NATIONAL

DEC 15 1967

BUREAU OF STANDARDS REPORT

9647

PERFORMANCE OF CONVERSION COATINGS ON MAGNESIUM ALLOYS

IN MARINE AND SALT SPRAY (FOG) ENVIRONMENTS

By

W. F. GerholdEngineering Metallurgy Section

To

Mater i al s Division

Naval Air Systems CommandDepartment of the NavyProject No. RRMA 2001

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

THE NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards 1 provides measurement and technical information services

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research and services:

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States for a complete and consistent system of physical measurements, coordinates that system with the

measurement systems of other nations, and furnishes essential services leading to accurate and uniformphysical measurements throughout the Nation’s scientific community, industry, and commerce. This

Institute comprises a series of divisions, each serving a classical subject matter area:

—Applied Mathematics—Electricity—Metrology—Mechanics—Heat—Atomic Physics—Physical

Chemistry—Radiation Physics— -Laboratory Astrophysics2—Radio Standards Laboratory,

2 which

includes Radio Standards Physics and Radio Standards Engineering—Office of Standard Refer-

ence Data.

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT3120410 9647BU WE PS RRMA 2001

p ^r , 5 ^957

PERFORMANCE OF CONVERSION COATINGS ON MAGNESIUM ALLOYSIN MARINE AND SALT SPRAY (FOG) ENVIRONMENTS

By

W. F. GerholdEngineering Metallurgy Section

Sponsor

Mater i a 1 s DivisionNaval Air Systems CommandDepartment of the Navy

IMPORTANT NOTICE

NATIONAL BUREAU OF STAI

for use within the Government. B

and review. For this reason, the|

whole or in part, is not authorizi

Bureau of Standards, Washington,

the Report has been specifically pi

Approved for public release by the

director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015

accounting documents intended

ibjected to additional evaluation

sting of this Report, either in

Office of the Director, National

the Government agency for whjch

ies tor its own use.

<^BS>U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

",

'

Performance of Conversion Coatings On Magnesium AlloysIn Marine and Salt Spray (Fog) Environments

References: (a) National Bureau of Standards Report No. 5261,u Performance of HAE and Dow 17 Anodic Coatings on

Magnesium Alloys In Marine and Salt Fog Environments, 11

dated Apri 1 26, 1957.

(b) Naval Air Systems Command (BUaer) letter Aer-AE-432/ 128,

dated 27 July 1954.

I ntroduct i on : Poor performance by HAE and Dow 17 anodic conversion coatingson sand cast magnesium alloy AZ91C in tidewater and in salt spray environ-

ments was reported in Reference (a). In that investigation it was found

that the rate of corrosive attack was greater on specimens coated by the Dow

17 process than on similar specimens coated by the HAE process. Specimens

were attacked more rapidly on one end than on the other. There wereindications that the poor performance may have been due to inhomogeneitiesand impurities in the cast specimens.

Since the initiation of the above tests, improvements have been reportedfor both the HAE and Dow 17 anodic coatings. These include variations in

thicknesses obtainable and in post (sealing) treatments. In addition othertypes of anodic conversion coatings have been developed. As a result

;

of

the past experience and of recent developments. Reference (b) requested that

additional tests be conducted to evaluate the protection given by conversioncoatings, both painted and unpainted, on wrought AZ31A-4124 and sand cast

AZ91C-T6 magnesium alloys in the salt spray and in marine environments.Because of the results reported in Reference (a), it was stipulated thatadditional tests for sand cast AZ91C be conducted on material with controlledamounts of iron, nickel and copper.

Mater i a 1

s

: The following conversion coatings were investigated:

Chemical ConversionCoat ? ngs

Dow 7Dow 19 (brush-on)

Dow 1

7

The alloys used in this investigation were wrought magnesium alloyAZ31A-H24 and sand cast magnesium alloy AZ91C-T6 (controlled purity). All

materials were furnished by the Magnesium Division, Dow Chemical Company,Midland, Michigan. The chemical composition of the alloys as furnished by

the Dow Chemical Company is as follows:

Anodic ConversionCoat i n<

HAESOK

%'

>

,» y

\ !

s

'’

.

