversion coatings provide varying degrees of surface protection for magnesium parts exposed to indoor atmosphere either in storage or in service under mild exposure conditions. An example is the extensive use of the dichromate treatment (see 5.2) as a final coating for machined surfaces of die cast magnesium components in the computer industry.
1.3 The traditional numbering of the coating is used throughout.
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation:B879–97 (Reapproved 2003)
Standard Practice for
Applying Non-Electrolytic Conversion Coatings on
Magnesium and Magnesium Alloys
This standard is issued under the fixed designation B 879; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
e NOTE—Sections X2.2 and X2.4.2 were editorially updated in May 2003.
1. Scope MIL-M-3171 Magnesium Alloy, Processes for Pretreat-
ment and Prevention of Corrosion on
1.1 This practice covers a guide for metal finishers to clean
DTD 911 (British), Protection of Magnesium-Rich Alloys
and then provide a paint base for the finishing of magnesium
Against Corrosion
and magnesium alloys using chemical conversion coatings.
DTD 5562 (British), Clear Baking Resin for Surface Seal-
Where applicable (for example, aerospace) secondary supple-
ing Magnesium
mentary coatings (for example, surface sealing) can be used
DTD 935 (British), Surface Sealing of Magnesium Rich
(see Appendix X1).
Alloys
1.2 Although primarily intended as a base for paint, chemi-
cal conversion coatings provide varying degrees of surface
3. Significance and Use
protection for magnesium parts exposed to indoor atmosphere
3.1 The processes described in this practice clean and
either in storage or in service under mild exposure conditions.
provide a paint base for the finishing of magnesium and
An example is the extensive use of the dichromate treatment
magnesium alloys. Service conditions will determine, to some
(see section 5.2) as a final coating for machined surfaces of die
degree, the specific process to be applied.
cast magnesium components in the computer industry.
1.3 The traditional numbering of the coating is used
4. Reagents
throughout.
4.1 The chemicals that are used to formulate and control the
1.4 This standard does not purport to address all of the
processing solutions are listed in Table 1. Commercial grade
safety concerns, if any, associated with its use. It is the
chemicalsaresatisfactory.Theconcentrationsstatedforchemi-
responsibility of the user of this standard to establish appro-
cals that are normally supplied at less than a nominal 100 %
priate safety and health practices and determine the applica-
strength are those typically available. Other strengths may be
bility of regulatory limitations prior to use.
used in the proportions that yield the specified processing
concentrations. Unless otherwise stated all solutions are made
2. Referenced Documents
up using water.
2.1 The following documents form a part of this practice to
the extent referenced herein.
5. Types of Coating
2.2 ASTM Standards:
5.1 Chrome Pickle (Traditional Number 1) Treatment (See
D 1732 Practices for Preparation of Magnesium Alloy Sur-
Practices D 1732):
faces for Painting
5.1.1 With slight variations this treatment can be applied to
2.3 SAE Standard:
3 all alloys and forms of magnesium. The treatment removes up
AMS 2475 Protective Treatments—Magnesium Alloys
to 15 µm of metal per surface, 30 µm per diameter. Therefore,
2.4 Military Specifications:
it may not be applicable to machined surfaces with close
tolerances. Parts with steel inserts may be processed, but some
slight etching of the steel surface may occur.
This practice is under the jurisdiction of ASTM Committee B08 on Metallic
and Inorganic Coatings and is the direct responsibility of Subcommittee B08.07 on
5.1.2 The color, luster, and etch produced by the treatment
Chromate Conversion Coatings.
will vary with the age and usage of the solution, alloy
Current edition approved Feb. 10, 2003. Published May 2003. Originally
approved in 1997. Last previous edition approved in 1997 as B 879–97.
Annual Book of ASTM Standards, Vol 02.05.
3 4
Available from Society of Automotive Engineers, 400 Commonwealth Drive, AvailablefromStandardizationDocumentsOrderDesk,Bldg.4SectionD,700
Warrendale, PA 15096. Robbins Ave., Philadelphia, PA 19111-5094, Attn: NPODS.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
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B879–97 (2003)
TABLE 1 Processing Chemicals
dichromate treatment, a prior immersion in acid fluoride
Acetic acid glacial, (CH COOH) solution is required to condition the magnesium surface. The
Aluminum sulfate (Al [SO ] ·14H O)
2 3 2
galvanic chromate treatment causes no appreciable dimen-
Ammonium bifluoride (NH HF )
4 2
sional change and is normally applied after machining.
Ammonium hydroxide (NH OH), 30 %
Ammonium phosphate monobasic (NH H PO )
5.3.2 Properly applied coatings vary from dark brown to a
4 2 4
Ammonium sulfate ([NH ] SO )
4 2 4
dense black color depending on the alloy. The treatment is
Ammonium sulfite ([NH ] SO ·H O)
4 2 3 2
particularly useful for application to optical equipment requir-
Calcium chromate (CaCrO )
Calcium fluoride (CaF )
ing a nonreflective black coating.
