pe£i(ZThe Macroscopic Detection of Corrosion

in Aluminum Aircraft Structures With

Thermal Neutron Beams and Film

Imaging Methods

A Report

Submitted to the U.S. Naval Air

Systems Command, Washington, DC

USN Contract No. N00019-76-IP-69007

December 7, 1977

Report Prepared

by

Donald A. Garrett

Reactor Radiation Division

Institute for Materials Research

National Bureau of Standards

Washington, DC 20234

Work Performed By

William Parker, Ph.D.

Martin Ganoczy

Michael Dorsey

Donald A. Garrett

The Macroscopic Detection of Corrosion

in Aluminum Aircraft Structures With

Thermal Neutron Beams and Film

Imaging Methods

A Report

Submitted to the U.S. Naval Air

Systems Command, Washington, DC

USN Contract No. N00019-76-IP-69007

Report Prepared

by

Donald A. Garrett

Reactor Radiation Division

Institute for Materials Research

National Bureau of Standards

Washington, DC 20234

Work Performed by

William Parker, Ph.

Martin Ganozcy

Michael Dorsey

Donald A. Garrett
bit A.
Table of Contents

Page

Abstract ]

1.0 Objectives 2

2.0 Background 2

3.0 Experimental Facilities at NBS 3

4.0 Experimental Procedures: General 3

5.0 Technical Approach 5

6.0 Exposure Series 1 Through 3: General 5

6.1-

A Exposure Series lA g

6.1-

B Exposure Series IB ]2

6.1-

C Exposure Series 1: Results and Conclusions ]2

6.2-

A Exposure Series 2A ]

6.2-

B Exposure Series 2B ]5

6.2-

C Exposure Series 2: Results and Conclusions '21

6.3-

A Exposure Series 3A 21

6.3-

B Exposure Series 3B 22

6.3-

C Exposure Series 3: Results and Conclusions 25

6.4-

A Exposure Series 4 28

6.4-

B Exposure Series 4; Results and Conclusions 29

7.0 Quantification of Corrosion 32

8.0 Remarks and Qualifications 32

9.0 Summary 33

10.0 Recommendations for Future Investigations at NBS 34

10.1 Converter Development 34

10.2 Real Time Imaging Methods for Corrosion Detection 35

10.3 Corrosion Quantification - Standard Penetrameter Development 35
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Tables

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Table lA Exposure Series lA: Radiographic Parameters 9

Table IB Exposure Series IB: Radiographic Parameters 12

Table 2A Exposure Series 2A: Radiographic Parameters 16

Table 2B Exposure Series 2B: Radiographic Parameters 16

Table 3A Exposure Series 3A: Radiographic Parameters 22

Table 3B Exposure Series 3B: Radiographic Parameters 25

Table 4 Exposure Series 4: Radiographic Parameters 28

Table 5 Penetrameter Calibration 29
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1 Schematic Diagram of the NBS Thermal Neutron Radiography

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2 C-2 Aircraft Vertical Fin Section

3A Aluminum Corrosion Penetrameter P-1

3B Aluminum Corrosion Penetrameter P-1 - Close Up View

4 Aluminum Corrosion Penetrameters P-2-1 through P-2-4

5 Exposure Series lA: Radiographic Data

6 Exposure Series IB: Radiographic Data

7 Exposure Series 2A: Radiographic Data

8 Exposure Series 2B: Radiographic Data

9 Exposure Series 3A: Radiographic Data

10 Exposure Series 3B: Radiographic Data

11 Exposure Series 4: Radiographic Data

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Abstrac t

The primary objective of this investigation was to determine the

feasibility of detecting corrosion in aluminum Naval aircraft components

with neutron radiographic interrogation and the use of standard corrosion penetrameters . Secondary objectives included the determination

of the effect of object thickness on image quality, the defining of

minimum levels of detectability and a preliminary investigation of a means

whereby the degree of corrosion could be quantified with neutron radiographic data.

All objectives were met and may be summarized as follows: (1)

Environment-induced corrosion can be detected with high sensitivity by

thermal neutron radiographic interrogation, (2) the fluence at the image

plane required to visualize corrosion with conventional imaging methods

e.g., a gadolinium converter in conjunction with a medium contrast

8 *2

industrial x-ray film at density of 2.5, is approximately 4 x 10 n-cm .