-,

- 2 -

E 1 ement

A] umi numCalciumCopperI ron

I ron

ManganeseN i c ke I

LeadS i 1 i con

TinZi nc

Bery Ilium

A1 loy

AZ31A-H24 AZ91C-T6Percent by Weight

0 )

2 8 9 040 032 < 0 01

0 010 0 0030 016 0 0032 .

0 004 (2 )

0 43 0 4?< 0 0005 < 0 001

< 0 001 < 0 001

< 0 001 0 012

< 0 01 < 0 01

1 04 0 570 0003

(1) Metal for all panels was melted down in a 2000 lb pot andpoured from two 600 lb ladles. Values given are theaverage obtained from analyses of both ladles.

(2) Wet chemical ana lysis, other analyses are spectroscopic.

The wrought AZ3 1 A-H24 alloy material submitted was in the form of

panels with dimensions of 0.064 in. x 4 in. x 9 i n . and 0.064 in. x 4

x 14 in. The sand cast AZ91C-T6 alloy submitted was in the form of

individual cast' panels, 0.375 in* x 3 in. x 8 in.

Specimen Preparation - General : The initial preparation of panel surfacesprior to coating was performed by the Metallurgical Laboratories of theMagnesium Division of the Dow Chemical Company, Midland, Michigan. Thesand cast AZ91C-T6 alloy panels were sandblasted and acid pickled. Thewrought AZ31A-H24 alloy panels were pickled in an acet i c-n i t rate solution(standard production pickle).

After the initial surface preparation by the Dow Chemical Company,the panels were submitted to NBS for codifications for identificationpurposes and distribution to the various coating processors. Coatingswere applied by the following processors.

i. %

"

i

- 3 -

Coat i ng Processor

Dow 7 Brooks and Perkins,Detroit, Michigan.

Dow 1

7

Dow Chemical Company,Metallurgical Laboratory,Mi dl and. Mi chi gan

.

HAE and CR-22 Frankford Arsenal,Pi tman-Dunn Laboratories,Philadelphia, Pennsylvania

SOK National Bureau of StandardsElectrodeposition SectionWashington, D. C.

Dow 19 (brush-on) National Bureau of StandardsWashington, D. C.

Details concerning the application of the various coatings are given

in Table 1

.

Specimen Preparation-intentional Damage : !n order to determine the

corrosion protection given by the repair of damaged coatings, one specimenof each alloy from each coating group was intentionally damaged by

scribing with a pointed instrument, several areas on each, deep enoughto penetrate the coating to the base metal. Dow 19 conversion coating wasthen applied to these areas on each specimen with a brush. Dow 19

(brush-on) coating was also applied to one surface of one of the uncoat^dwrought AZ3 1 A-H24 alloy specimens and to one surface on each of two of

the uncoated sand cast AZ91 C-T6 magnesium alloy samples.

Specimen Preparation - Painting : Paint coatings were applied by NBS to

some of the specimens from each of the coating groups. The following paintsystems were used on both sand cast AZ91 C-T6 and wrought AZ31 A-H24magnes i um alloy specimens:

Paint Systems

(1) Wash pr i mer. Specification MI L-C-85 1 - , One coat.

(2) Zinc chromate primer, Speci f i cat i on MIL-P-7962, Two coats.

(3) Aery 1 1 1 c-n i trocel 1 ul ose 1 acquer - Two coats

.

(a) Top surfaces, light gull gray camouflage

Spec! f i cation MS L—L— 19538-(b) Bottom surfaces, gloss white, Speci f i cat i on M! L-L-

1

9537*

'VO

. ft.

- k -

Exposure - General ; For exposure, specimens were divided into four lots

as fpl lows : (l) controls, (2 ) salt spray tests, (3) marine atmospheretests and (4) tidewater tests. A listing of painted and unpaintedspecimens exposed in each of the three test environments is given in

Tables 2 and 3* The control specimens were stored at NBS in desiccatedcab i nets

.

