ASTM B879-97(2013)
(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
3.1 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 (Reapproved 2013)
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.
1. Scope 2.3 SAE Standard:
AMS 2475Protective Treatments—Magnesium Alloys
1.1 This practice covers a guide for metal finishers to clean
2.4 Military Specifications:
and then provide a paint base for the finishing of magnesium
MIL-M-3171Magnesium Alloy, Processes for Pretreatment
and magnesium alloys using chemical conversion coatings.
and Prevention of Corrosion on
Where applicable (for example, aerospace) secondary supple-
DTD 911(British), Protection of Magnesium-Rich Alloys
mentary coatings (for example, surface sealing) can be used
Against Corrosion
(see Appendix X1).
DTD5562(British),ClearBakingResinforSurfaceSealing
1.2 Although primarily intended as a base for paint, chemi-
Magnesium
cal conversion coatings provide varying degrees of surface
DTD 935(British), Surface Sealing of Magnesium Rich
protection for magnesium parts exposed to indoor atmosphere
Alloys
either in storage or in service under mild exposure conditions.
An example is the extensive use of the dichromate treatment
3. Significance and Use
(see 5.2) as a final coating for machined surfaces of die cast
3.1 The processes described in this practice clean and
magnesium components in the computer industry.
provide a paint base for the finishing of magnesium and
1.3 The traditional numbering of the coating is used
magnesium alloys. Service conditions will determine, to some
throughout.
degree, the specific process to be applied.
1.4 The values stated in SI units are to be regarded as
4. Reagents
standard. No other units of measurement are included in this
4.1 Thechemicalsthatareusedtoformulateandcontrolthe
standard.
processing solutions are listed in Table 1. Commercial grade
1.5 This standard does not purport to address all of the
chemicalsaresatisfactory.Theconcentrationsstatedforchemi-
safety concerns, if any, associated with its use. It is the
cals that are normally supplied at less than a nominal 100%
responsibility of the user of this standard to establish appro-
strength are those typically available. Other strengths may be
priate safety and health practices and determine the applica-
used in the proportions that yield the specified processing
bility of regulatory limitations prior to use.
concentrations. Unless otherwise stated all solutions are made
up using water.
2. Referenced Documents
2.1 The following documents form a part of this practice to
5. Types of Coating
the extent referenced herein.
5.1 Chrome Pickle (Traditional Number 1) Treatment (See
2.2 ASTM Standards:
Practices D1732):
D1732Practices for Preparation of Magnesium Alloy Sur-
5.1.1 With slight variations this treatment can be applied to
faces for Painting
all alloys and forms of magnesium. The treatment removes up
to 15 µm of metal per surface, 30 µm per diameter. Therefore,
it may not be applicable to machined surfaces with close
This practice is under the jurisdiction of ASTM Committee B08 on Metallic
tolerances. Parts with steel inserts may be processed, but some
and Inorganic Coatings and is the direct responsibility of Subcommittee B08.07 on
slight etching of the steel surface may occur.
Conversion Coatings.
Current edition approved May 1, 2013. Published May 2013. Originally
ε1
approved in 1997. Last previous edition approved in 2008 as B879 – 97(2008) .
DOI: 10.1520/B0879-97R13. Available from Society of Automotive Engineers (SAE), 400 Commonwealth
For referenced ASTM standards, visit the ASTM website, www.astm.org, or Dr., Warrendale, PA 15096-0001, http://www.sae.org.
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Available from Standardization Documents Order Desk, DODSSP, Bldg. 4,
Standards volume information, refer to the standard’s Document Summary page on Section D, 700 Robbins Ave., Philadelphia, PA 19111-5098, http://
the ASTM website. dodssp.daps.dla.mil.
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B879 − 97 (2013)
TABLE 1 Processing Chemicals
requires no external current but utilizes the relatively high
Acetic acid glacial, (CH COOH) potential difference between suitably racked magnesium com-
Aluminum sulfate (Al [SO ] ·14H O)
2 3 2
ponents and steel tank walls or other cathodes. As with the
Ammonium bifluoride (NH HF )
4 2
dichromate treatment, a prior immersion in acid fluoride
Ammonium hydroxide (NH OH), 30 %
Ammonium phosphate monobasic (NH H PO ) solution is required to condition the magnesium surface. The
4 2 4
Ammonium sulfate ([NH ] SO )
4 2 4
galvanic chromate treatment causes no appreciable dimen-
Ammonium sulfite ([NH ] SO ·H O)
4 2 3 2
sional change and is normally applied after machining.
Calcium chromate (CaCrO )
Calcium fluoride (CaF )
5.3.2 Properly applied coatings vary from dark brown to a
Calcium sulfate (CaSO ·2H O)
4 2
dense black color depending on the alloy. The treatment is
Chromic acid (CrO )
particularly useful for application to optical equipment requir-
Ferric nitrate (Fe[NO ] ·9H O)
3 3 2
Glycolic acid (HOCH COOH), 70 % ing a nonreflective black coating.