Assuming that a transportable neutron radiography system were capable of

4 -2 -

producing a flux of 1.5 x 10 n-cm -sec at an L/D ratio of 40:1, an

exposure of 7.4 hours would be required, (3) at an L/D ratio of 40:1,

the corrosion signatures of both surfaces of a thick object, e.g., a

wing or airfoil, must be interrogated individually. This is due to the

fact that geometrical unsharpness obliterates signature detail on the

surface opposite the cassette. (4) The possibility of corrosion quantification does exist. This conclusion is based on an investigation

with standard corrosion plaque penetrameters , and (5) although the use

of LiF/ZnS light emitting converters is the most efficient method by

which thermal neutron beams can be imaged, the results are inadequate

for unambiguous signature analysis with presently available converters.

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1 . 0 Objecti ves

The primary objective of these investigations was to determine the

feasibility of detecting corrosion in aluminum Naval aircraft components

with neutron radiographic interrogation and the use of standard corrosion

penetrameters . Secondary objectives included the determination of

the effect of object thickness on corrosion image quality, the defining of

minimum levels of detectability and a preliminary investigation of a

means whereby the degree of corrosion could be quantified with neutron

radiographic data.

2.0 Background

The nondestructive detection and visualization of environmentinduced corrosion in aircraft presents an important maintenance problem

and economic impact in both the civilian and military sectors. Since

there is no reliable instrumental method by which corrosion can be

detected in inaccessible areas of aircraft structures, visual and destructive

inspection methods must presently be employed.

Corrosion in aircraft generally is induced by the presence of

moisture and oxygen. The corrosion products so generated oftentimes,

but not always, contain hydrogen in the forms of hydrated oxides or

hydroxides. In the case of salt-water induced corrosion, chlorine may

also be present. Because of the high attenuation of thermal neutrons by

hydrogen and chlorine, it should be possible to visualize corrosion

areas using neutron radiographic i nterrograti on.

The basic procedures governing the neutron radiographic inspection

methods employed in these studies are outlined in this report. In

addition, the results of preliminary experiments at the National Bureau

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of Standards to establish minimum levels of corrosion detectability and

proposed methods to quantify the degree of corrosion are presented.

3. 0 Experimental Facilities at the National Bureau of Standards

The Thermal Neutron Radiography Facility (TNRF) at the NBS lOMW

Research Reactor was employed for these investigations. At a power

7 -2

level of lOMW, the reactor provided a thermal neutron flux of 10 n-cm

-sec ^ at the image plane for an L/D ratio of 40:1. The L/D ratio of

the facility was variable from a minimum of 40:1 to a maximum of 500:1.

The neutron beam is highly thermal! zed, having a gold-cadmium ratio of

500:1. During the course of these studies, the neutron beam shutter was

manually operated. A schematic diagram of the facility is illustrated

in Figure 1.

4. 0 Experimental Procedures: General

All radiographic data generated in these investigations was obtained

using direct neutron imaging methods. Manual film development was

performed throughout with Eastman Kodak standard industrial radiography

chemistry. A 7 in. -thick section cut from an uncorroded C-2 aircraft

vertical fin was used as a standard object to be radiographed. The fin,

which was obtained from the NAVAIR Rework Facility at North Island, San

Diego, CA is illustrated in Figure 2.

Standard corrosion penetrameters were fabricated from aircraftgrade aluminum which had been corroded under controlled environments for

predetermined periods of time. The penetrameter samples were obtained

from the Naval Air Development Center, Warminster, PA. Two types of

penetrameters were employed in these investigations.

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FIGURE

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SCHEMATIC

DIAGRAM

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NBS

THERMAL

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RADIOGRAPHY

FACILITY
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FIGURE 2. C-2 VERTICAL AIRFOIL

- 5

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The first was fabricated from aircraft-grade aluminum wliich had

been subjected to a super-saturated salt water spray for 6.5 hrs.

Nominal step thicknesses were 0.125 in. and 0.87 in. Two views of this

penetrameter are illustrated in Figures 3A and 3B. This penetrameter

/ will be referred to as P-1 in this report.

The second set of four penetrameters was fabricated from 0.062 in.-

thick ai rcraft-grade aluminum plates which had been subjected to a salt

water spray/S02 gas environment for zero, four, eight and fifteen days..

This set which will be referred to as P-2-1 through P-2-4 in this

report, is illustrated in Figure 4. No effort was made to correlate the

corrosion penetrameters with real life conditions. They were employed

only to provide corrosion under standardized conditions.

5. 0 Technical Approach

The tasks outlined in the program objectives were divided into four

systematic experiments, each with one or more objectives. The objectives

of each exposure series at the radiographic parameters employed, and the

experimental results are illustrated.

