ASTM B879-97
(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
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 3.2) 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.
General Information
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Standards Content (Sample)
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Designation: B 879 – 97
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.
1. Scope DTD 911 (British), Protection of Magnesium-Rich Alloys
Against Corrosion
1.1 This practice covers a guide for metal finishers to clean
DTD 5562 (British), Clear Baking Resin for Surface Seal-
and then provide a paint base for the finishing of magnesium
ing Magnesium
and magnesium alloys using chemical conversion coatings.
DTD 935 (British), Surface Sealing of Magnesium Rich
Where applicable (for example, aerospace) secondary supple-
Alloys
mentary coatings (for example, surface sealing) can be used
(see Appendix X1).
3. Significance and Use
1.2 Although primarily intended as a base for paint, chemi-
3.1 The processes described in this practice clean and
cal conversion coatings provide varying degrees of surface
provide a paint base for the finishing of magnesium and
protection for magnesium parts exposed to indoor atmosphere
magnesium alloys. Service conditions will determine, to some
either in storage or in service under mild exposure conditions.
degree, the specific process to be applied.
An example is the extensive use of the dichromate treatment
(see section 5.2) as a final coating for machined surfaces of die
4. Reagents
cast magnesium components in the computer industry.
4.1 The chemicals that are used to formulate and control the
1.3 The traditional numbering of the coating is used
processing solutions are listed in Table 1. Commercial grade
throughout.
chemicals are satisfactory. The concentrations stated for chemi-
1.4 This standard does not purport to address all of the
cals that are normally supplied at less than a nominal 100 %
safety concerns, if any, associated with its use. It is the
strength are those typically available. Other strengths may be
responsibility of the user of this standard to establish appro-
used in the proportions that yield the specified processing
priate safety and health practices and determine the applica-
concentrations. Unless otherwise stated all solutions are made
bility of regulatory limitations prior to use.
up using water.
2. Referenced Documents
5. Types of Coating
2.1 The following documents form a part of this practice to
5.1 Chrome Pickle (Traditional Number 1) Treatment (See
the extent referenced herein.
Practices D 1732):
2.2 ASTM Standards:
5.1.1 With slight variations this treatment can be applied to
D 1732 Practices for Preparation of Magnesium Alloy Sur-
all alloys and forms of magnesium. The treatment removes up
faces for Painting
to 15 μm of metal per surface, 30 μm per diameter. Therefore,
2.3 SAE Standard:
it may not be applicable to machined surfaces with close
AMS 2475 Protective Treatments—Magnesium Alloys
tolerances. Parts with steel inserts may be processed, but some
2.4 Military Specifications:
slight etching of the steel surface may occur.
MIL-M-3171 Magnesium Alloy, Processes for Pretreat-
4 5.1.2 The color, luster, and etch produced by the treatment
ment and Prevention of Corrosion on
will vary with the age and usage of the solution, alloy
composition, and heat treatment of the alloy. The most desir-
able paint base is a matte grey to yellow-red, iridescent coating
This practice is under the jurisdiction of ASTM Committee B-8 on Metallic and
Inorganic Coatings and is the direct responsibility of Subcommittee B08.07 on
which exhibits a pebbled etch finish when viewed under low
Chemical Conversion Coatings.
magnification (5 to 103). Bright brassy coatings, showing a
Current edition approved Jan. 10, 1997. Published March 1997.
relatively smooth surface with only occasional rounded pits
Annual Book of ASTM Standards, Vol 02.05.
Available from Society of Automotive Engineers, 400 Commonwealth Drive, under low magnification are unsatisfactory as a paint base but
Warrendale, PA 15096.
are acceptable for protection during shipping and storage.
Available from Standardization Documents Order Desk, Bldg. 4 Section D, 700
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.
NOTICE: This standard has either been superceded and replaced by a new version or discontinued. NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information. Contact ASTM International (www.astm.org) for the latest information.
B879–97
TABLE 1 Processing Chemicals
particularly useful for application to optical equipment requir-
Acetic acid glacial, (CH COOH) ing a nonreflective black coating.
Aluminum sulfate (Al [SO ] ·14H O)
2 3 2
5.4 Chromic Acid Brush-On (Traditional Number 19) Treat-
Ammonium bifluoride (NH HF )
4 2
ment:
Ammonium hydroxide (NH OH), 30 %
Ammonium phosphate monobasic (NH H PO )
4 2 4
5.4.1 This treatment can be applied to parts that require
Ammonium sulfate ([NH ] SO )
4 2 4
touchup. It is generally used in refinishing procedures or where
Ammonium sulfite ([NH ] SO ·H O)
4 2 3 2
Calcium chromate (CaCrO ) parts or assemblies are too large to be immersed. It is effective
Calcium fluoride (CaF )
on most alloys and causes negligible dimensional changes.
