ASTM B879-97(2008)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 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.5 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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Standards Content (Sample)
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Designation: B879 − 97 (Reapproved2008)
Standard Practice for
Applying Non-Electrolytic Conversion Coatings on
Magnesium and Magnesium Alloys
This standard is issued under the fixed designation B879; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
´ NOTE—The units statement in subsection 1.4 was corrected editorially in April 2008.
1. Scope D1732Practices for Preparation of Magnesium Alloy Sur-
faces for Painting
1.1 This practice covers a guide for metal finishers to clean
2.3 SAE Standard:
and then provide a paint base for the finishing of magnesium
AMS 2475Protective Treatments—Magnesium Alloys
and magnesium alloys using chemical conversion coatings.
2.4 Military Specifications:
Where applicable (for example, aerospace) secondary supple-
MIL-M-3171 MagnesiumAlloy, Processes for Pretreatment
mentary coatings (for example, surface sealing) can be used
and Prevention of Corrosion on
(see Appendix X1).
DTD 911(British), Protection of Magnesium-Rich Alloys
1.2 Although primarily intended as a base for paint, chemi-
Against Corrosion
cal conversion coatings provide varying degrees of surface
DTD5562(British),ClearBakingResinforSurfaceSealing
protection for magnesium parts exposed to indoor atmosphere
Magnesium
either in storage or in service under mild exposure conditions.
DTD 935(British), Surface Sealing of Magnesium Rich
An example is the extensive use of the dichromate treatment
Alloys
(see 5.2) as a final coating for machined surfaces of die cast
magnesium components in the computer industry.
3. Significance and Use
1.3 The traditional numbering of the coating is used
3.1 The processes described in this practice clean and
throughout.
provide a paint base for the finishing of magnesium and
magnesium alloys. Service conditions will determine, to some
1.4 The values stated in SI units are to be regarded as
degree, the specific process to be applied.
standard. No other units of measurement are included in this
standard.
4. Reagents
1.5 This standard does not purport to address all of the
4.1 Thechemicalsthatareusedtoformulateandcontrolthe
safety concerns, if any, associated with its use. It is the
processing solutions are listed in Table 1. Commercial grade
responsibility of the user of this standard to establish appro-
chemicalsaresatisfactory.Theconcentrationsstatedforchemi-
priate safety and health practices and determine the applica-
cals that are normally supplied at less than a nominal 100%
bility of regulatory limitations prior to use.
strength are those typically available. Other strengths may be
used in the proportions that yield the specified processing
2. Referenced Documents
concentrations. Unless otherwise stated all solutions are made
2.1 The following documents form a part of this practice to
up using water.
the extent referenced herein.
5. Types of Coating
2.2 ASTM Standards:
5.1 Chrome Pickle (Traditional Number 1) Treatment (See
Practices D1732):
This practice is under the jurisdiction of ASTM Committee B08 on Metallic
5.1.1 With slight variations this treatment can be applied to
and Inorganic Coatings and is the direct responsibility of Subcommittee B08.07 on
all alloys and forms of magnesium. The treatment removes up
Conversion Coatings.
to 15 µm of metal per surface, 30 µm per diameter. Therefore,
Current edition approved April 1, 2008. Published April 2008. Originally
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approved in 1997. Last previous edition approved in 2003 as B879–97(2003) .
DOI: 10.1520/B0879-97R08E01.
2 3
For referenced ASTM standards, visit the ASTM website, www.astm.org, or Available from Society of Automotive Engineers (SAE), 400 Commonwealth
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Dr., Warrendale, PA 15096-0001, http://www.sae.org.
Standards volume information, refer to the standard’s Document Summary page on AvailablefromStandardizationDocumentsOrderDesk,Bldg.4SectionD,700
the ASTM website. Robbins Ave., Philadelphia, PA 19111-5094, Attn: NPODS.
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B879 − 97 (2008)
TABLE 1 Processing Chemicals
5.3.1 This treatment can be used for all alloys and is
Acetic acid glacial, (CH COOH) specifically used for those alloys which do not react or form
Aluminum sulfate (Al [SO ] ·14H O)
2 3 2
satisfactory conversion coatings in other baths. The treatment
Ammonium bifluoride (NH HF )
4 2
requires no external current but utilizes the relatively high
Ammonium hydroxide (NH OH), 30 %
Ammonium phosphate monobasic (NH H PO ) potential difference between suitably racked magnesium com-
4 2 4
Ammonium sulfate ([NH ] SO )
4 2 4
ponents and steel tank walls or other cathodes. As with the
Ammonium sulfite ([NH ] SO ·H O)
4 2 3 2
dichromate treatment, a prior immersion in acid fluoride
Calcium chromate (CaCrO )
Calcium fluoride (CaF )
solution is required to condition the magnesium surface. The
Calcium sulfate (CaSO ·2H O)
4 2
galvanic chromate treatment causes no appreciable dimen-
Chromic acid (CrO )
sional change and is normally applied after machining.