6. 0 Exposure Series 1 Through 3: General

The general objectives of this series of exposures were:

(1) To determine the feasibility of utilizing neutron radiographic

techniques to visualize corrosion areas in ai rcraft-grade

aluminum employing several thermal neutron f i Im-converter

imaging techniques and a standard corrosion penetrameter.

(2) To assess the effect of object thickness on the penetrameter

image quality.

(3) To observe the effect of analog edge enhancement on the

penetrameter image quality.

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FIGURE 3A. ALUMINUM CORROSION PENETRAMETER

FIGURE 3B. ALUMINUM CORROSION PENETRAMETER CLOSE-UP VIEW

- 7 -

FIGURE 4. ALUMINUM CORROSION PENETRAMETERS P-2-1 THROUGH P-2-4

-6-

In exposure Series 1 through 3, the penetrameter , object and geometry

outlined below are identical, only. the radiographic parameters are

changed in each series. Radiographic exposures were made of:

(A) The penetrameter P-1 alone.

(B) The C-2 \/ertical fin section alone.

(C) The C-2 vertical fin section with penetrameter P-1 facing the

cassette.

(D) The C-2 vertical fin with the penetrameter P-1 facing the

collimated neutron source.

(E) Analog edge enhanced image of C above.

(F) Analog edge enhanced image of D above.

6.1-A Exposure Series 1

The radiographic parameters employed in the Exposure Series lA are

outlined in Table lA below. This series of exposures serves to illustrate

the radiographic data obtained utilizing a reactor-based system having a

high (200:1) L/D ratio and employing medium-contrast (Kodak Type M)

film. The radiographic data are illustrated in Figures 5A through 5F.

Converter

Table lA: Exposure Series lA: Radiographic Parameters

0.0005 in. Vapor Deposited Gd

Film Type Kodak Type M

L/D Ratio 200:1

8

Exposure Time/Fluence 50 min./6 x 10 n-cm

Neutron Beam Cd Ratio 500:

Penetrameter P-1

Object C-2 Vertical Fin Section

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PENETRAMETER

(A)

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(B)

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PENETRAMETER FACING CASSETTE

(C)

C-2 VERTICAL FIN

PENETRAMETER FACING SOURCE

(D)

Fiq. 5: Continued

- 11 -

6.1-B Exposure Series 1_B

The objective of this series of exposures was to determine the

effect on the penetrameter image quality when the L/D ratio was reduced

from 200:1 to 43:1. These experiments provided comparative data of the

results obtainable with a reactor-based system having a high L/D ratio

to a source-based system having a low L/D ratio.

Radiographic parameters employed in Exposure Series IB are outlined

in Table IB below.

Table IB: Exposure Series IB: Radiographic Parameters

Converter

Film Type

L/D Ratio

Exposure Time/Fluence

Neutron Beam Cd Ratio

Penetrameter

Object

0.0005 in. Vapor Deposited Gd

Kodak Type M

43:1

90 sec./4 X 10® n-cm ^ (1)

500:1

P-1

C-2 Vertical Fin Section

The radiographic data derived from Exposure Series IB are illustrated

in Figures 6A through 6F.

(1) Significant Parameter Change

6. 1-C Exposure Series 1 : Pvesul ts and Conclusions

(a) Salt water induced corrosion signatures in aircraft-grade aluminum

can be visualized with neutron radiography using a 200:1 L/D ratio.

The effect of the 7 in. -thick fin thickness is to degrade the image

quality. However, the signature can still be discerned as illustrated

in Figures 5C and 5D.

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iiivMfdiulfc fiDeip'-itentJifr

ori<»i U\J ItOflS a po^eii

i

f.^*j(nf fldt ebB'Tft^^ ,rf zj i'ui-’jiitf nf1 <i !f rf4'** nt T 944 twl7 v

boIe^jBiilf^ af b9nni3i*rt> 4d tltf t»e rt, :6na're 9/1J /i9v5>-^., ;?f?«wp

/ 02 tjn* D2 ? ^u0i l n^

' 4

=ra

-

.i''*r*-E. mf?

akfib
Fiq. 6: Exposure Series IB: Radiogranhic Data

PENETRAMETER

(A)

C-2 VERTICAL FIN

PENETRAMETER FACING CASSETTE

(C)

C-2 VERTICAL FIN

PENETRAMETER FACING SOURCE

- 13 - (D)

F i q . 6

Conti nued

ANALOG EDGE ENHANCEMENT PENETRAMETER

FACING CASSETTE

(E)

ANALOG EDGE ENHANCEMENT PENETRAMETER

FACING SOURCE

in

- 14 -

(b) The time required for the exposure was 50 min. for a L/D ratio of

200 : 1 .

