ASTM B879-17(2022)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: B879 − 17 (Reapproved 2022)
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 2.2 ASTM Standards:
D1732Practices for Preparation of Magnesium Alloy Sur-
1.1 This practice covers a guide for metal finishers to clean
faces for Painting
and then provide a paint base for the finishing of magnesium
2.3 SAE Standard:
and magnesium alloys using chemical conversion coatings.
AMS 2475Protective Treatments—Magnesium Alloys
Where applicable (for example, aerospace) secondary supple-
2.4 Military Specifications:
mentary coatings (for example, surface sealing) can be used
MIL-M-3171Magnesium Alloy, Processes for Pretreatment
(see Appendix X1).
and Prevention of Corrosion on
1.2 Although primarily intended as a base for paint, chemi-
DTD 911(British), Protection of Magnesium-Rich Alloys
cal conversion coatings provide varying degrees of surface
Against Corrosion
protection for magnesium parts exposed to indoor atmosphere
DTD5562(British),ClearBakingResinforSurfaceSealing
either in storage or in service under mild exposure conditions.
Magnesium
An example is the extensive use of the dichromate treatment
DTD 935(British), Surface Sealing of Magnesium Rich
(see 5.2) as a final coating for machined surfaces of die cast
Alloys
magnesium components in the computer industry.
3. Significance and Use
1.3 The traditional numbering of the coating is used
throughout.
3.1 The processes described in this practice clean and
provide a paint base for the finishing of magnesium and
1.4 The values stated in SI units are to be regarded as
magnesium alloys. Service conditions will determine, to some
standard. No other units of measurement are included in this
degree, the specific process to be applied.
standard.
1.5 This standard does not purport to address all of the
4. Reagents
safety concerns, if any, associated with its use. It is the
4.1 Thechemicalsthatareusedtoformulateandcontrolthe
responsibility of the user of this standard to establish appro-
processing solutions are listed in Table 1. Commercial grade
priate safety, health, and environmental practices and deter-
chemicalsaresatisfactory.Theconcentrationsstatedforchemi-
mine the applicability of regulatory limitations prior to use.
cals that are normally supplied at less than a nominal 100%
1.6 This international standard was developed in accor-
strength are those typically available. Other strengths may be
dance with internationally recognized principles on standard-
used in the proportions that yield the specified processing
ization established in the Decision on Principles for the
concentrations. Unless otherwise stated all solutions are made
Development of International Standards, Guides and Recom-
up using water.
mendations issued by the World Trade Organization Technical
Barriers to Trade (TBT) Committee.
5. Types of Coating
5.1 Chrome Pickle (Traditional Number 1) Treatment (See
2. Referenced Documents
Practices D1732):
2.1 The following documents form a part of this practice to
the extent referenced herein.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on
This practice is under the jurisdiction of ASTM Committee B08 on Metallic the ASTM website.
and Inorganic Coatings and is the direct responsibility of Subcommittee B08.07 on Available from Society of Automotive Engineers (SAE), 400 Commonwealth
Conversion Coatings. Dr., Warrendale, PA 15096-0001, http://www.sae.org.
Current edition approved May 1, 2022. Published June 2022. Originally Available from Standardization Documents Order Desk, DODSSP, Bldg. 4,
approved in 1997. Last previous edition approved in 2017 as B879 – 17. DOI: Section D, 700 Robbins Ave., Philadelphia, PA 19111-5098, http://
10.1520/B0879-17R22. dodssp.daps.dla.mil.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
B879 − 17 (2022)
TABLE 1 Processing Chemicals
5.3 Galvanic Chromate (Traditional Number 9) Treatment
Acetic acid glacial, (CH COOH) (see Practices D1732):
Aluminum sulfate (Al [SO ] ·14H O)
2 3 2
5.3.1 This treatment can be used for all alloys and is
Ammonium bifluoride (NH HF )
4 2
specifically used for those alloys which do not react or form
Ammonium hydroxide (NH OH), 30 %
Ammonium phosphate monobasic (NH H PO )
satisfactory conversion coatings in other baths. The treatment
4 2 4
Ammonium sulfate ([NH ] SO )
4 2 4
requires no external current but utilizes the relatively high
Ammonium sulfite ([NH ] SO ·H O)
4 2 3 2
potential difference between suitably racked magnesium com-
Calcium chromate (CaCrO )
Calcium fluoride (CaF )
ponents and steel tank walls or other cathodes. As with the
Calcium sulfate (CaSO ·2H O)
4 2
dichromate treatment, a prior immersion in acid fluoride
Chromic acid (CrO )
solution is required to condition the magnesium surface. The
Ferric nitrate (Fe[NO ] ·9H O)
3 3 2
Glycolic acid (HOCH COOH), 70 % galvanic chromate treatment causes no appreciable dimen-
Hydrofluoric acid (HF), 60 %
sional change and is normally applied after machining.
Magnesium fluoride (MgF )
5.3.2 Properly applied coatings vary from dark brown to a
Magnesium nitrate (Mg[NO ] ·6H O)
3 2 2
Magnesium sulfate (MgSO ·7H O)
4 2 dense black color depending on the alloy. The treatment is
Manganese sulfate (MnSO ·5H O)
4 2
particularly useful for application to optical equipment requir-
Nitric acid (HNO ), sp gr 1.42
ing a nonreflective black coating.