Ferric nitrate (Fe[NO ] ·9H O)
3 3 2
Glycolic acid (HOCH COOH), 70 %
2 5.3.2 Properly applied coatings vary from dark brown to a
Hydrofluoric acid (HF), 60 %
dense black color depending on the alloy. The treatment is
Magnesium fluoride (MgF )
particularly useful for application to optical equipment requir-
Magnesium nitrate (Mg[NO ] ·6H O)
3 2 2
Magnesium sulfate (MgSO ·7H O)
4 2 ing a nonreflective black coating.
Manganese sulfate (MnSO ·5H O)
4 2
Nitric acid (HNO ), sp gr 1.42 5.4 Chromic Acid Brush-On (Traditional Number 19) Treat-
Phosphoric acid (H PO ), 85 %
3 4
ment:
Potassium fluoride (KF)
5.4.1 This treatment can be applied to parts that require
Potassium bifluoride (KHF )
Sodium bifluoride (NaHF )
touch up. It is generally used in refinishing procedures or
Sodium bisulfate (NaHSO )
where parts or assemblies are too large to be immersed. It is
Sodium carbonate (Na CO )
2 3
effective on most alloys and causes negligible dimensional
Sodium dichromate (Na Cr O ·2H O)
2 2 7 2
Sodium hydroxide (NaOH)
changes.
Sodium metasilicate (Na SiO ,orNa SiO ·4H O)
2 3 2 3 2
5.4.2 Coatings produced by this treatment can vary from a
Sodium nitrate (NaNO )
brassy iridescence to a dark brown depending upon treatment
Sulfuric acid (H SO ), sp gr 1.84
2 4
time.Prolongedtreatmentproducespowderycoatings.Forbest
adhesion, dark brown coatings are preferred.
5.5 Chromate Treatment (see DTD 911):
it may not be applicable to machined surfaces with close
5.5.1 This treatment is suitable for all magnesium alloys.
tolerances. Parts with steel inserts may be processed, but some
The treatment causes no dimensional change and is normally
slight etching of the steel surface may occur.
applied after machining. The pickling procedures and the
5.1.2 The color, luster, and etch produced by the treatment
composition of the treating solution generally vary with the
will vary with the age and usage of the solution, alloy
alloy being processed.
composition, and heat treatment of the alloy. The most desir-
5.5.2 The coating will vary from dark brown to light
ablepaintbaseisamattegreytoyellow-red,iridescentcoating
reddish-brown depending on the alloy.
which exhibits a pebbled etch finish when viewed under low
magnification (5 to 10×). Bright brassy coatings, showing a
5.6 Chrome-Manganese Treatment :
relatively smooth surface with only occasional rounded pits
5.6.1 This treatment provides an improved paint base com-
under low magnification are unsatisfactory as a paint base but
pared with the chrome pickle treatment and protection on all
are acceptable for protection during shipping and storage.
standard alloys except EK41A, HM31A, HM21A, HK31A,
and M1A on which the coating does not form. The treatment
5.2 Dichromate (Traditional Number 7) Treatment (see
causes no appreciable dimensional change, and normally is
Practices D1732):
applied after machining. It is suitable for close clearance parts.
5.2.1 This treatment provides an improved paint base com-
Parts containing inserts of bronze, brass, steel, or cadmium
pared with the chrome pickle treatment, and for temporary
plated steel should not be treated unless the dissimilar metals
protection on all standard alloys except, EK41A, HM31A,
are masked or it is demonstrated that the treatment will not
HM21A, HK31A, WE54, WE43, and M1A on which the
adversely affect them.
coating does not form. The treatment causes no appreciable
dimensional changes, is normally applied after machining, and 5.6.2 The bath generally gives dark brown to black films on
both cast and wrought magnesium alloys. Treatment of alumi-
is suitable for close clearance parts. Parts containing inserts of
bronze, brass, steel, or cadmium plated steel should not be num containing alloys may require bath temperatures above
50°C.
treated unless the dissimilar metals are masked or it is
demonstrated that the treatment will not adversely affect them.
5.7 SemiBright Pickle (Traditional Number 21) Treatment—
For assemblies containing aluminum inserts or rivets, the acid
This treatment provides a semibright silvery surface on mag-
fluoride treatment (see 7.2.3) should replace the hydrofluoric
nesium parts that prevents tarnishing and corrosion for indoor
acid treatment in part preparation.
storage up to six months in non-air-conditioned environments.