(c) With the penetrameter on either side of the fin, analog edge

enhancement aids in identifying the corrosion signature.

(d) When the L/D ratio is reduced to that of a field system, e.g.

43:1, The corrosion signature can only be visualized when the

penetrameter was placed on the cassette side of the fin as shown

in Figures 6C and 6D, making it necessary to inspect each side of

an airfoil of 7 in. thickness individually.

(e) The time required for the exposure was 90 sec. for a L/D ratio

of 43:1.

(f) With a L/D ratio of 43:1, image edge enhancement aids in

visualizing the corrosion penetrameter when placed on the

source side of the film, however, paint could possibly

produce a similar signature.

6.2-A Exposure Series 2A

In the group 2 series of exposures the 0.0005 in. -thick Gd converter

has been replaced by a Gd202S converter. The radiographic parameters

employed in this series of exposures is given in Table 2A below.

15 -
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1*^

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1 -t«J«rrsif?^n9q art; tiJ iw (^

..,B,C= Uf*>H ,fr V, Oi'f^fc' UVJ itty.

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moAz ih nr’T «rt.T 'M2 ‘>nmpa^ sHt

j

1., i1nti5 i-saq^fv? ot :> f ^r.,«4i/T: ,Q*A b#»n 03

• ,ji»rfe»;uf '^ltw^' »u 't UoV’rs rta ^ f:

fi IP' -tjaft 'JT* 2sy i 'U-.i-^ -fo'1 ba^tu^ro-i

' -=^ f o

nl 3br,fi J|>5*(W ?pt>a ^C<1II 1 - ? 10

^ ^ rtitw (.1)

ant no boDfiTq <iO(*v/ '>?-r^'ji^Jvia{; f»Qrit>’<14W oiiJ grtsKau?!.^

s^fe?«eo<jt bfuo.-j .rftrpq ,iav*av<'.;.i i ^1 > a^uoa

I ^ ,artu^w%>-- t «ii. -aiq

a^uaoqxB A-v 3

.-,.tiu''^n.-i (.;} jIdW' ni 6000.4) efirti ^n* :vO,. : --iff --^v T. .v-'^O w^i nl

afrtliMpofUi *<»t '‘A;^ •• '^•»*f*l»3

'!"- ._ 'V-:3

-ci.Tart A'k ahia) in H9V»$ ##m • Aa> > &i hON(of.qaia^^
Table 2A: Exposure Series 2A: Radiographic Parameters

Converter

Film Type

L/D Ratio

Exposure Time/Fluence

Neutron Beam Cd Ratio

Penetrameter

Object

Gd202S (2)

Kodak Type M

200:1

15 min./2 x 10® n-cm'^ (2)

500:1

P-1

C-2 Vertical Fin

The radiographic data resulting from exposure series 2A are

illustrated in Figures 7A through 7F.

(2) Significant Parameter Change

6.2-B Exposure Series 2B

The objective of Exposure 2B was to examine the radiographic penetrameter P-1 using a Gd202S converter, when the L/D ratio was reduced

from 200:1 to 43:1. The radiographic parameters employed in this exposure

series are outlined in Table 2B below.

Table 2B: Exposure Series 2B: Radiographic Parameters

Converter

Film Type

L/D Ratio

Exposure Time/Fluence

Neutron Beam Cd Ratio

Penetrameter

Object

The radiographic data from thi

Figures 8A through 8F.

Gd202S

Kodak Type M

43:1 (3)

45 sec./2 X 10® n-cm ^ (3)

500:1

P-1

C-2 Vertical Fin Section

exposure series are illustrated in

(3) Significant Parameter Changes
ilr»naff’885TS^ s ioaoflitj AS 9t«eT

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9t«ap«)t9 e!<l) nt baynfqif-) 8As.^»rwi»M 3r.'.'3tiBOttl*.i «ai f ;£A oJ 1:00$ mpil

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Fig. 7: Exposure Series 2A: Radiograohic Data

PENETRAMETER

(A)

C-2 VERTICAL FIN

(B)

C-2 VERTICAL FIN

PENETRAMETER FACING CASSETTE

(C)

C-2 VERTICAL FIN

PENETRAMETER FACING SOURCE

(D)

- 17 -

Fig. 7: Continued

- 18 -

Fig. 8: Exposure Series 2B; Radiographic Data

PENETRAMETER

(A)

C-2 VERTICAL FIN

(B)

C-2 VERTICAL FIN

PENETRAMETER FACING CASSETTE

C-2 VERTICAL FIN

PENETRAMETER FACING SOURCE

(C) (D)

-19

Fig. 8: Continued

-20

6.2-

C Exposure Series 2; Results and Conclusions

(a) Corrosion signatures can be visualized using Gd202S converters

with the penetrameter placed on either side of the 7 in. -thick

fin, using an L/D ratio of 200:1. The apparent contrast was

not as high as that obtained with Gd foil, as illustrated in

Figures 7C and 7D.

