NASA COtJTRACTOR REPORT

NASA CR-161431

STUDY OF ETCHANTS FOR CORROS I ON-RES I STANT METALS, SPACE SHUTTLE EXTERNAL TANK

By J. R. Simmons Ma&-Marietta Aero6pace Michod Operations New Orleans, LA

{ Y ASA-CR-16 1431) STUDY OF E'ICYANZS FOE N&o-254 14 COBCOSIOI-BESISTAIT METALS, SEACE SHUTTLE EXT2RRAL TANK ( .art in flariet ta Aerospace) 26 p BC AD3/HF A01 C S C L 11P Uoclas

~anuary L A , AJUV G3/26 2L4ld

Prepared for

NASA-George C . Marshall Space Flight Center Marshall Space Flignt Center, Alabama 35812

https://ntrs.nasa.gov/search.jsp?R=19800016918 2018-06-08T16:07:35+00:00Z

NASA CR-161431

Study of Etchants for Corrosion- Resistant Metals, Space Shuttle External Tank

January 11, 1980

APPROVED :

J. P. Knadler, I11 Chief , NDE Branch EH13

M. C. McIlwain Chief, Metals Processes Branch EH42

MATERIALS ENGINEERING

TEST REPORT

NO. ETTR-52

R e v i s i o n A

METAL ETCHANTS PRIOR TO PENETRANT INSPECTION

JANUARY l l , 1 9 8 0

PREPARED BY: R. Simnons

M a t e r i a l s E n g i n e e r i n g

APPROVED BY: Yd

0. A. B o l s t a d , Ch ie f M a t e r i a l s E n g i n e e r i n g

MARTIN MAR1 ETTA AEROSPACE MICHOUD OPERATIONS

NEW ORLEANS, LA

iii

TABLE OF CONTENT?

Introduction

Objectives

Conclusions

Recomnendat i ons

Material s

Specimens

Etchant Solutions

Standard HN03/HF

FeC1 3/HN03/H3PQ

HCI/HNC3/NiC1 */FeCl 3/Cr03

HCl /H202

LW HNOj/HF NO. 1

LW HN03/HF NO. 2

Pasa-Jel 1 101

Test Procedures

Discussion o f Test Results and Conclusions

References

LIST OF TABLES

Table Page

I

I I

I11

I v V

VI

VI I

VIII

I X

X

XI

XI I

XI11

Ni tric/Hydrofluoric Solution A

Ni t r ic /Hydrof lwr ic Solution B

Ni tric/Hydrofluoric Solution C

Ni tri c/Hydrofl uaric Solution D

PasaJell 101

FeCl 3/HN03/H3P04

HC1 /HNO3/Nj C12/FeC1 3/C&

Low HN03/tiF No. 1

HCl /HZ02

LOW HN(33/HF NII. 2A

Low .HE03!.HF No. 2B

Low ' H ~ O ~ / H F N O . 2C

Low 'HN03/HF NO. 20

1.0 IFTRODUCTION

All metal parts, materials, and welds which are mechani-

cally disturbed and require penetrant inspection must be etched

prior to that inspection step. Etching removes metal smears that

might mask defects.

Metals used in the Space Shuttle External Tank(ET) which re-

quire etching prior to penetrant inspection are aluminum alloys,

austenitic stainless steels, nickel base alloys, and titanium

alloys (annealed). Etchants for austenitic stainless steels,

nickel base alloys, and titanium alloys were selected for evalua-

tion in this study after a review of applicable M C , Eosing,

McDonnell Douglas and Rockwell International specifications ( ~ e f -

erences 1-3 0) .

2.0 OBJECTIVES

The objectives of this program were to study and formulate

acceptable etchant concentrations and application and removal

procedures for etching austenitic stainless steel, nickel base alloys and titanium alloys (annealed) employed on the ET.

The etchant solutions were to be capable of removing a minimum

of 0.4 mils of surface material in less than one hour, at a

rate such that control could be exercised,

3.0 CONCLUSIONS

3.1 There was no intergranular attack of 6AL-4V titanium,

21-6-9, A-286, or Inconel 718 by the solutions tested, given the

respective processing conditions.

3.2. 6AL-4V:

The standard HN03/HF solution, paragraph 5.2.1, can be

effectively utilized to remove 0.4 mils surface material in less

than one hour; see tables I thru IV for etchant solution, technique,

and rate.

3.3. Inconel 718:

Two solutions, HCl/HNO3/NiCl2/FeCl3/CrO3, paragraph 5.2.3,

and HC1/H202, paragraph 5 .2 .4 , can be effectively utilized to

remove 0.4 mils surface material in less than one hour; see

tables VII and IX for respective etchant solution, technique, and

rate.

3.4. A-286 and 21-6-9:

Three solutions FeC13/HN03/HjP04, paragraph 5.2.2, HC1/

HNO3/NiCl2/FeCl3/CrO3. paragraph 5.2.3, and HC1/H202, paragraph

5.2.4, can be effectively utilized to remove 0.4 mils surface

material in less than one hour; see tables VI, VII, and I X for

respective etchant solution, technique, and rate.