Calcium sulfate (CaSO ·2H O)
4 2
5.4 Chromic Acid Brush-On (Traditional Number 19) Treat-
Chromic acid (CrO )
ment:
Ferric nitrate (Fe[NO ] ·9H O)
3 3 2
Glycolic acid (HOCH COOH), 70 %
5.4.1 This treatment can be applied to parts that require
Hydrofluoric acid (HF), 60 %
touch up. It is generally used in refinishing procedures or
Magnesium fluoride (MgF )
Magnesium nitrate (Mg[NO ] ·6H O) where parts or assemblies are too large to be immersed. It is
3 2 2
Magnesium sulfate (MgSO ·7H O)
4 2
effective on most alloys and causes negligible dimensional
Manganese sulfate (MnSO ·5H O)
4 2
changes.
Nitric acid (HNO ), sp gr 1.42
Phosphoric acid (H PO ), 85 %
5.4.2 Coatings produced by this treatment can vary from a
3 4
Potassium fluoride (KF)
brassy iridescence to a dark brown depending upon treatment
Potassium bifluoride (KHF )
time.Prolongedtreatmentproducespowderycoatings.Forbest
Sodium bifluoride (NaHF )
Sodium bisulfate (NaHSO )
4 adhesion, dark brown coatings are preferred.
Sodium carbonate (Na CO )
2 3
5.5 Chromate Treatment (see DTD 911):
Sodium dichromate (Na Cr O ·2H O)
2 2 7 2
Sodium hydroxide (NaOH) 5.5.1 This treatment is suitable for all magnesium alloys.
Sodium metasilicate (Na SiO ,orNa SiO ·4H O)
2 3 2 3 2
The treatment causes no dimensional change and is normally
Sodium nitrate (NaNO )
applied after machining. The pickling procedures and the
Sulfuric acid (H SO ), sp gr 1.84
2 4
composition of the treating solution generally vary with the
alloy being processed.
5.5.2 The coating will vary from dark brown to light
composition, and heat treatment of the alloy. The most desir-
reddish-brown depending on the alloy.
able paint base is a matte grey to yellow-red, iridescent coating
5.6 Chrome-Manganese Treatment:
which exhibits a pebbled etch finish when viewed under low
5.6.1 This treatment provides an improved paint base com-
magnification (5 to 103). Bright brassy coatings, showing a
pared with the chrome pickle treatment and protection on all
relatively smooth surface with only occasional rounded pits
standard alloys except EK41A, HM31A, HM21A, HK31A,
under low magnification are unsatisfactory as a paint base but
and M1A on which the coating does not form. The treatment
are acceptable for protection during shipping and storage.
causes no appreciable dimensional change, and normally is
5.2 Dichromate (Traditional Number 7) Treatment (see
applied after machining. It is suitable for close clearance parts.
Practices D 1732):
Parts containing inserts of bronze, brass, steel, or cadmium
5.2.1 This treatment provides an improved paint base com-
plated steel should not be treated unless the dissimilar metals
pared with the chrome pickle treatment, and for temporary
are masked or it is demonstrated that the treatment will not
protection on all standard alloys except, EK41A, HM31A,
adversely affect them.
HM21A, HK31A, WE54, WE43, and M1A on which the
5.6.2 The bath generally gives dark brown to black films on
coating does not form. The treatment causes no appreciable
both cast and wrought magnesium alloys. Treatment of alumi-
dimensional changes, is normally applied after machining, and
num containing alloys may require bath temperatures above
is suitable for close clearance parts. Parts containing inserts of
50°C.
bronze, brass, steel, or cadmium plated steel should not be
5.7 SemiBright Pickle (Traditional Number 21) Treatment:
treated unless the dissimilar metals are masked or it is
5.7.1 This treatment provides a semibright silvery surface
demonstrated that the treatment will not adversely affect them.
on magnesium parts that prevents tarnishing and corrosion for
For assemblies containing aluminum inserts or rivets, the acid
indoor storage up to six months in non-air-conditioned envi-
fluoride treatment (see 7.2.3) should replace the hydrofluoric
ronments. Extended storage times can be obtained by using air
acid treatment in part preparation.
conditioning. This process causes negligible dimensional
5.2.2 Coatings vary from light to dark brown depending
change. It is a simple, economical way to apply an attractive
upon the alloy. On AZ91C-T6 and AZ92A-T6 castings the
shelf-life finish and is a good base for clear lacquers. The
coating is grey.
treatment greatly reduces or eliminates “filiform or worm-
5.3 Galvanic Chromate (Traditional Number 9) Treatment
tracking” corrosion usually experienced when clear paints are
(see Practices D 1732):
used directly over polished metal surfaces.