(b) The exposure time required using a Gd202S converter was 15

min. for a L/D ratio of 200:1.

(c) Image edge enhancement aided in identifying the corrosion

signature on either side of the fin with a L/D ratio of 200:1.

(d) When the L/D ratio was reduced to 43:1 using a Gd202S converter,

the corrosion signature could only be visualized with the

penetrameter placed between the cassette and the fin as

illustrated in Figure 8C. All image detail was eliminated due

to geometrical unsharpness when the corrosion penetrameter was

placed on the source side of the fin as shown in Figure 8D.

(e) The exposure required 45 sec. to complete.

(f) Edge enhancement did aid in the visualization of the penetrameter when it was placed between the fin and cassette,

however, played little role in the detection of the corrosion

signature when the penetrameter was placed on the source side

of the cassette.

6.3-

A Exposure Series 3A

The principal objectives of this exposure series was to determine

the effect on penetrameter P-1 image quality when the Gd202S converter

21
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* ^ i "
and Kodak Type M film were replaced by a ^LiF/Zns converter and lightsensitive Kodak Type Blue Brand film. The radiographic parameters of

interest in this series of exposures is given in Table 3A below.

Table 3A: Exposure Series 3A: Radiographic Parameters

6 , Converter

Film Type

L/D Ratio

Exposure Time/Fluence

Neutron Beam Cd Ratio

Penetrameter

Object

'LiF/Zns (4)

Kodak Blue Brand (4)

200:1

5 sec. /I X 10^ n-cm

^

500:

P-1

C-2 Vertical Fin Section

Radiographic data from Exposure 3A are illustrated in Figure 9A

through 9F.

(4) Significant Parameter Change

6.3-B Exposure Series 3B

This series of exposure was made to observe the effect of reducing the

system L/D ratio from 200:1 to 43:1 on the image quality of the P-1 penetrameter. The ^LiF/Zns in combination with Kodak Blue Brand film imaging

method remained the same as in Series 3A. The radiographic parameters

employed in these series are presented in Table 3B below.

22 -
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Fig. 9: Exposure Series 3A: Radiographic Data

PENETRAMETER

(A)

C-2 VERTICAL FIN

PENETRAMETER FACING CASSETTE

C-2 VERTICAL FIN

PENETRAMETER FACING SOURCE

(C) (D)

- 23 -

fig. 9: Continued

- 24 -
>T3T3-i<AWI fit'
Table 3B: Exposure Series 3B: Radiographic Parameters

Converter

Film Type Kodak Blue Brand

Exposure Time/Fluence

L/D Ratio 43:1 (5)

0.2 sec. /I X 10^ n-cm ^ (5)

Neutron Beam Cd Ratio 500:

Penetrameter P-1

Object C-2 Vertical Fin Section

The radiographic data derived from these exposures is illustrated in

Figures lOA through lOF.

(5) Significant Parameter Change

6.3-C Exposure Series 3: Results and Concl usions

verter in conjunction with Kodak Blue Brand film and a L/D

ratio of 200:1, it was possible to visualize the corrosion

signature when the penetrameter was placed between the vertical

fin and cassette. The image quality was so poor however, that

it could easily be confused with the scintillator mottling as

illustrated in Figure 9C.

(b) The exposure time required was 5 sec.

(c) When the corrosion penetrameter was placed on the source side

of the fin section, it was impossible to differentiate accurately

between the corrosion signature and the background mottling.

(d) Edge image enhancement provided no useful purpose in the

visualization of the corrosion signature with the penetrameter

on either side of the film.

(a) Using a Nuclear Enterprise NE 421 ^LiF/Zns light-emitting con25 -
•A»4

k

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.

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-

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J6<iJ . T<iv.4te’.!?p tooo :'L»> hnfe oH

1

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' ti* ' ll

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*j

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frbi. oAt t<1J t *iiitf <bj

''. i"

v tsj6ius»e I, ' ifis

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smV* rif k tj.,*i:ija I uo nil\i I • In?. ^i:b3 (t>) . /

i»WinriJ»..aci ^lU Mh/ wje^gt* 't ,-l*tiiiv
Fig. 10: Exposure Series 3B: Radiographic Data

PENETRAMETER

(A)

C-2 VERTICAL FIN

(B)

C-2 VERTICAL FIN

PENETRAMETER FACING CASSETTE

(C) _26-

C-2 VERTICAL FIN

PENETRAMETER FACING SOURCE

(D)

%

r'

Fia. 10: Continued

(e) When the L/D ratio was reduced to 43:1, it was impossible to

accurately distinguish between the corrosion signature and the

background mottling with the penetrameter on either side of

the vertical fin.

