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 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
6.3 Previously Applied Chemical Finishes:
disposal of solvents.
6.3.1 Magnesium base alloys are often supplied with a
6.1.2 Solvent Cleaning—Grease or oil may be removed by
chrome pickle treatment to protect them during shipment,
means of vapor degreasing, ultrasonic cleaning, solvent
storage, and machining. The coating from this treatment
washing, or an emulsion cleaning process that utilizes a
remaining on unmachined areas will impair the film produced
mineral oil distillate and an emulsifying agent. Chlorinated
by any subsequent chromate treatment and therefore must be
solvents, petroleum spirits, naphths, lacquer thinner, and simi-
removed.
larsolventsthatdonotattackmagnesiummaybeused.Methyl
6.3.2 Previously applied coatings may be removed with the
alcohol(CH OH)shouldnotbeusedbecauseitmayreactwith
alkaline cleaners recommended in 6.1.4.
the magnesium surface.
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
B879 − 17 (2022)
required to remove aged finishes. Rinse well in water between replenishments, the additions of chromic acid required to
acid and alkaline pickling. rejuvenatebecomeexcessive,andthebathshouldbediscarded
6.3.4 Thechromicacidbrush-ontreatment(see5.4)maybe and replaced.
applied over the chrome pickle finish or over previously 6.5.3 Nitric Acid Pickle—Nitric acid pickle is used to clean
applied brush-on coatings without removing the previously
and brighten alloys on which chromate films are not desired.
applied coating. The solution contains 70 mL/L nitric acid. Immersion time is
30to60sat20to27°C.
6.4 Acid Pickling:
6.4.1 Generalpicklingtoremoveoxidelayers,oldchemical
NOTE 1—This pickle will remove 12.5 to 25 µm per metal surface. Use
ceramic, rubber-lined, or aluminum containers.
finishes, burned-on drawing and forming lubricants, and other
water insoluble or non-emulsifiable substances is preferably
6.5.4 Sulfuric Acid Pickle—The sulfuric acid pickle is used
carried out using a chromic acid type pickle as described in
on magnesium sand castings to remove the effects of blasting
6.5.1 or 6.5.2.
operations. Use the pickle before any machining operations
6.4.2 Pickle sand and permanent mold castings that have
since the amount of metal removed is likely to exceed
been mechanically cleaned as described in 6.1.3 in the sulfuric
permissible tolerances.The solution contains 32 mL/Lsulfuric
acid pickle described in 6.5.4 or in the nitric-sulfuric acid
acid in water. Immersion time is 10 to 15 s at 20 to 32°C or as
pickle described in 6.5.5.The pickling operations should be of
required to remove approximately 50 µm of metal per surface.
such duration that 50 µm of surface is removed whenever
6.5.5 Nitric-Sulfuric Acid Pickle—As an alternative for the
dimensional tolerances permit.
sulfuric acid pickle (see 6.5.4) the nitric-sulfuric acid pickle
6.4.3 Magnesium sheet and plate up through 12.5 mm thick
may be used.The solution contains 80 mL/Lnitric acid and 20
should have the mill scale removed to ensure a clean surface
mL/L sulfuric acid. Conditions of operation and time of
for the subsequent treatments. Where dimensional tolerances
immersion are the same as for the sulfuric acid pickle.
permit, remove 12 to 25 µm per metal surface. The acetic-
6.5.6 Chromic-Nitric-HydrofluoricAcid Pickle—Thispickle
nitrate pickle described in 6.5.8, or an equivalent solution,
may be used on any casting, but is especially effective on die
should be used.
castings. It will remove metal at approximately 12.5 µm per
6.4.4 Picklediecastingsthataretobepickledformaximum
minute per surface. The solution contains 280 g/L chromic
corrosion resistance and a more uniform surface for applying
acid, 25 mL/L nitric acid, and either 7.5 mL/L HF or 6.5 g/L
subsequent coatings in the chromic-nitric-hydrofluoric acid
ammonium bifluoride. Immersion time is 0.5 to 2 min at 21 to
pickle described in 6.5.6 or the phosphoric acid pickle de-
32°C.
scribed in 6.5.7.
6.5.7 Phosphoric Acid Pickle—The phosphoric acid pickle
6.5 Acid Pickling Solutions: may be used for pickling all castings, particularly die castings.
It is especially effective at removing segregated aluminum
6.5.1 ChromicAcid Pickling—Chromicacidpicklingcauses
from the surface ofAZ91A,AZ91B, andAZ91D alloys. It has
nodimensionalchangeandthereforemaybeusedonpartswith
been used for some wrought alloys, such as HK31A. Dip the
close tolerances. It may be used for removal of previously
applied chemical finishes (see 6.3.3). It is satisfactory for the parts for 10 to 15 s in 50 to 85 v/o phosphoric acid and allow
to drain and react in air for 30 to 60 s. Without rinsing, dip in
removal of surface oxidation and corrosion products and for
general cleaning. It is not satisfactory for the removal of sand sodiumhydroxide80to120g/Lfor30sat20to25°C.Follow
with a cold-water rinse. Sodium hydroxide corrects the pow-
or effects of blasting and should not be used on parts that
contain copper inserts unless the copper can be completely dering effect of the phosphoric acid. Metal loss is 12.5 to 25
µm. Contain the phosphoric acid solution in tanks with rubber,
masked off. Excessive concentrations of chloride, sulfate, and
fluoride must be avoided in the solution since these will cause glass, ceramic or lead lining.