Exposure - Salt Spray ; Three separate salt spray exposure tests wereconducted on coated specimens of each alloy at the National Bureau of

Standards. Specimens were subjected to the mist formed by atomizing a

20°!o NaCl solution maintained at 95° F + 2°,

-3° in a standard salt-fogchamber. The specimens were inclined 15° from the vertical.

Exposure - Karine Atmosphere : Marine atmosphere exposure tests were con-ducted in the 80 foot lot at the Kure Beach, N. C. test site of the

International Nickel Company, Inc. The specimens were inclined in the

test racks at an angle of 30 ° from the horizontal and faced towards the

ocean. Exposure periods are given in Tables 2 and 3-

Exposure - Tidewater : Tidewater exposure tests were conducted in sea waterat the Harbor Island, N. C. test site of the I nternat i ona 1 Nickel Company.Specimens were exposed vertically and were totally immersed in the sea

water at high tide and totally exposed to the atmosphere at low tide.

Exposure periods for specimens from each alloy and coating groups are givenin Tables 2 and 3

»

Results - Painted and Unpainted Specimens : Corrosion products wereremoved from the unpainted exposed specimens by immersing them in a 20°jo

chromic acid solution. The immersion time employed was dependent uponthe concentration of corrosion products present on any particular specimen*The paint coatings were stripped by immersing the exposed painted specimensin acetone to remove the lacquer and then into a proprietary agent to removethe primer coatings. Subsequent to the above procedures the panels wereexamined visually for estimations of the corrosion damage. The resultsobtained on examinations of unpainted and painted specimens are given in

Table 2 for the wrought AZ3!A°H24 magnesium alloy and in Table 3 for thesand cast AZ91C-T6 magnesium alloy.

Results - Salt Spray (Fog) Tests : The results obtained from 3 separatesalt spray (fog) tests are given in Tables 2 and 3* Figures 1 and 2 showthe typical attack on representative specimens after 1000 hours exposure.The thick HAE coating with the post treatment was judged to be the bestfor the wrought AZ3lA*H24 magnesium alloy while the thick Dow 17 coatingwith the post treatment was judged best for the sand cast AZ91C-T6magnes i urn alloy.

~ 5 -

Deterioration of all of the coatings on either alloy was characterized

by localized failures of the coating followed by subsequent pitting of the

base metal. This is shown in figure 1 for the wrought AZ3 1 A-H24 magnesiumalloy and in figure 2 for the sand cast AZ91C-T6 magnesium alloy. The

declining order of the coatings, arranged according to the protection

given is i n Tab 1 e 4.

Paint failures were noted on wrought AZ31A-H24 magnesium alloy speci-

mens that had been given the Dow 7 and the thick SOK surface treatments.Similar paint failures were also noted on the sand cast AZ9 1 C-T6 magnesiumalloy specimens that had been surface treated with thick Dow 17 (post

treated and not post treated). These failures occurred one on each of

two of the three separate tests conducted.

Results " Marine Atmosphere Tests : Results obtained from visual examinationof the sand cast AZ9 1 C-T6 specimens exposed for 2k months in the marineatmosphere revealed no apparent deterioration regardless of surfacetreatment. Similar specimens, after 110 months exposure, still retainedwhat appeared to be the rough cast surfaces, however the coatings hadvisibly deteriorated on all specimens except that on the specimen withthe CR-22 surface treatment. This specimen still retained a portion of

the green coat i ng on approximately one^third of the surface. The generalappearance of these specimens as a whole is best described as gray to

grayish-white with no apparent deterioration of the metallic surfaces.

Of the surface treated wrought AZ3

1

A-H24 magnesium alloy specimensexposed for 2k months in the marine atmosphere, the one with the thickHAE coating plus post treatment was judged to be the best. For thosespecimens that had been exposed 110 months, the surfaces that had beentreated with the thick HAE coating and the post treatment were still

judged to be the best. The coatings had completely deteriorated and the

specimens were coated with a film of pale yellow corrosion products.

Deterioration of^the wrought magnes i urn alloy specimens was initiatedby localized failure of the coated surfaces which subsequently resultedin pitting of the base metal at these failed areas. Corrosion pitsobserved on all of the coated specimens were generally shallow in depth.