(f) The exposure time required for this radiograph was approximately

6.4-A Exposure Series 4: General

These investigations were conducted in an attempt to define the

lower limit of corrosion detectability in aircraft-grade aluminum utilizing

reactor-based neutron radiographic procedures. A set of four calibrated

corrosion penetrameters P-2 were fabricated from 0.062 in. -thick aluminum

plates which had been subjected to a salt water spray/S02 gas environment

for 0, 4, 8 and 15 days. These penetrameters are illustrated in Figure 4.

Thermal neutron radiographs were made of the seven inch thick C-2 vertical

fin with calibrated penetrameters placed on the cassette side of the C-2

vertical fin.

The radiographic parameters employed for this set of exposures are

given in Table 4 below.

0.2 sec.

Table 4: Exposure Series 4: Radiographic Parameters

Converter 0.0005 in. -Thick Vapor Deposited Gd

Film Type Kodak Type M

L/D Ratio 100:

Exposure Time/Fluence 30 mi n. /1 8 x 10^ n-cm

Neutron Beam Cd Ratio 500: 1

Object C-2 Vertical Fin

Penetrameters P-2-1 through P-2-4

The penetrameter calibrations are outlined in Table 5 below.

28 -
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;

,.' "

/; . -

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t »! f9w,

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if Hir ' t t C'M A. t I i Vn
Table 5: Penetrameter Calibration

Penetrameter Number Exposure Period to Salt Water

P-2-1

Spray/S02 Gas Environment (Days)

0

P-2-2 4

P-2-3 8

P-2-4 15

The radiographic data illustrated in Figures 11 A, IIC, HE and IIG

are video reproductions of penetrameters subjected to the corrosive

environment for periods of 0, 4, 8 and 15 days respectively. The data

illustrated in Figures IIB, HD, HE and HG illustrate the edge enhanced

reproductions respectively of these radiographs. It should be noted that

the artifacts which appear as white dots in the neutron radiographic

images were the result of dirt or other foreign neutron absorbing matter

on the inside of the airfoil and were not images of the corrosion penetrameter.

6.4-B Exposure Series 4: Results and Conclusions

(a) Thermal neutron radiographs of the four calibrated corrosion

penetrameters P-2-1 through P-2-4 are illustrated in Figures

HA, HB, HC and HD. These penetrameters were corroded with a

salt water spray/S02 gas mixture for periods of 0, 4, 8 and 15

days respectively.

(b) In the corrosion radiographs illustrated in Figures HA and HC,

the corrosion cannot be distinguished from the background

noise as shown in the edge enhanced illustrations in

29 -
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a*^J ;iJI ,0M ,8r ni b!,^6^Jauri? .

bs4a»5

^

n^uort^ Bo vi.{9vU3<»M2r» ano'ri^Dubo tqtt

n^TtlUSi^ 'sCJ Af ri,‘»b -JKk 'Esqqi flDfrtW 9rti 1

'=j'6ni Api isrtit'o 10 i'Mb Bo .Huisi artJ 9*i9W

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rg .'Tsiawfcit \

V

riAoigii| jA»:i kiB » taha^ yiu^oqxB B

mj f jjTvt'to^ b5»*sidil6a -Mwl srtJ Bi* i'*' u tQorbST A0i't>afl fi‘«T!af^T (6) ' ,

ibaife^ rf ^ tofjajrinjtni^^

1^ j

'' s? ^ Hjfw bAbO’rlOtl aiTiW f ^ OU f8U

d> boo 8 > ,0 To ?t<il*»a<j '»'Tut‘ »t*' it^iw .’Fo^^

.YlaVFJjoqiS'T ^^b

'4 ' '

.Oil t>fi6 AM sS'UQfB Ai iwi <(jo bai noFaoiiab #rti Al' (d)

Lit)uoie;liiftd uto'tB <>*wi/irM9'‘ i U ad ionnsj AO^^oTflio^ arti

rrrr f W:Mk*MtA’? aobs ni ,* 6' »arod'
Fig. 11: Exposure Series: Radiographic Data

(A) (B)

(C) (D)

- 30 -

Fig. 11: Continued

(E) (F)

(G) (H)

-31

Figures llE and IIF. It is likely that the mottled background

noise is due to the neutron response to the fin paint.