6.5.8 Acetic-Nitrate Pickle—The acetic-nitrate pickle is
etching or film formation rather than cleaning. The solution
contains180g/Lofchromicacid.Treatmenttimeis1to15min suitable for the removal of mill scale and other surface
contamination from sheet to ensure maximum effectiveness of
at 88 to 95°C. Lower temperatures are permissible provided
the treatment time is suitably increased. the protective coating. This solution can be used on wrought
forms and on solution heat treated castings. Castings in the
6.5.2 Chromic-NitratePickle—Thechromic-nitratepickleis
not generally used for the removal of corrosion products of as-cast condition (-F) or in the solution heat treated and aged
condition (-T6) should not be pickled in this solution because
surface oxidation but may be substituted for the chromic acid
pickle described in 6.5.1. It is mainly used for the removal of a loose grey smut forms. Treat castings in the -F or -T6
conditions in the chromic-nitric-hydrofluoric acid pickle (see
burned on graphite lubricants as described in 6.2.Itis not
suitablefortheremovalofsandfromcastingsorforcorrecting 6.5.6).Forbestresults,thepickleshouldbeallowedtoremove
10 to 25 µm of metal per surface. The treatment may not
theeffectsofblasting.Itshouldnotbeusedforpartscontaining
copperinsertsunlesstheinsertscanbecompletelymaskedoff. therefore be suitable for the treatment of parts with close
tolerances. The solution contains 200 mL/Lacetic acid and 50
Thesolutioncontains180g/Lchromicacidand30g/Lsodium
nitrate. Immersion time is 2 to 20 min at 15 to 30°C which g/L sodium nitrate. Immersion time at 20 to 30°C is 0.5 to 1
min.
normally removes 12.5 µm of metal/surface. Lack of chemical
actionandapHof1.7orhigherindicatesdepletionofthebath. 6.5.9 Glycolic-Nitrate Pickle—In cases where spray pick-
TheadditionofsufficientchromicacidtorestoreapHof0.5to ling is used or fumes are a problem, the glycolic-nitrate pickle
0.7 will rejuvenate the solution. After three to four may be substituted for the acetic-nitrate pickle. This will also
B879 − 17 (2022)
reduce acid loss from vaporization. The solution contains 250 6.6.5 Thoroughly rinse parts in cold water after treatment,
mL/L commercial glycolic acid, 200 g/L magnesium nitrate and strip the fluoride film in hot chromic acid as detailed in
6.5.1. If fluoride anodizing is followed by an hard anodizing
and 37.5 mL/L nitric acid. Immersion time at 20 to 30°C is 3
process, such as the HAE treatment, removal of the fluoride
to 4 min.
film is not necessary.
6.5.10 Pickling Prior to Spot Welding—A special cleaning
andpicklingsequenceisusedtopreparepartsforspotwelding.
NOTE 2—Chromium-based cleaners—Chromate based cleaners shall
The treatment results in a surface that has a very low surface not be used in any cleaning or surface preparation step when a chromium
or chromate-free finish is desired.
resistanceandthatwillretainthislowvalueforsometime.The
sequence consists of: alkaline cleaning and rinsing as in 6.1.4;
7. Application of Coatings
alkali neutralization in a dilute acid solution such as 0.5 to 1.0
volume % sulfuric acid or a 7.5 to 15 g/L sodium acid sulfate
7.1 Chrome Pickle (Traditional Number 1) Treatment (see
solution;treatmentinachromic-sulfuricacidpicklecontaining
Practices D1732):
180 g/L chromic acid and 0.5 mL/L sulfuric acid. Immersion
7.1.1 Cleaningofpartstoreceivethistreatmentshouldbein
time is 3 min at 20 to 30°C.
accordance with the procedures outlined in Section 6.