Paint failures were observed on most of the painted wrought AZ31A-H21+

magnesium alloy oanels after 2k months exposure in the marine atmosphere.There were no paint failures on panels with the following coatings: thickDow 17 (post treated and not post treated), thick HAE (post treated) andCR-22. Paint failures were more prevalent on the specimens with the thinHAE coating. Generally the adhesion of the paint coatings was exception-ally good on all surface treated sand cast AZ9 IC-T6 magnesium alloyspecimens except on those with the thin SOK coating. After 110 months

'

*

- 6 -

exposure In the marine atmosphere, the paint coatings on all of the specimensappeared to have sustained deterioration from erosion of the painted surfaces.There were paint failures along the bottom edge of all of the surface treatedwrought AZ3IA-H24 magnesium alloy specimens; however, the CR-22 surfacetreated specimen was the only one with no paint failures at other areas.

There were no paint failures on the sand cast AZ91C-T6 magnesium alloyspecimens that had been given the CR-22 or the thick SOK coating except at

areas along the bottom edge of the specimens. Paint coating failures on

all of the other sand cast specimens were of a local nature and few in

number (less than 10); however, the paint coating that was applied to the

specimens with the thin and thick Dow 17 surface treatments had eroded to

the primer coating over approximately one-fifth of the surface of the specimen.

Results - Tidewater Exposure Tests ; The results obtained from visual exami-nation of the surface treated wrought AZ3 1 A-H24 and sand cast AZ91C-T6magnesium alloys after varying exposure periods in tidewater are given in

Tables 2 and 3* respectively.

These results indicate that the thick MAE coating with the post treat-ment provides the greatest protection for the wrought AZ31A-H24 alloy.Localized coating failures and subsequent pitting of the base metal varyingin degree were observed on all of the surface treated specimens after 3^+

days exposure; however, perforation of the base material had not occurreduntil after an exposure period in excess of 90 days. Examination of speci-mens exposed for 1 66 days revealed that the pitting corrosion had perforatedall of the specimens. The severity of attack and the number of perforationsvaried with the surface treatment given the specimens. The attack on

representative specimens after 1 66 days exposure in the tidewater is shownin figure 3 . The least corroded areas are located at the top of thespecimens. These areas were the first to emerge and the last to submergeduring tidal changes. Accordingly this area would most likely dry quickerand remain dry longer than the other areas and would therefore not have as

high a corrosion rate. While the specimen having the thick HAE coatingand post treatment had been perforated at local areas, it may be seenthat a major portion of the surface had not been attacked.

The thick HAE coating plus post treatment and the CR-22 coatingprovided the greatest protection for sand cast AZ91C-T6 magnesium alloyspecimens. The thin HAE surface treatment also gave good protection to thesand cast alloy but to a slightly lesser degree. Corrosion damage on

specimens of this alloy was not of sufficient intensity to cause penetrationof the specimens. However, attack on all specimens was more severe alongone edge and at areas adjacent to this edge than at other areas on thespecimens. Figure k shows representat i ve specimens of each coating typeafter 203 days immersion in the tidewater racks.

- 7 -

Visual examination of the specimens whose coatings had been scoredand then repaired with the Dow 19 (brush-on) coating revealed that after

34 days tidal immersion there was some attack within the scored areas.

This attack was noted on all of the wrought AZ3 1 A-H24 magnesium alloyspecimens and on the sand cast AZ91 C-T6 magnesium alloy specimens with the

thick HAE coating (post treated and not post treated).

The paint coatings on the Dow 7j the thin HAE and the thin and thickSOK surface treated specimens were the least protective for the wroughtAZ31A-H24 magnesium alloy. Failures were observed on specimens exposed for

203 days in tidewater. Similarly^ failures of the paint coatings on the

thin Dow 17 coating were observed after 455 days of tidal immersion. Thepaint coating on the thick HAE coating gave good protection after 240 days

immersion in tidewater^ except at areas where marine fouling was observed.There were no failures after 455 days of tidal immersion on wroughtmagnesium alloy specimens with the thick Dow 17 coating (post treated and

not post treated)^ the thick HAE coating (post treated) or the CR-22coat i ng.