(c) Environment induced corrosion can be observed without image

enhancement in the 8 and 15 day samples illustrated in Figures

lie and IID respectively. Image enhancement serves to render

the corrosion images more observable as illustrated in Figures

IIG and IIH.

7. 0 Quantification of Corrosion

Based on the experience gained in these preliminary investigations,

empirical quantification of corrosion appears to be a possibility,

either by isodensity scanning of neutron radiographs and comparison of

scans of the corrosion areas with sound aluminum, or by digital image

enhancement techniques. This conclusion is based on the fact that

differences in corrosion density in the P-1 penetrameter could be visualized and that a minimum level of detectability could be established with

the plaque penetrameters P-2-1 through P-2-4. Penetrameter P-1 was more

easily seen then the P-2 series because of the greater degree of induced

corrosion.

8. 0 Remarks and Qualifications

It should be noted that the approach taken in these investigations

was a very practical one in that the radiographic data obtained using

ideal laboratory conditions was taken as a base, e.g., relatively high

resolution imaging methods were employed. Comparative radiographic data

were then obtained by varying the radiographic parameters to simulate a

low-resolution system as might be the case in a transportabl e field unit

neutron radiography system.

32
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In the preparation of this report the conclusions reached were

derived from actual radiographic data and not from photographic reproductions.

Photographic reproduction of high density radiographs was very difficult to accomplish for this report, e.g., the overexposure of Figure 5

compared to Figure 6 is not real, but is due to the photographic reproducti on.

The data obtained in this work was limited to the use of filmconverter imaging systems. No attempt was made to evaluate scintillatorcoupled real time imaging systems.

! 9.0 Summary

The significant results obtained from these investigations at the

National Bureau of Standards may be summarized conclusively in the

following statements.

(1) Envi ronment- i nduced corrosion in aircraft-grade aluminum can.

be detected with a high degree of sensitivity with thermal

neutron radiographic interrogation.

(2) The fluence (integrated neutron flux) at the image plane

required to visualize corrosion with conventional imaging

methods e.g., gadolinium converter in conjunction with a

medium contrast industrial x-ray film at density of 2.5, is

8 “2

approximately 4 x 10 n-cm . Assuming that a transportable

4

neutron radiography system were capable of producing 1.5 x 10

-2 -

n-cm -sec at a L/D ratio of 40:1, an exposure of 7.2 hrs

would be required.

(3) At a L/D ratio of 40:1, the corrosion signature of both

surfaces of a thick object, e.g., a wing or airfoil, must be
4

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interrogated individually. 'his is due to the fact that

geometrical unsharpness obliterates signature detail on the

surface opposite the cassette.

(4) The possibility of corrosion quantification does exist, based

on investigation with standard corrosion plaque penetrameters

(5) Although the use of ^LiF/ZnS light emitting converters is the

most efficient method by which thermal neutron beams can be

imaged, the results are inadequate for unambiguous corrosion

signature analysis using presently available converters. The

reasons for this are described below.

10.0 Recommendations for Future V/ork at NB$

10.1 Converter Development

The most efficient light-emitting neutron imaging system in so far

g as speed is concerned consists of a physical mixture of LiF and ZnS

powders. The components are held in suspension with a binder and settled

on a thin aluminum substrate. The nuclear reaction which takes place is

g Li(n,T)a, the tritons and alpha particles producing light in the ZnS

scintillator.

The neutron radiographic images produced when this type of imaging

screen is placed in intimate contact with light-sensitive film is quite

grainy, exhibiting poor resolution. This is due, in fact, to several

factors which include:

(a) The fact that the matrix is composed of powders having a

finite grain size. If this grain is too coarse, a granular

image wi 1 1 result.

34
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(b) The ^LiF/ZnS is partially tr insparent to light generated

within the matrix, making it possible for light generated at

one point to be scattered and emitted at a neighboring site.

(c) Light sensitive films generally tend to exhibit greater grain

than industrial x-ray films.

We propose that a funded R & D program be initiated at NBS to

develop a neutron-sensitive scintillating converter which will overcome

the aforementioned problems of the presently available continuous ^LiF/ZnS

thermal neutron imaging screens. Specifically, (1) we propose to size

g the LiF/ZnS components so that only the finest grains available are

employed to mix the final matrix using a fluorocarbon binder and (2) to

break the initial continuous screen into a myriad of discrete information

centers, or cells in order to eliminate the cross-talk generated as a

result of the light transmission within the scintillator screen matrix.