Phosphoric acid (H PO ), 85 %
3 4
Potassium fluoride (KF)
5.4 Chromic Acid Brush-On (Traditional Number 19) Treat-
Potassium bifluoride (KHF )
Sodium bifluoride (NaHF )
ment:
Sodium bisulfate (NaHSO )
5.4.1 This treatment can be applied to parts that require
Sodium carbonate (Na CO )
2 3
touch up. It is generally used in refinishing procedures or
Sodium dichromate (Na Cr O ·2H O)
2 2 7 2
Sodium hydroxide (NaOH)
where parts or assemblies are too large to be immersed. It is
Sodium metasilicate (Na SiO ,orNa SiO ·4H O)
2 3 2 3 2
effective on most alloys and causes negligible dimensional
Sodium nitrate (NaNO )
changes.
Sulfuric acid (H SO ), sp gr 1.84
2 4
5.4.2 Coatings produced by this treatment can vary from a
brassy iridescence to a dark brown depending upon treatment
5.1.1 With slight variations this treatment can be applied to time.Prolongedtreatmentproducespowderycoatings.Forbest
all alloys and forms of magnesium. The treatment removes up
adhesion, dark brown coatings are preferred.
to 15 µm of metal per surface, 30 µm per diameter. Therefore,
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
5.6 Chrome-Manganese Treatment:
magnification (5 to 10×). Bright brassy coatings, showing a
5.6.1 This treatment provides an improved paint base com-
relatively smooth surface with only occasional rounded pits
pared with the chrome pickle treatment and protection on all
under low magnification are unsatisfactory as a paint base but
standard alloys except EK41A, HM31A, HM21A, HK31A,
are acceptable for protection during shipping and storage.
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
coating is grey. simple, economical way to apply an attractive shelf-life finish
B879 − 17 (2022)
and is a good base for clear lacquers. The treatment greatly rough polishing. Sand, shot, or grit blasting leaves surface
reduces or eliminates “filiform or worm-tracking” corrosion contamination that will greatly increase the corrosion rate of
usually experienced when clear paints are used directly over the magnesium on exposure to salt water or humid environ-
polished metal surfaces. ment. If these methods are used, specific pickling procedures
must be employed after blasting (see 6.4.2).
5.8 Phosphate Treatment:
6.1.4 Alkaline Cleaning—Cleaning prior to application of
5.8.1 Phosphate treatments can provide a satisfactory paint
treatments other than the chrome pickle treatment (see 5.1),
baseonmagnesiumformanyapplicationswhenitisnecessary
whenusedforprotectionduringshipmentorstorage,shouldbe
to avoid the use of chromates. Commercial iron phosphate
done in an alkaline cleaner recommended for steel or in a
treatments applied by spray or dipping have been successfully
cleaning solution as specified in 6.1.4.1. Maintain the solution
used on magnesium die castings for automotive and other
pH above 8.0.Alkaline cleaning prior to the application of the
consumer product applications. The suitability of a particular
chrome pickle treatment (see 5.1), when used for protection
phosphatizing process for magnesium should be verified by
duringshipmentandstorageonly,maybeomittedprovidedthe
testing. Iron phosphate treatments containing nickel or copper
partsarefreeofgrease,oil,andotherdeleteriousdepositsatthe
salts as accelerators are detrimental to the corrosion resistance
time of application. Alkaline cleaning solutions containing
of magnesium and should not be used.
more than 2% sodium hydroxide will etch ZK60A, ZK60B,
5.8.2 Phosphate treatments do not provide interim stand-
and some other magnesium alloys producing a change in
alone protection against atmospheric oxidation and tarnish
dimensions. If such a dimensional change is undesirable, use
equal to that provided by some chromate conversion coatings.
cleaners with lower alkali content.
5.9 Plasma Electrolytic Oxidation (PEO)—Thisprocessisa
6.1.4.1 Alkaline cleaning may be carried out in solutions of
combination of the co-deposition on Magnesium and Magne-
proprietary cleaners. In this case the operating conditions
siumalloysfromanelectrolyteandtheoxidationofthesurface
should be as specified by the supplier. In no case should a
ofthemetalsinquestionwithaplasma.Shortpulsesofplasma
cleaner having pH lower than 8.0 be used. Most recommended
discharges result in the metals being processed to oxidize and
cleanersareusedbysimpleimmersion.Afteralkalinecleaning,
then melt the generated Magnesium oxides onto the surface of
rise parts thoroughly in cold running water. No water breaks
the metals being treated. The melting seals up any holes in the
should be observed in the rinse.
surface oxides to prevent corrosion, increase ware and provide
6.1.5 Electrolytic Cleaning—Use of anodic current for
for a surface that will accept secondary organic coatings (that
cleaningisnotgenerallyrecommendedbecauseofthepossible
is, commercial paints).
formation of oxide films, pitting of the magnesium surface, or
5.10 All Organic Hydrocarbon Acid—This process chemi-
both. However, electrolytic cleaning using cathodic current at
cally reacts the Magnesium or Magnesium alloy, or both, in 1 to 4 A/dm may be carried out in properly formulated
questionwithvariousorganicacidsthatthenattachthemselves
cleaners.
to the surface of the metal in question, seals the surface of the
6.2 Graphite Lubricant Removal:
metaltopreventanyoxidationandproducesasurfacethatwill
6.2.1 Remove graphite-based lubricants from hot formed
easily accept applied secondary coatings (that is, commercial
magnesiumsheetpartsbysoakingthepartsfor10to20minin
paints).
100g/Lsodiumhydroxidemaintainedat88to100°C.ThepH
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 an
...