5.3.1 This treatment can be used for all alloys and is
specifically used for those alloys which do not react or form 5.8 Phosphate Treatment:
satisfactory conversion coatings in other baths. The treatment 5.8.1 Phosphate treatments can provide a satisfactory paint
requires no external current but utilizes the relatively high base on magnesium for many applications when it is necessary
potential difference between suitably racked magnesium com- to avoid the use of chromates. Commercial iron phosphate
ponents and steel tank walls or other cathodes. As with the treatments applied by spray or dipping have been successfully
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B879–97 (2003)
used on magnesium die castings for automotive and other formation of oxide films, pitting of the magnesium surface, or
consumer product applications. The suitability of a particular both. However, electrolytic cleaning using cathodic current at
phosphatizing process for magnesium should be verified by 1 to 4 A/dm may be carried out in properly formulated
testing. Iron phosphate treatments containing nickel or copper cleaners.
salts as accelerators are detrimental to the corrosion resistance
6.2 Graphite Lubricant Removal:
of magnesium and should not be used.
6.2.1 Remove graphite-based lubricants from hot formed
5.8.2 Phosphate treatments do not provide interim stand-
magnesium sheet parts by soaking the parts for 10 to 20 min in
alone protection against atmospheric oxidation and tarnish
100 g/L sodium hydroxide maintained at 88 to 100°C. The pH
equal to that provided by some chromate conversion coatings.
should be above 13.0.Add wetting agent (0.75 g/L), if needed,
for the removal of heavy films of mineral oil. Then rinse parts
6. Part Preparation
thoroughly in cold water and immerse for 3 min in a chromic-
6.1 Cleaning—General:
nitrate pickle as specified in 6.5.2. Repeat the cycle until all
6.1.1 Before considering the use of solvent degreasing,
parts are clean.
consult federal and state safety and environmental laws and
6.2.2 Because of the difficulty of removing graphite from
regulations. Many of the commonly used solvents are now
chrome pickled sheet, such sheet should not be used for
beingbannedfromuse.Exposuretotheirvapor(VOC)isbeing
forming unless the chrome pickle is removed as outlined in 6.3
strictly regulated for health, safety, and environmental reasons.
before forming.
Obtain current safe exposure levels for various solvents before
6.3 Previously Applied Chemical Finishes:
use. Follow all federal, state, and local regulations for the
6.3.1 Magnesium base alloys are often supplied with a
disposal of solvents.
chrome pickle treatment to protect them during shipment,
6.1.2 Solvent Cleaning—Grease or oil may be removed by
storage, and machining. The coating from this treatment
means of vapor degreasing, ultrasonic cleaning, solvent wash-
remaining on unmachined areas will impair the film produced
ing, or an emulsion cleaning process that utilizes a mineral oil
by any subsequent chromate treatment and therefore must be
distillate and an emulsifying agent. Chlorinated solvents,
removed.
petroleumspirits,naphths,lacquerthinner,andsimilarsolvents
6.3.2 Previously applied coatings may be removed with the
that do not attack magnesium may be used. Methyl alcohol
alkaline cleaners recommended in 6.1.4.
(CH OH) should not be used because it may react with the
6.3.3 If the finish is difficult to remove, immerse the part in
magnesium surface.
the chromic acid pickle given in 6.5.1.Alternate immersion in
6.1.3 Mechanical Cleaning—Mechanical cleaning may
the alkaline cleaner and the chromic acid pickle may be
consist of sand, shot, pumice, grit or vapor blasting, sodium
required to remove aged finishes. Rinse well in water between
carbonate slurry, sanding, hard bristle brushing, grinding and
acid and alkaline pickling.
rough polishing. Sand, shot, or grit blasting leaves surface
6.3.4 The chromic acid brush-on treatment (see 5.4) may be
contamination that will greatly increase the corrosion rate of
applied over the chrome pickle finish or over previously
the magnesium on exposure to salt water or humid environ-
applied brush-on coatings without removing the previously
ment. If these methods are used, specific pickling procedures
applied coating.
must be employed after blasting (see 6.4.2).
6.4 Acid Pickling:
6.1.4 Alkaline Cleaning—Cleaning prior to application of
6.4.1 General pickling to remove oxide layers, old chemical
treatments other than the chrome pickle treatment (see 5.1),
finishes, burned-on drawing and forming lubricants, and other
whenusedforprotectionduringshipmentorstorage,shouldbe
water insoluble or non-emulsifiable substances is preferably
done in an alkaline cleaner recommended for steel or in a
carried out using a chromic acid type pickle as described in
cleaning solution as specified in 6.1.4.1. Maintain the solution
6.5.1 or 6.5.2.
pH above 8.0.Alkaline cleaning prior to the application of the
6.4.2 Pickle sand and permanent mold castings that have
chrome pickle treatment (see 5.1), when used for protection
been mechanically cleaned as described in 6.1.3 in the sulfuric
duringshipmentandstorageonly,maybeomittedprovidedthe
acid pickle described in 6.5
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