Failures of the paint coatings applied over the Dow 7* thick Dow 17

(post treated and not post treated) and the thin HAE coatings were notedon the sand cast AZ91C-T6 magnesium alloy specimens after 204 days of

tidal immersion.

Summary : Salt spray (fog) tests and marine environmental tests wereconducted on painted and unpainted surface treated wrought AZ3 1 A-H24 andsand cast AZ9 IC-T6 (controlled purity) magnesium alloys. The surfacetreatments investigated were: Dow 7? Dow 17,? Dow 19? HAE^ CR-22 and SOKconversion coatings. Variations in those treatments included thin andthick coatings for the Dow 17? HAE and SOK coatings. In addition posttreatments (or sealing treatments) were given to the thick coated Dow 17and the thick coated HAE specimens. The Dow 19 (brush-on) coating wasapplied to one surface of specimens of both alloys that had been pickledonly and to surface treated areas on specimens of both alloys where thecoating had been intentionally damaged. Specimens given the Dow 19

treatment were exposed in tidewater only.

The salt spray tests were conducted for a period of 1^000 hours.Exposures in tidewater ranged from 3 ^+ days to 455 days for the wroughtAZ3 1 A“H24 alloy ard 3^+ days to 204 days for the sand cast AZ91C-T6 alloy.The marine atmosphere exposures were for periods of 24 months and 110months

.

*:

c - *

The results obtained show:

8 -

(1) The thick HAE coating with the post treatment gave the mostprotection to the wrought AZ31A-H24 magnesium alloy in all

three environments,

(2) The thick Dow 17 coating (post treated) gave the mostprotection to the sand cast AZ91C-T6 magnesium alloy in the

salt spray tests.

( 3 ) The CR-22 coating adhered to the sand cast AZ91C-T6 alloyfor the longest time in the marine atmosphere. Moreover,there were no visible indications of corrosive attack on anyof the specimens of this alloy after 110 months exposureregardless of the surface treatment employed,

(A) The thick HAE coating (post treated) and the CR-22 coatinggave the most protection to the sand cast AZ9 IC-T6 alloy in

t i dewater

.

( 5 ) The declining merit of the coatings, arranged according to theprotection given specimens in salt spray tests, was as follows

Wrought AZ31A-H2A magnesium alloy

Thick HAE (post treated)CR-22Thick SOKThick Dow 17 (post treated)Thin SOKThick Dow 1

7

Thin HAEThin Dow 17

Thick HAEDow 7

Sand cast AZ91C-T6 magnesium alloy

Thick Dow 17 (post treated)Thin HAE

Thick HAE (post treated)CR-22Thin SOKThick Dow 1 7Dow 7Thick SOKThi ck HAE

Thin Dow 1

7

; •

.

it\

(6) The declining order of the coatings, arranged according to

the protection given specimens in tidewater, was as follows

Wrought AZ31A-H2A magnesium ailoy

Thick HAE (post treated)CR-22Thick SOKDow 7Thin SOKThin HAE

Thick HAEThin Dow 17

Thick Dow 1

7

Thick Dow 17 (post treated)

Sand cast AZ91C-T6 magnesium alloy

Thick HAE (post treated)CR-22Thin HAEThick Dow 17 (post treated)Dow 7Thin SOKThin Dow 17

Thick Dow 17

Thick SOKThi ck HAE

( 7 ) The declining order of the coatings, arranged according tothe protection given specimens in the marine atmosphere,was as foil ows

:

Wrought AZ31A-H24 magnesium alloy

Thick HAE (post treated)Thick Dow 17 (post treated)Thick Dow 17

Dow 7Thick SOKThin HAE

Thin SOKThin Dow 17

CR-22Thi ck HAE

..