6.5.11 Hydrofluoric-Sulfuric Acid Pickle—This pickle is
7.1.2 Procedures for Wrought Parts—Thesolutionfortreat-
used (particularly in brightening die castings) when the 12.5
ingwroughtpartscontains180g/Lsodiumdichromateand190
µm metal loss caused by the phosphoric acid pickle cannot be
mL/L nitric acid. The bath is operated at 20 to 45°C and
tolerated. This solution contains 15 to 20 volume % hydroflu- immersion times can range from 30 s to 2 min depending on
oric acid and 5 volume % sulfuric acid. Immersion time at the activity of the solution. Immerse the part and agitate in the
roomtemperatureis2to5min.Afterthedip,rinsethepartsin solution under the conditions indicated above. Following
treatment, drain the parts for at least 5 s and then rinse
cold water.
thoroughlyincoldrunningwater.Dryingmaybefacilitatedby
6.6 Fluoride Anodizing Cleaning (see MIL-M-3171 and
a dip in hot water or with a hot air blast.Avoid heating above
DTD 911):
200°C.Undersomeconditions,thecoatingmaybeappliedby
6.6.1 Fluoride Anodizing Cleaning—This electrolytic pro-
spraying the solution over the part rather than immersing the
cess was developed for the cleaning of magnesium parts prior
part in the solution.
to conversion coating. While the process is normally used to
7.1.3 Procedures for Sand, Permanent Hold and Die
clean raw sand castings, it is also particularly useful for the
Castings—Chrome pickling of the widely used sand, perma-
cleaning of components because the process does not signifi-
nent mold and die castings of high aluminum content (AZ91
cantly alter dimensions.
types) requires modified solutions and procedures. The
6.6.2 The electrolytic bath contains 150 to 250 g/L ammo-
aluminum-rich phase present in these alloys can cause prob-
nium bifluoride in a tank lined with non-conducting hard lems ranging from failure to react to selective reaction leaving
rubber or suitable plastic material.Apower source is required a loose dark aluminum-rich smut deposit on the surface.
Preventive action includes the addition of fluoride and sulfate
capableofdeliveringaprogressivelyincreasingacoutputupto
tothesolution,adjustmentoftemperatureandtreatmenttimes,
a maximum of 120 V. Current requirements will vary depend-
and the use of pretreatment pickles (chromic-nitric-
ing upon the size of the installation, but a minimum of 150
hydrofluoric, phosphoric, or hydrofluoric-sulfuric) where nec-
A/m ononeelectrode,whenthebathisbeingusedtocapacity
essary.
isrecommended.Thepowerisappliedvianormalbusbarsand
7.1.3.1 The solution for treating these types of castings
clamps.All sections of fixing clamps immersed in the electro-
contains of sodium bifluoride 15 g/L, sodium dichromate 120
lyte must be of magnesium alloy, and parts to be cleaned
g/L, aluminum sulfate 10 g/L, nitric acid 90 mL/L. Operating
should not contain any dissimilar metal inserts.
temperatures can range from 21 to 60°C. Preheat die castings,
6.6.3 Parts are fixed in good electrical contact with the bus
permanent mold, and aged sand castings by dipping in hot
bars and located in the tank so that they are at least 0.25 m
water (80 to 95°C) followed by immediate immersion in the
below the electrolyte surface and there is approximately equal
treating solution. A 10 s immersion is sufficient if the bath is
surface area on each electrode. Alternating current is applied
operated at 50 to 60°C. Lower temperatures require longer
and the voltage progressively increased until 120Vis reached.
treating times.
Current flow is heavy at first but rapidly diminishes as the
7.1.3.2 Excessive treatment time will produce a powdery
surface impurities are removed and an insulating coating of
coating. Failure to preheat the part in hot water may result in
magnesiumfluorideisformed.Thetreatmentiscompletewhen
failureofthecoatingtoform.Ifaseparatesolutionfordiecast,
the current density falls below 50 A/m .
permanent mold, and aged sand castings is not available, the
6.6.4 Certain alloys, however, may be etched at 120 V,
bath used for wrought parts may be used. Sand castings in the
particularly if the ammonium bifluoride concentration is to-
solution heat treated condition may be chrome pickled in this
wards the lower end of the range. For these alloys the process
solution at room temperature with no preheating. After
should be terminated at about 90 V. The magnesium surface,
treatment, drain the parts and rinse for at least 5 s and then
when satisfactorily anodized, should be a uniform gray-white
rinse thoroughly in cold water. Drying may be facilitated by a
color. dip in hot water.
B879 − 17 (2022)
7.1.3.3 Optional Hydrofluoric Sulfuric Pre-Pickle— for 30 s. Immerse all other wrought and cast alloys for 5 min.
Hydrofluoric acid 30 to 40 volume %, sulfuric acid 5 volume Following treatment, give the parts a thorough rinse in cold
%, 2 to 5 min, at room temperature, followed by a cold water running water. Drag-over of fluoride ion will render the
rinse. dichromate solution inoperative.
7.1.3.4 Dilute Chrome Pickle Treatment—The hexavalent
7.2.3 Acid Fluoride Treatment—Use this treatment for all
chromium content of this bath is about 20% lower than the
parts containing aluminum inserts, rivets, etc. and as an
commonly used chrome pickle baths. This results in lower
alternative treatment to the hydrofluoric acid treatment (see
initial cost, less dragout loss, and lower effluent treatment
7.2.2). It is the preferred treatment for AZ31B and AZ31C
costs.The paint base created is equivalent to that of a standard
alloy parts. It
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