Hydrofluoric acid (HF), 60 %
5.4 Chromic Acid Brush-On (Traditional Number 19) Treat-
Magnesium fluoride (MgF )
Magnesium nitrate (Mg[NO ] ·6H O)
3 2 2 ment:
Magnesium sulfate (MgSO ·7H O)
4 2
5.4.1 This treatment can be applied to parts that require
Manganese sulfate (MnSO ·5H O)
4 2
touch up. It is generally used in refinishing procedures or
Nitric acid (HNO ), sp gr 1.42
Phosphoric acid (H PO ), 85 %
3 4
where parts or assemblies are too large to be immersed. It is
Potassium fluoride (KF)
effective on most alloys and causes negligible dimensional
Potassium bifluoride (KHF )
Sodium bifluoride (NaHF ) changes.
Sodium bisulfate (NaHSO )
5.4.2 Coatings produced by this treatment can vary from a
Sodium carbonate (Na CO )
2 3
brassy iridescence to a dark brown depending upon treatment
Sodium dichromate (Na Cr O ·2H O)
2 2 7 2
Sodium hydroxide (NaOH) time.Prolongedtreatmentproducespowderycoatings.Forbest
Sodium metasilicate (Na SiO ,orNa SiO ·4H O)
2 3 2 3 2
adhesion, dark brown coatings are preferred.
Sodium nitrate (NaNO )
Sulfuric acid (H SO ), sp gr 1.84
5.5 Chromate Treatment (see DTD 911):
2 4
5.5.1 This treatment is suitable for all magnesium alloys.
The treatment causes no dimensional change and is normally
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
5.6 Chrome-Manganese Treatment:
magnification (5 to 10×). Bright brassy coatings, showing a
relatively smooth surface with only occasional rounded pits 5.6.1 This treatment provides an improved paint base com-
pared with the chrome pickle treatment and protection on all
under low magnification are unsatisfactory as a paint base but
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
5.6.2 The bath generally gives dark brown to black films on
dimensional changes, is normally applied after machining, and
both cast and wrought magnesium alloys. Treatment of alumi-
is suitable for close clearance parts. Parts containing inserts of
num containing alloys may require bath temperatures above
bronze, brass, steel, or cadmium plated steel should not be
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
simple, economical way to apply an attractive shelf-life finish
coating is grey.
and is a good base for clear lacquers. The treatment greatly
5.3 Galvanic Chromate (Traditional Number 9) Treatment
reduces or eliminates “filiform or worm-tracking” corrosion
(see Practices D1732):
usually experienced when clear paints are used directly over
5.3.1 This treatment can be used for all alloys and is
polished metal surfaces.
specifically used for those alloys which do not react or form
satisfactory conversion coatings in other baths. The treatment 5.8 Phosphate Treatment:
B879 − 97 (2013)
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-
6.2.1 Remove graphite-based lubricants from hot formed
alone protection against atmospheric oxidation and tarnish
magnesiumsheetpartsbysoakingthepartsfor10to20minin
equal to that provided by some chromate conversion coatings.
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
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
formingunlessthechromepickleisremovedasoutlinedin6.3
strictlyregulatedforhealth,safety,andenvironmentalreasons.
before forming.
Obtain current safe exposure levels for various solvents before
use. Follow all federal, state, and local regulations for the 6.3 Previously Applied Chemical Finishes:
disposal of solvents. 6.3.1 Magnesium base alloys are often supplied with a
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
remaining on unmachined areas will impair the film produced
washing, or an emulsion cleaning process that utilizes a
mineral oil distillate and an emulsifying agent. Chlorinated by any subsequent chromate treatment and therefore must be
removed.
solvents, petroleum spirits, naphths, lacquer thinner, and simi-
larsolventsthatdonotattackmagnesiummaybeused.Methyl 6.3.2 Previously applied coatings may be removed with the
alkaline cleaners recommended in 6.1.4.
alcohol(CH OH)shouldnotbeusedbecauseitmayreactwith
the magnesium surface. 6.3.3 If the finish is difficult to remove, immerse the part in
the chromic acid pickle given in 6.5.1.Alternate immersion in
6.1.3 Mechanical Cleaning—Mechanicalcleaningmaycon-
the alkaline cleaner and the chromic acid pickle may be
sist 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 Thechromicacidbrush-ontreatment(see5.4)maybe
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).
6.1.4 Alkaline Cleaning—Cleaning prior to application of
6.4 Acid Pickling:
treatments other than the chrome pickle treatment (see 5.1),
6.4.1 Generalpicklingtoremoveoxidelayers,oldchemical
whenusedforprotectionduringshipmentorstorage,shouldbe
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 is preferably
cleaning solu
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