NBSIR 76-1118 ! /C

Examination of Ruptured

Tubing from a Hot Water

Generator at Bolling Air Force

Base

I. J. Feinberg

Mechanical Properties Section

Metallurgy Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

Failure Analysis Report

July 1976

Issued September 1976

Prepared for Naval Facility Engineering Command

Department of the Navy

Washington Navy Yard

Washington, D. C. 20374

NBSIR 76-1 118

EXAMINATION OF RUPTURED

TUBING FROM A HOT WATER

GENERATOR AT BOLLING AIR FORCE

BASE

I. J. Feinberg

Mechanical Properties Section

Metallurgy Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

Failure Analysis Report

July 1976

Issued September 1976

Prepared for Naval Facility Engineering Command

Department of the Navy

Washington, Navy Yard

Washington, D. C. 20374

U S. DEPARTMENT OF COMMERCE, Elliot L. Richardson, Secretary

Edward O. Vetter, Under Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

TABLE OF CONTENTS

Page

SUMMARY iii

1. INTRODUCTION. 1

1.1 Reference 1

1.2 Background Information 1

1.3 Material Submitted 1

2. PURPOSE 2

3. RESULTS OF EXAMINATIONS, TESTS AND ANALYSES 2

3.1 Visual and Macroscopic Examination 2

3.2 Microscopic Examination 2

3.2.1 Observations On Water-Side 2

3.2.2 Observations On Fire-Side 3

3.2.3 Observations On Rupture Surface 3

3.3 Chemical Analyses 3

4. DISCUSSION 3

4.1 Visual and Macroscopic Examination 3

4.2 Microscopic Examination 3

4.2.1 Observations On Water-Side 3

4.2.2 Observations on Fire-Side 4

4.2.3 Observations On Rupture Surface 4

4.3 Chemical Analyses 4

5. CONCLUSIONS 5

6 . ACKNOWLEDGEMENTS 5

REFERENCES 5

x
TABLE OF CONTENTS (continued)

TABLES

1. Chemical Analysis of Steel in Tube Sample

2. Chemical Analysis of Exterior Scale

3. Chemical Analysis of Interior Scale

FIGURES

1. Photograph Showing Rupture and Location of Test Sections

2. Photomacrograph Showing Interior Scale Deposits

3. Photomacrograph Showing Tube Wall Thinning

4. Photomicrographs of Structures Adjacent To Rupture in Tube Wall

5. Photomicrographs Showing Tube Wall Crevice Corrosion and Pitting

Attack

6. Photomicrograph of Rupture Profile Showing Corrosive Attack

Adjacent to Heated Fire-Side of Tube Wall

7. SEM Photomicrograph Showing Oxide Deposit on Rupture Face

8. Photomicrograph of Tube Wall Specimen Showing Rupture Profile and

Corrosion Pit

li
SUMMARY

A failure analysis was carried out on a ruptured hot water generator

tube. Scale deposits were found on the inside and outside of the tube.

Apparently, these deposits promoted crevice corrosion and pitting

corrosion, which resulted in wall thinning. Furthermore, pitting attack

created a stress-raising notch. Rupture evidently occurred when

generator operating pressure exceeded the remaining strength of the

deteriorated tube.

Examination of Ruptured Tubing From A

Hot Water Generator At

Bolling Air Force Base

1. INTRODUCTION

1.1 Reference

Naval Facilities Engineering Command, Building 57, Washington Navy

Yard, Washington, D.C. 20374. This examination was conducted at the

request of Mr. George Anadale, Code 511, of the Naval Facilities

Engineering Command, under Work Request No. N62477-76-WR00277 . The

request was dated 25 May 1976 and was received at NBS on 9 June 1976.

1. 2 Background Information

Information in this section was supplied on 25 June 1976 by Navy

Lt. Paul D. Steinke, C.E.C., who is with the resident engineer in charge

of Construction at Bolling Air Force Base. The information was given at

the site of the generator at the Base.

Hot water generator (boiler) no. 2, Building No. 18, Bolling Air

Force Base was installed and not used for approximately two years before

becoming operational. The boiler became operational in November 1975.

The boiler initially operated without chemical treatment of the

feed water.

The first of a number of tube failures in this boiler occurred in

February 1976. After the failures, tube interiors were cleaned with

hydrochloric acid followed by neutralization with soda ash (anhydrous

sodium carbonate) and tube exteriors were brushed. Chemical treatment

of feed water was also provided at this time.

1.3 Material Submitted

One length of tube from the hot water generator was submitted for

examination. According to Lt. Steinke, this length of tube was cut

from a tube that failed after the earlier failures. This tube had been

subjected to approximately 8 to 10 weeks of continuous operation before

the installation of the feed water chemical treatment system. Rupture

occurred about two weeks after installation of the system. Thus, the

tube had an operational service life of about 10 to 12 weeks.