10

(8) Visual examination of the surface treated AZ91C-T6 magnesiumalloy after 110 months exposure in the marine atmosphere did

not reveal any indications of corrosive attack on the basemetal. With the exception of the CR-22 coating, no visibletrace of any of the coatings was noted after 36 monthsexposure. No traces of the Dow 7 and the thick SOK coatingswere visible after a one year exposure. The declining orderof merit for these coatings, based on coating longevity,

was as foil ows

:

CR-22Thick Dow 17 (post treated)Thick Dow 17

Thick HAE (post treated)Thick HAEThin HAE

Thin Dow 1

7

Thin SOKThick SOKDow 7

( 9 ) In one of the three salt spray tests performed, paint failuresoccurred on one of the sand cast AZ91C-T6 alloy specimens gjventhe thick Dow 17 coating and in another test paint failureswere noted on a specimen given the thick Dow 17 (post treated)coating. On wrought AZ3 1 A-H24 alloy specimens paint failureswere noted on all of the Dow 7 coated panels in all three testsperformed and on two (one each from two of the three tests)thick SOK coated panels.

(10) After 24 months In the marine atmosphere, paint failures werenoted on all of the surface treated wrought AZ31A-H24 alloyspecimens except those given the thick Dow 17 (post treated),the thick HAE (post treated) and the CR-22 coatings. Afterthe same exposure period, the sand cast AZ91C-T6 alloyspecimens with the paint over-lay on the thin SOK coating had

scattered areas of paint failures.

(11) The declining order of coatings with paint over-ldys, arrangedaccordirg to the protection given wrought AZ31A-H24 alloyspecimens in tidewater, was as follows:

1. Thick Dow 17 (post treated and not post treated)Thick HAE (post treated)CR-22

2 . Thin Dow 17

I

. . x

" ' '

-

1

1

3. Thick HAE (failed at fouled areas).

b . Dow 7Thin HAE

Thin SOKThick SOK

The paint over-lays on the Dow 7j the thick Dow 17 (posttreated and not post treated) and the thin HAE coatingsgave the least protection to the sand cast AZ91C-T6magnes i um alloy.

USCOMM-NBS-DC

J

•

Table 1 . Details of the various surface treatments given wrought magnesiumalloy AZ3 1 A-H24 and sand cast magnesium alloy AZ91C-T6. All specji^

mens were given the pickle treatment prior to application of a

surface treatment. Pickle solution used was dependent upon the

alloy, as noted below.

Surface Treatment Process Deta i 1

s

1. Bare pickled surface

2 . Dow 7

A. AZ31A-H2A -- Acet i c-N i t rate pickle solutionat 70 - 80

0F. Glacial Acetic Acid - 25 1/2

fl. oz. Sodium Nitrate - 6 2/3 oz. Water to

make 1 gallon.B* AZ91C-T6 — Chromic-Nitric - HF pickle

solution. Chromic acid - 37 1/2 oz. Nitricacid (70°/o) - 3 1/^ fl. oz. Hydrofluoricaci d ( 60$>) - 1 f 1 . oz. Water to make 1

gal Ion.

A. Panels were immersed for one minute (approx.)in a hydrofluoric acid (60$> HF) solution,20 fl. oz./gal Ion of water, rinsed in coldwater

.

B. Immersed for 30 minutes in a boilingsolution of;

Sodium di chromate - 20 oz.

Calcium fluoride - 1/3 oz.Water to make 1 gallon; rinsed in cold,then hot water and dried.

3. Dow 17 Bath composition:Ammonium acid fluoride - 40 oz.

Sodium di chromate - 13*3 oz.Phosphoric acid ($5$) - 11.5 fl. oz.

Water to make 1 gallon.Temperature for operation - l80° F.

Voltage - DC

A. Thin coating(a) AZ3 1 A-H24 - 75 volts at 20 amp/sq ft,

for 2 1/2 mi nutes

.

(b) AZ91C-T6 - 73 volts at 20 amp/sq ft,

for 3 1/2 minutesB . Thick coat i ng

(a) AZ31A-H24 - volts at 20 amp/sq ft,

for 15 minutes(b) AZ91C-T6 — 90 volts at 20 amp/sq ft,

for 2 1 mi nutes

.

C. Post Treatment on Dow 17.? thick coating.Immersed for 15 minutes in a 5^ solutionof sodium tetras i 1 i cate (water glass)

maintained at 200 - 212° F.

• •

c:.'= .

s

.