3.5. Cab-0-Sil was a satisfactory thickener for solutions

FeC13/HN03/H3P04, paragraph 5.2.2, and HC1/HN03/NiC12/FeC13/Cr~3,

~aragraph 5.2.3, reducinq etch rate.

The MMC specification governing etchinq of studied materials

should be appropriately revised per the study conclusions.

5.0 HATERIALS

5.1 S c imns Metal specimens, 14" x 3'' were made from 0.25" 6AL-4V t i t a n m, Inconel, 0.063 A-286 CRES and 0.040" 21 -6-9 CRES. Specimens were a lka l ine cleaned p r i o r t o etchant tests.

5,2 Etchant Solutions -- Etchant solut ions were made up from technical and reagent grade chemicas t o obtain the given make-up f o r each solut ions

, excppt Pasa-Jell 101 which i s a propr ietary solution. These solut ions are described as f o l 1 ows . 5.2.1 Standard HN@/HF

5.2.1.1 Solut ion Make-Up

Chemical - A B C D

HN03 (42cBb), fl oz/gal 40 60 40 60

Water, DM Ral ance

Temperature, OF 75/140

5.2.1.2 Portions o f solut ions B and C were also thickened w i th Cab-0-Sil . _to brush consistenc.~ and used a t 750F.

Solution Make-Up Chemical

Amount

Water, DM Balance

Temperature, OF 7511 40

5.2.2.2 A port ion of the above solut ion was also thickened wi th Cab-0-Sil and used a t 750F.

Solut ion 'Make-Up Chemicii

Amount

FeC13, fl oz/gal

Ct-03, or lga l

Water, DM

Temperature, OF

4 6

51

Balance

751140

5.2.3.2 A por t ion o f t he above so lu t ion was a iso thickened w i t h Cab-O-Sil and used a t 753F.

Solut jon Make-Up: 50% HC1 (2OoBe') and 50% Hz02 (30% Stabi l ized) .

5.2.5 Low HNOQIHF No. 1

5.2.5.1 Solut i;;z;bl;iUp

HNO3, f l oz/gal

Amount

HF, f l ozlgal 3.8

Water, DM Bal ance

Dissolved T i , oz/gal .3

Temperature, OF 75/140

5.2.5.2 A por t ion o f t he above solut ion was thickened w i th C a b - M i l and used a t 750F.

5.2.6 Low HN031HF No. 2

I r o n [Fe), oz/gal -- - 3.4 1.0 3.4

Wetting Agent, Dyneslun --- --- --- --- (See 5.2.7.3) 31-32 31-32

Water, DM Balance

Temperature, OF 7 50F

5.2.6.2 A po r t i on o f so lu t ion B was thickened w i th Cab-0-Sil and used a t 750F.

5.2.7.3 The surface tension o f two por t ions of solut ions B and C were reduced w i t h Ultrawet K (A t lan t ic -R ich f ie ld ) t o 31-32 dyneslcm and t h e solut ions used a t 750F.

5.2.7 Pasa-Jell 101 - This i s a propr ie tary blend of mineral acids, act i \ *atorr and inh jb i to rs . It i s o f brush consistercy and used a t 750F. (Products Research and Chemical Corporati on;

6.0 TEST PROCEDURES

Duplicate specimens o f each metal were weighed p r i o r t o and a f t e r etching i n each so lu t ion a t d i f f e r e n t temperatures and times. The etch ra tes were then determined by welght di f ference. Results are recorded i n Table I through Table X I I I . The absence o f in tergranular a t tack was detemined by d i r e c t observation and 10X magnification.

7 -0 DlSCUSSION OF RESULTS AND CONCLUSIONS

7.1 Intergranular Attack - There was no intergranular at tack o f any metal surfaces tested by t h e solut ions and respect ive processing condit ions.

7.2 Thickening Aqents - Two th ickening agents were t r i e d ; barium su l fa te and Cab-0-Sil . 7.2.1 Barium s u l f a t e was qu ick l y discounted a f t e r t he f i r s t use. A l a rge amount i s required t o provide a thickened version which remains thickened a f t e r appl i ca t ion . The resul tant mixture i s very unreact ive. Reducing the amount o f barium sulfate t o achieve reasonable etch ra tes r e s u l t s i n a mixture t h a t separates too eas i ly f o r p rac t ica l use.

7.2.2 Cab-0-Sil was found sa t is fac tory fo r so lut ions 5.2.2, 5.2.3, 5.2.5 and 5.2.6 (Tables V I , V I I , V I I I and X I ) . IL appears t h a t fo r so lu t ions 5.2.1 (Tables I 1 and 111) wherp the HFIHNO? r a t i o i s greater than 1 t o 7, too much o f the HF i s neutra- l i z e d by [ab-0-Sil ( s t l i c a ) . A l l so lu t ions were thickened a t the t ime o f use and not storcd.