Rupture of the subject tube occurred midway between tube supports.

The supports were approximately six feet apart. The tube sample was

flame cut from a section of the tube which was originally straight when

installed. Original dimensions were nominally 1.25 inch O.D. and

No. 13 guage (0.094 inch) wall thickness.

1
A sample of scale from the tube labeled "interior" (water-side)

and another labeled "exterior" (fire-side) were submitted to NBS for

analyses at the same time.

2.

PURPOSE

The Naval Facilities Engineering Command requested the NBS

Mechanical Properties Section to perform a failure analysis to determine

the cause of failure of the tube. The following tasks were specifically

requested

1. Chemical analysis of the tube metal and scale on the inside

and outside of the tube.

2. A metallographic and fracture analysis.

3. An examination by Scanning Electron Microscope (SEM) with

micrographs

4. A visit to the boiler site for first-hand observation of the

equipment and possible photographs.

3.

RESULTS OF EXAMINATIONS, TESTS, AND ANALYSES

3 . 1 Visual and Macroscopic Examination

Macroscopic examination of the tube sample revealed a tight longitudinal rupture, approximately 0.75 inch long that is shown between

arrows B and C in Figure 1. The rupture was in the heated fire-side wall

(side on which the flame impinged) . The diameter of the tube sample

adjacent to the rupture was approximately 1.4 inch. The sample was slightly bowed. An adherent scale was present on the heated-fire-side

surface of the tube sample adjacent to the rupture. The sample was flame-cut from the parent tube, which is evident from the irregular

contour of the sample end at F, Figure 1. Adherent scale which was

found on the water-side of the tube sample is shown in Figures 2a and 2b.

Thinning of the tube wall that occurred is shown in Figures 3a and 3b.

3 . 2 Microscopic Examination

3.2.1 Observations on Water-Side

The original microstructure of the tube wall, as observed

in unaffected areas, consisted of pearlite in a ferrite matrix. Some

decomposition of the pearlite was found at a metal-scale interface on the

tube water-side (see Figure 4a) . Crevice corrosion and pitting attack were both revealed in microscopic examination [1]* Crevice corrosion of

the water-side of the tube wall is shown in Figure 5a. Pitting attack of

the water-side adjacent to the rupture is shown in Figures 5b, 5c and 8.

* Bracketed numbers refer to references on page 5.

2
3.2.2

Observations On Fire-Side

In examination of a fire-side area a penetration of oxides

and decomposed pearlite were found adjacent to a metal-scale interface.

Decarburization was also observed in this area (see Figure 4b)

Corrosive attack penetrated into the heated fire-side of the tube wall

adjacent to the rupture and decarburization also occurred in this zone

(see Figure 6)

3.2.3

Observations On Rupture Surface

An oxide deposit was present over the entire length of the

rupture surface. An SEM micrograph, Figure 7, shows the deposit midway

along the rupture face.

3.

Chemical Analyses

The results of a conventional chemical analysis of the steel in the

sample are given in Table 1* The results of semi-quantitative and

qualitative spectrochemical analyses of the exterior scale and the interior

scale are given in Tables 2 and 3, respectively.

4. DISCUSSION

4.1 Visual and Macroscopic Examination

Visual and macroscopic examination of the tube sample revealed that

the following damage to the tube occurred in service in addition to

rupturing

a. Bowing and bulging of the tube.

b. Scaling of the fire-side of the tube.

c. Scaling of the water-side of the tube.

d. Thinning of the tube wall.

4 . 2 Microscopic Examination

4.2.1 Observations On Water-Side

The original microstructure, pearlite in a ferrite matrix, is

a normal boiler tube steel microstructure. Decomposition of the pearlite

observed at a metal-scale interface adjacent to the rupture is the result

of a deterioration of heat transfer characteristics because of scale

accumulation. Excessive scaling will create a localized hot spot.

3
Crevice corrosion and pitting corrosion which were observed

both proceed by the same mechanism - galvanic corrosion. Scale or deposits frequently lead to crevice corrosion in boiler tubing by

creating anodic and cathodic areas. The anodic areas are subject to

relatively rapid attack, mainly because they are small in relation to

surrounding cathode areas. The general thinning of the tube wall in

the water-side adjacent to the rupture, as shown in Figures 3a and 3b,

resulted from crevice corrosion. In pitting corrosion, a small area is

attacked because it becomes anodic to the rest of the surface, or because of a highly localized concentration of a corrosive contaminant

in the water. Pitting that occurs at relatively few and scattered sites

can result in rapid perforation because of the large ratio of cathode-toanode area. Cracking of the scale, as shown in Figure 2a, is considered

to have been conducive to pitting attack of the tube water-side.