»

Table 1* (Continued)

Surface

4. HAE

T reatmen

t

Process Deta i 1

s

Bath composition:oz/ ga

1

Potassium hydroxide 20.0Aluminum hydroxide 4.5Trisodium phosphate 4.5Anhydrous potassium fluoride 4.5Potassium manganate 2.5

A. Thin coating (AZ31A-H24 and AZ91C-T6).1. Hot alkaline cleaner.2 . Cold water r i nse.

3. Chromic acid, 20$>, solution at

room temperature for 2 minutes.4. Cold water r i nse.

5. Processed in HAE solution (above),

55 to 60° C, 1 8 minutes, 7 volts DC

at 40 amp/sq ft.

6 . Cold water r i nse.

7. Dip in 0.2$> solution of sodiumdi chromate at 80 - 90° C for 2

minutes, no r i ns i ng, a i r dried.B . Thick coat i ng

.

1. AZ31A-H24..(a) Hot alkaline cleaner.(b) Cold water rinse.

(c) Processed in HAE solution (above),room temperature, voltage 0 - 58 ,

15 amp/sq ft for 8 minutes.(d) Cold water rinse.(e) Dip in b i f 1 uor i de-di chromate

solution, 1 minute, air dried.No rinsing after dip in bifluoride-dichromate solution.

( 1 ) Solution -

Ammonium bi fluoride - 13 oz/gal.Sodium dichromate - 2.5 oz/gal.

(f) Aged in humid atmosphere at 175° F

and 83io relative humidity for

3 hours.

2. AZ91C-T6The same treatment as described under 1,

except that the panels were dipped in a

20°lo chromic acid solution at roomtemperature for 3 minutes and rinsed in

cold water between (b) and (c).

• — - - -

'

Table 1. (Continued)

Surface Treatment Process Detai 1

s

CR-22

SOK (Alkaline-chromate)

C. Thick coating plus post treatment.AZ31A-H24 and AZ91C-T6.1. Hot alkaline cleaner.2 . Cold water r i nse

.

3. Processed in HAE solution (above), roomtemperature, voltage 0 - 80 , 15 amp/sq ft

for 75 mi nutes

.

4. Cold water r i nse

.

5. Dip in b i f 1 uor i de-chromate solution for1 minute and air dried without rinsing.

6 . Aged in humid atmosphere at 175° F and

83 relative humidity for 3 hours.

7. Another 1 minute dip in the bifluoride-di chromate solution and air driedwi thout rinsing.

8. Aged at 175° F and 83^ relativehumidity for 4 hours.

AZ31A-H24 and AZ91C-T6A. Hot alkaline cleaner.B . Col d water ri nse.

C. Dip in acid pickle (90 parts water, 8 partsnitric acid (cone.) and 2 parts sulfuricacid (cone.),

D . Cold water r i nse.

E. Dip in 20$ chromic acid solution.F . Cold water rinse.G. Processed in CR-22 bath, 90° C, voltage 0 -

320 , 15 amp/sq ft, 12 minutes.Bath composition:

Hydrofluoric acid ( 52$>)

Phosphoric acid ( 85$>)

Chromic tricxideAmmonium hydroxide

- 25 ml/1

.

- 50 ml/1

.

- 25 g/1.- 160 ml/1

.

A. Bath composition:Sodium hydroxide - 0.75 molarSodium dichromate - 0.75 molar

B. Temperature 70° C

C. Current - AC at 120 amp/sq ft.

D. Cleaner

(1) Hot alkal i ne cleaner

(2) AZ31A-H24 “ solution containing250 g. chromic acid and 20 g. calciumni trate per 1 i ter

.

(3) AZ91C-T6 - solution containing 37*5 oz.

chromic acid, 3*25 fl. oz. nitric acid

(cone.) and 1 fl. oz . hydrofluoricacid (60$) per gallon.

,

--

Table 1„ (Continued)

Surface Treatment Process Deta i 1

s

Thi n coat S ng:

Specimens were anodized in A for

10 minutes^ rinsed in hot waterand air dr i ed„

Thi ck coat? ng:

Specimens were anodized in A for

30 minutes^ rinsed in hot waterand air dr i ed.

.

Table

2.

Results

of

visual

examination

of

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3.