7.3 Etchant E f fec ts - The c r i t e r i o n for selected etchants was t h a t each should be capable of removing . 4 mi l / s ide i n less than one hour.

7.3.1 Standard N i t r i c /Hyd ro f l uo r i c Acids Solut ions (See Tables I through IV).

a) The c r i t e r i o n was met f ~ r 6AL-4V t i tan ium. b) The c r i t e r i o n was not w t for A-286 and Inconel 718, except f o r A-286

a t 1400F (Table I!' ccndit ions). No t e s t was performed on 21-6-9, which i s expected t o behave 1 i ke A-286.

7.3.2 Pasa-Jell 101 (Table V ) . This so lu t i on was t o t a l l j ine f fec t ive on 6A1-4V t i tanium, A-286, 21-6-9 and Incone: 718.

7.3.3 FeC13/HN03/H3P04 (Table VI).

a) The c r i t e r i o n was met fo r A-286, 21-6-9, by both immersion and brush app l ica t ion (non-thickened and thickened w i t h Cab-0-Sil ? , although the etch r a t e o f the thickened version was considerably reduced.

b) This so lu t ion was t o t a l l y ine f fec t ive on 6AL-4V and Inconel 718.

7.3.4 HC1/HN03/NiC12/FeC1 31Cr-03 (Table V I I)

a) The c r i t e r i o n was met f o r A-286, 21 -6-9, and Inconel 718 by both imnersion and brush appl i cat ion (non-thickened and thickened w i t h Cab-0-Sil ). The etch r a t e of the thickened version was a lso reduced.

b) An a c t i v a t i o n step (depassivation) i s rsqui red i n HC1 so lu t i on (30-50 oz/ga! ) p r i o r t o etching.

c) This so lu t i on was i n e f f e c t i v e on 6AL-4V t i tan ium.

7.3.5 Low HN03/HF (Table V I I I )

a) This so lu t i on g rea t l y exceeded the c r i t e r i a f o r 6AL-4V t i t an ium by immersion and should not be considered safe f o r use on t i tan ivm.

b) The thickened version (Cab-0-Sil; was sa t i s fac to ry f o r 6AL-4V t i tan ium. c ) The c r j t e r i o n i s not met fo r A-286 a t 750F o r Inconel 7;5 a t 750F o r

1 40°F. d) The so lu t ion could be heated f o r use on A-286.

7.3.6 Low HN03/HF (Tables K through X I I I )

a) The c r i t e r i o n was met f o r 6AL-4V t i tan ium. The etch r a t e var ied considerably, and for the so lu t ion w i t h low HF and h igh Fe bui ld-up (Table X I I I ) i t was reduced below the c r i t e r i o n .

b) Bnrsh application of the non-thickened version was satisfactory but the thickened version was ineffective (Table XI).

c) Reducing the surface tension reduced the etch rate (Table XI).

7.3.7 HClIH202 [Table IX)

a) The criterion is met in less than two minutes immersion for A-286, 21-6-9 and Inconel 718.

b) This is a rather unstable solution but can be used ef.i:tively by immersion or brush app! ication .

8.0 REFERENCES

1. Martin Marietta Aerospace STP 5007, Cleaning, Descal ing, and Passivation of Corrosion and Heat Resistant Steels an3 Nickel Alloys.

2. Boeing BAC 5751, Cleaning, Descaling and Surface Preparation of Various A1 loys.

3 . Boeing 6AC 5753, Cleaning, Descaling and Surface Preparation of Titanium and Titanium Alloys.

4. Boeing BAC 5758, Cleaning, Descaling and Surface Preparation of Nickel and Cobalt Base A1 loys.

5. Boeing BAC 5759, Chemical Milling of Steel.

6. Boeing BAC 5792, Chemical Milling of Nickel andcobalt Alloys.

7. McDonnell Doug1 as DPS 41003, Surface Treatment of Corrosion Resistant Steel s .

8. McDonnell Douglas DPS 41013, Surface Treatment of Heat and Corrosion Resistant Nickel Base Alloys.

9. McDonnell Douglas DPS 41450, Cleaning and Etching Titanium.

10. Rockwell International MPPRA 1103-003-50, Etching of Machined Metal Surfaces for Penetrant Inspection.

11. Martin Marietta Aer~space STP 5015, Cleaning Titanium Alloys.

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Mate-up: .6 gal HC1, .08 gal HI403 (4208e'). 1 1b N1C12 6H20, 1.8 l b FeC13, .40 1b Cr03/gal; ba1ance:HCl

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Metal Etched

Titanium 6A1-4V

A-286

Inconel 718

21 -6-9

A-286

Inconel 718

21 -6-9 '

Thickened wi th CAB-0-SIL

A-286

Inconel 718

21 -6-9

d Metal 5 mfnutes

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1.05

1.03

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TABLE VIII:

Mate-up: 12.8 f l or HNOj (420f3e1 ), 3.8 f l oz HF (70X)lgal; balance: demineralized water

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140oF

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75OF

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