5.2.2 Coatings vary from light to dark brown depending
Extendedstoragetimescanbeobtainedbyusingaircondition-
upon the alloy. On AZ91C-T6 and AZ92A-T6 castings the
ing. This process causes negligible dimensional change. It is a
coating is grey.
simple, economical way to apply an attractive shelf-life finish
5.3 Galvanic Chromate (Traditional Number 9) Treatment and is a good base for clear lacquers. The treatment greatly
(see Practices D1732): reduces or eliminates “filiform or worm-tracking” corrosion
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B879 − 97 (2008)
usually experienced when clear paints are used directly over 6.1.4.1 Alkaline cleaning may be carried out in solutions of
polished metal surfaces. proprietary cleaners. In this case the operating conditions
should be as specified by the supplier. In no case should a
5.8 Phosphate Treatment:
cleaner having pH lower than 8.0 be used. Most recommended
5.8.1 Phosphate treatments can provide a satisfactory paint
cleanersareusedbysimpleimmersion.Afteralkalinecleaning,
baseonmagnesiumformanyapplicationswhenitisnecessary
rise parts thoroughly in cold running water. No water breaks
to avoid the use of chromates. Commercial iron phosphate
should be observed in the rinse.
treatments applied by spray or dipping have been successfully
6.1.5 Electrolytic Cleaning—Use of anodic current for
used on magnesium die castings for automotive and other
cleaningisnotgenerallyrecommendedbecauseofthepossible
consumer product applications. The suitability of a particular
formation of oxide films, pitting of the magnesium surface, or
phosphatizing process for magnesium should be verified by
both. However, electrolytic cleaning using cathodic current at
testing. Iron phosphate treatments containing nickel or copper
1 to 4 A/dm may be carried out in properly formulated
salts as accelerators are detrimental to the corrosion resistance
cleaners.
of magnesium and should not be used.
6.2 Graphite Lubricant Removal:
5.8.2 Phosphate treatments do not provide interim stand-
alone protection against atmospheric oxidation and tarnish 6.2.1 Remove graphite-based lubricants from hot formed
equal to that provided by some chromate conversion coatings.
magnesiumsheetpartsbysoakingthepartsfor10to20minin
100 g/Lsodium hydroxide maintained at 88 to 100°C. The pH
shouldbeabove13.0.Addwettingagent(0.75g/L),ifneeded,
6. Part Preparation
for the removal of heavy films of mineral oil. Then rinse parts
6.1 Cleaning—General:
thoroughly in cold water and immerse for 3 min in a chromic-
6.1.1 Before considering the use of solvent degreasing,
nitrate pickle as specified in 6.5.2. Repeat the cycle until all
consult federal and state safety and environmental laws and
parts are clean.
regulations. Many of the commonly used solvents are now
6.2.2 Because of the difficulty of removing graphite from
beingbannedfromuse.Exposuretotheirvapor(VOC)isbeing
chrome pickled sheet, such sheet should not be used for
strictlyregulatedforhealth,safety,andenvironmentalreasons.
formingunlessthechromepickleisremovedasoutlinedin6.3
Obtain current safe exposure levels for various solvents before
before forming.
use. Follow all federal, state, and local regulations for the
disposal of solvents. 6.3 Previously Applied Chemical Finishes:
6.1.2 Solvent Cleaning—Grease or oil may be removed by 6.3.1 Magnesium base alloys are often supplied with a
means of vapor degreasing, ultrasonic cleaning, solvent wash- chrome pickle treatment to protect them during shipment,
ing, or an emulsion cleaning process that utilizes a mineral oil storage, and machining. The coating from this treatment
distillate and an emulsifying agent. Chlorinated solvents, remaining on unmachined areas will impair the film produced
petroleumspirits,naphths,lacquerthinner,andsimilarsolvents by any subsequent chromate treatment and therefore must be
that do not attack magnesium may be used. Methyl alcohol removed.
(CH OH) should not be used because it may react with the
3 6.3.2 Previously applied coatings may be removed with the
magnesium surface.
alkaline cleaners recommended in 6.1.4.
6.1.3 Mechanical Cleaning—Mechanicalcleaningmaycon-
6.3.3 If the finish is difficult to remove, immerse the part in
sist of sand, shot, pumice, grit or vapor blasting, sodium
the chromic acid pickle given in 6.5.1.Alternate immersion in
carbonate slurry, sanding, hard bristle brushing, grinding and
the alkaline cleaner and the chromic acid pickle may be
rough polishing. Sand, shot, or grit blasting leaves surface
required to remove aged finishes. Rinse well in water between
contamination that will greatly increase the corrosion rate of
acid and alkaline pickling.
the magnesium on exposure to salt water or humid environ-
6.3.4 Thechromicacidbrush-ontreatment(see5.4)maybe
ment. If these methods are used, specific pickling procedures
applied over the chrome pickle finish or over previously
must be employed after blasting (see 6.4.2).
applied brush-on coatings without removing the previously
6.1.4 Alkaline Cleaning—Cleaning prior to application of
applied coating.
treatments other than the chrome pickle treatment (see 5.1),
...
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