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Table 4. Declining order of conversion coatings arranged according to

the protection given wrought AZ3 1 A-H24 and sand cast AZ91C-T6magnesium alloys in salt spray ^ Marine atmosphere and tide-water environments.

Salt Spray Marine Atmosphere T i dewater

Wrought AZ31A-H24 Magnesium Alloy

HAE, thick + post MAE, thick + post HAE> thick*|

post

treatment treatment treatmentCR-22 Dow ]

r

J J thick + post CR-22t reatment

SOK, thick Dow )'], thick SOK, thickDow 1 7* thick* post Dow 7 Dow 7

treatmentSOK, thin SOK, thick SOK, thinDow 17j> thick HAE, thin HAE^ thinHAE, thin SOK,, thin HAE, thickDow 17* thin Dow 17^ thin Dow 17* thin

HAE, thick CR-22^ a ' Dow 1 7> thickDow 7 HAE, thick Dow 17j> thick + post

treatment

Sand-cast AZ91C-T6 Magnesium Alloy^)

Dow 1 7> thick* post CR-22 HAE, thick +|

posttreatment t reatment

HAE, thin Dow 17^ thick* post CR-22

t reatmentHAE, thick + post Dow 17, thick HAE, thin

treatmentCR-22 HAE, thick + post Dow 17 ^ thick + post

t reatment t reatmentSOK, thin HAE, thick Dow 7Dow 17* thick HAE, thin S0K_, thinDow 7 Dow 17* thin Dow 17* thinSOK, th i ck SOK^ thin Dow 17* thickHAE, thick SDK, thick SOK, thickDow 17> thin Dow 7 HAE, thick

(a) Most of the. green coating maintained after 110 months exposure; howeverspecimen pitted at failed areas. Pits were larger in diameter than on

other specimens with coatings ranked above.(b) Listing of the coatings on specimens exposed in the marine atmosphere

is based on durability. Corrosion protection given by the coatings on

this alloy was not readily determinable in that even after 110 monthsexposure there were no visible indications of corrosive attack.

USCOMM-NBS-DC

w.

.

.

• - *

.

'

.

5• -i:

Bare CR-22SOK,

thinSOK,

thi ck Dow 7

Dow 17>

thi n

Dow 17>

thick plus

Dow 17> post HAE,

thick treatment thin

HAE,

thickHAE, plus postthick treatment

Figure 1. Appearance of representative surface treated wrought AZ31A-H24magnesium alloy specimens after 1000 hours exposure in saltspray and after =cl ean i ng to remove corrosion products andconversion coatings. X ]/k.

Bare CR-22SOK,

thi n

SOK,

thick Dow 7

fCv'v .« • :v‘

Ms

.* ' 5 A",'*-

,<v ;

Dow 17>

thi n

Dow 1 7,thi ck

Dow 17>

thick plus

posttreatment

HAE,

thin

HAE,

thi ck

HAE,

thick plus

posttreatment

Figure 2. Representative surface treated sand cast AZ91C-T6 magnesiumalloy specimens after 1000 hours exposure in the salt sprayand after cleaning to remove corrosion products andconversion coatings. X 1/3.

4 . -i*

Bare CR-22

SOK,

thin

SOK,

thi ck Dow 7

Dow 17, Dow 17,

thin thick

Dow 17>

thick plus

post HAE,

treatment thin

HAE,

thi ck

HAE, plus post

thick treatment

Figure 3 . Effect of 166 days exposure in the tidewater on surface treated

wrought AZ31A-H21+ magnesium alloy specimens. Speci mens ^ are shown

after removal of corrosion products and conversion coatings. X 1/4

Bare CR-22

SOK,

thi n

SOK,

thi ck Dow 7

Dow 17, HAE,

thi ck pi us thi ck,

Dow 1 7,thi n

Dow 17,

thi ckpost

t reatment

HAE,

thi n

HAE,

thi ck

plus posttreatment

Figure A. Effect of 203 days

sand cast AZ91C-T6shown after removacoatings. X 1/3.

exposure in the tidewater on surface treated

magnesium alloy specimens. Specimens are

of corrosion products and conversion

.

k- V H

i ! !