Calcium sulfate (CaSO ·2H O)
4 2
5.4.2 Coatings produced by this treatment can vary from a
Chromic acid (CrO )
Ferric nitrate (Fe[NO ] ·9H O)
3 3 2
brassy iridescence to a dark brown depending upon treatment
Glycolic acid (HOCH COOH), 70 %
time. Prolonged treatment produces powdery coatings. For best
Hydrofluoric acid (HF), 60 %
adhesion, dark brown coatings are preferred.
Magnesium fluoride (MgF )
Magnesium nitrate (Mg[NO ] ·6H O)
3 2 2
5.5 Chromate Treatment (see DTD 911):
Magnesium sulfate (MgSO ·7H O)
4 2
Manganese sulfate (MnSO ·5H O)
5.5.1 This treatment is suitable for all magnesium alloys.
4 2
Nitric acid (HNO ), sp gr 1.42
The treatment causes no dimensional change and is normally
Phosphoric acid (H PO ), 85 %
3 4
applied after machining. The pickling procedures and the
Potassium fluoride (KF)
Potassium bifluoride (KHF ) composition of the treating solution generally vary with the
Sodium bifluoride (NaHF )
alloy being processed.
Sodium bisulfate (NaHSO )
Sodium carbonate (Na CO ) 5.5.2 The coating will vary from dark brown to light
2 3
Sodium dichromate (Na Cr O ·2H O)
2 2 7 2
reddish-brown depending on the alloy.
Sodium hydroxide (NaOH)
5.6 Chrome-Manganese Treatment:
Sodium metasilicate (Na SiO ,orNa SiO ·4H O)
2 3 2 3 2
Sodium nitrate (NaNO )
5.6.1 This treatment provides an improved paint base com-
Sulfuric acid (H SO ), sp gr 1.84
2 4
pared with the chrome pickle treatment and protection on all
standard alloys except EK41A, HM31A, HM21A, HK31A,
and M1A on which the coating does not form. The treatment
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 nonairconditioned environ-
fluoride treatment (see 7.2.3) should replace the hydrofluoric
ments. 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
5.8 Phosphate Treatment:
specifically used for those alloys which do not react or form
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
dichromate treatment, a prior immersion in acid fluoride
used on magnesium die castings for automotive and other
solution is required to condition the magnesium surface. The consumer product applications. The suitability of a particular
galvanic chromate treatment causes no appreciable dimen-
phosphatizing process for magnesium should be verified by
sional change and is normally applied after machining. testing. Iron phosphate treatments containing nickel or copper
5.3.2 Properly applied coatings vary from dark brown to a salts as accelerators are detrimental to the corrosion resistance
dense black color depending on the alloy. The treatment is of magnesium and should not be used.
NOTICE: This standard has either been superceded and replaced by a new version or discontinued. NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
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B879–97
5.8.2 Phosphate treatments do not provide interim stand- 6.2.1 Remove graphite-based lubricants from hot formed
alone protection against atmospheric oxidation and tarnish magnesium sheet parts by soaking the parts for 10 to 20 min in
equal to that provided by some chromate conversion coatings. 100 g/L sodium hydroxide maintained at 88 to 100°C. The pH
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
being banned from use. Exposure to their vapor (VOC) is being
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.
petroleum spirits, naphths, lacquer thinner, and similar solvents
that do not attack magnesium may be used. Methyl alcohol
6.3.2 Previously applied coatings may be removed with the
(CH OH) should not be used because it may react with the alkaline cleaners recommended in 6.1.4.
magnesium surface.
6.3.3 If the finish is difficult to remove, immerse the part in
6.1.3 Mechanical Cleaning—Mechanical cleaning may
the chromic acid pickle given in 6.5.1. Alternate immersion in
consist of sand, shot, pumice, grit or vapor blasting, sodium
the alkaline cleaner and the chromic acid pickle may be
carbonate slurry, sanding, hard bristle brushing, grinding and
required to remove aged finishes. Rinse well in water between
rough polishing. Sand, shot, or grit blasting leaves surface
acid and alkaline pickling.
contamination that will greatly increase the corrosion rate of
6.3.4 The chromic acid brush-on treatment (see 5.4) may be
the magnesium on exposure to salt water or humid environ-
applied over the chrome pickle finish or over previously
ment. If these methods are used, specific pickling procedures
applied brush-on coatings without removing the previously
must be employed after blasting (see 6.4.2).
applied coating.
6.1.4 Alkaline Cleaning—Cleaning prior to application of
6.4 Acid Pickling:
treatments other than the chrome pickle treatment (see 5.1),
6.4.1 General pickling to remove oxide layers, old chemical
when used for protection during shipment or storage, should be
finishes, burned-on drawing and forming lubricants, and other
done in an alkaline cleaner recommended for steel or in a
water insoluble or non-emulsifiable substances i
...
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