We have in mind to deposit the converter matrix in individual cells

using chemically milled meshes having cells of approximately 0.001 in.

on a side separated by 0.005 in. walls to minimize cross-talk.

10.2 Real-Time Imagi ng Methods for Corrosi on Detection

Pending the outcome of the screen-development work, we propose that

an evaluation of state-of-the-art real-time imaging systems be conducted

for corrosion detection under field conditions. It would be most cost

effective to evaluate the available real-time imaging systems on a lease

basis.

This method has the advantage of speed based on the assumption that a

suitable fast imaging screen can be developed which would provide the required resolution.

35
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10.3 Corrosi on Quanti f i cation Standa i;;d Penetrameter Devel opment

The quantification of detectable corrosion could produce an economic

I impact on military aircraft maintenance costs based on the assumption that

short exposure times can be achieved. By being able to quantify the

corrosion, it should be possible to predict the time at which a defective

area of skin or component should be replaced, rather than replacement on

a preventative maintenance basis. This is contingent on the fact that

a fast, high resolution imaging screen can be developed which would

result in reasonable exposure times.

We propose that several avenues of investigations towards this end

be pursued, e.g.

(1) Automated radiographic image analysis based on the isodensity

pattern comparison with standard corrosion penetrameters or

edge enhanced shadow analysis.

(2) Microchemistry of actual aircraft corrosion samples to aid in

the fabrication of standard corrosion penetrameters for use

as Standard Reference Materials.

(3) Subtraction of x-ray data from neutron data to eliminate the

radiographic interferences caused by structural airframe

thickness variation.

36
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J|V V-73) NATIONAL BU f( E A U O F S T A N O A R DS

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(i-ITLE OF PAPER:

•the macroscopic detection of corrosion in aluminum

'AIRCRAFT STRUCTURES WITH THERMAL NEUTRON BEAMS AND FILM

IMAGING METHODS

^ D E D for NB5 Pu’olicotio”' (Definitions: Nf.<S Manual)

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1. laJHUCATlON OR KHPORTNO. 2. (lov't Accession

No.

I^T.r. AND SUHTHT-H

'-RE MACROSCOPIC DETECTION OF CORROSION IN ALUMINUM AIRCRAFT

STRUCTURES WITH THERMAL NEUTRON BEAMS AND FILM IMAGE METHODS

KUTHOR(S)

lonaTd A . . Garrett

JRRFORMING ORGANIZATION NAME ANO ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

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SUFPl.IiMENTARY NOTES

AllSTRACiT (A 200-word or less factual summary of most si^ificant information. If document includes a significant

bibliography or literature survey, mention it here.) ^ The primary objective of this investigation was to determine the feasibility of

'tecting corrosion in aluminum Naval aircraft' components with neutron radiographic

iterrogation and the use of standard corrosion penetrameters. Secondary objectives in

iuded the determination of the effect of object thickness on image quality, the definintjj

'minimum levels a dectability and a preliminary investigation of means whereby the de-

•ee of corrosion could be quantified with neutron radiographic data.

All objectives v/ere met and may be summarized as follows: (1) Environment-induced

Drrosion can be detected with high sensitivity by thermal neutron radiographic interroiticn, (2) the fluence at the image plane required to visualize corrosion with conven-

'^onal imaging methods e.g., a gadolinium converter in conjunction v/ith a medium contras

;idustrial x-ray film at density of 2.5, is approximately 4x1 0^ n-cm“^. Assuming that a I’ansportable neutron radiography system v/ere capable of producing a flux of 1.5x10^

•cm"^-sec"^ at an L/D ratio of 40:1, an exposure of 7.2 hours would be required, (3) at

] L/D ratio of 40:1, the corrosion signatures of both surfaces of a thick object, e.g.,

v/ing or airfoil, must be interrogated individually. This is due to the fact that geo5trical unsharpness obliterates signature detail on the surface opposite the cassette.

1) The possibility of corrosion quantification does exist. This conclusion is based on

1 investigation with standard corrosion plaque penetrameters, and (5) although the use °LiF/ZnS light emitting converters is the most efficient method by whigh therm.al

eiitron beams can be imaged, the results are inadeouate for unambiguous signature anaiys

r

itb_p42eseft-tly avaiT a-b-l e--c-&nv^t&i^s^ . . ,

- i KEY WORDS fs/x to twelve entries; alphebeticol order; capitalize only the first letter of the first key ivora unless a pioper

; name; separated by semicolons)

Aircraft corrosion; corrosion:

corrosion qu^nti f iV;:itinn • hiHf

radiography

corrosion characterization; corrosion detection;

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1 with neutron beams ; neutron

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