From microscopic observations it is evident that crevice

corrosion on the water-side greatly reduced tube wall thickness. In

addition, pitting attack further reduced tube wall thickness in localized

areas and created stress-raising notches.

4.2.2 Observations On Fire-Side

Decarburization observed on the fire-side of the tube

indicates that the tube was at some time exposed to an oxidizing

atmosphere (flame). This atmosphere reacted with and removed carbon

from the pearlite constituent in the steel resulting in a localized

weakening of the metal structure. Corrosive attack on the fire-side

also reduced tube wall thickness in the rupture zone.

4.2.3 Observations On Rupture Surface

The oxide deposit observed on the fracture surface resulted

from the seepage of detritus from within the tube. An S.E.M. fractographic examination of the rupture surface could not be made because of

the deposit.

4 .Chemical Analyses

The chemical composition of the steel in the tube was found to comply

with the requirements for ASME SA 178A, Grade A, as reported to be specified

by the Naval Facility Engineering Command for this installation. The carbon

content of the exterior scale points to an unfavorable operating condition.

The high carbon content (about 70%) is indicative of uncombusted fuel. This

condition resulted in the accumulation of the fire-side scale that is shown

in Figures 4b, 5c and 6. An accumulation of exterior scale on boiler tubes

detrimentally affects heat transfer characteristics and can result in

localized hot spots. Sulfur, sodium and vanadium found in the exterior

scale were apparently derived from the fuel oil in which they are frequently

found. These elements are present in complex compounds formed during the

combustion of fuel oil. These compounds may have contributed to the

corrosive attack shown in Figure 6 [2].

4
Compounds containing calcium, iron, magnesium, silicon and other

cations that were found in analysis of the interior scale are deposited

from most untreated water. It could not be determined from this

examination whether the interior scale was old (deposited before feed

water treatment) or new (deposited after feed water treatment) . The

contribution of scale to corrosive attack of the tube water-side has

been noted in sub-section 4.2.1.

5. CONCLUSIONS

From evidence gathered in this examination, more than one factor

was involved in the rupture of the subject boiler tube. These factors were as follows:

1. Scaling of the tube water-side which led to crevice corrosion

and thinning of the tube wall.

2. Pitting attack of the tube water-side which created a stressraising notch.

3. Corrosive attack and deterioration of the fire-side wall.

These factors reduced the thickness and strength of the tube wall

in the rupture zone until it could no longer sustain generator operating

pressure and finally rupture of the tube occurred. Most of the damage

before rupture occurred on the water-side of the tube.

ACKNOWLEDGEMENTS

The metallographic specimens were prepared by Mr. L. C. Smith,

Mechanical Properties Section, Metallurgy Division. The photographic

prints were also made by Mr. Smith.

REFERENCES

[1] ASM Metals Handbook, Vol. 10, 8th Edition, 1973, American Society

for Metals, Metals Park, Ohio 44073; Water-Side Corrosion in

Boilers , p. 534.

[2] Ibidem

Oil-Ash Corrosion, p. 537.

5
Table 1

Chemical Analysis of Steel in Tube Sample

Carbon 0.10%

Manganese 0.36

Phosphorus <0.005

Sulfur 0.019

Silicon <0.01

Nickel 0.03

Chromium 0.10

' \ Molybdenum <0.01

Copper 0.11

6
Table 2

Chemical Analysis

Semi-Quantitative Analysis

Carbon

Sulfur

Qualitative Spectrochemical Analysis

Detected Constituents

Iron

Sodium

Calcium, Vanadium, Silicon

Nickel

Aluminum, Magnesium, Manganese

Strontium, Barium

Zirconium

Titanium, Copper, Lead

Cobalt, Molybdenum, Chromium

Silver

Tin

Zinc

Boron

of Exterior Scale

69.9%

0.109

Concentrational Estimates of

0.3-3

0 . 2-2

0.03-.3 each

0 . 02-.2

0.01-.1 each

0.003-.03 each

0 . 002-.02

0.0005-.005 each

0.0003-.003 each

0 . 00001-.0001

T <0.001

ND <0.05

ND <0.002

T - Trace

ND - Not Detected

7
Table 3

Chemical Analysis of Interior Scale

Semi-Quantitative Analysis

2.4 %

0.98%

Carbon

Sulfur

Qualitative Spectrochemical Analysis and Concentrational Estimates of

Detected Constituents

Calcium

Iron

Magnesium

Silicon

Copper, Manganese

Sodium, Stontium

Zinc

Nickel, Vanadium, Aluminum

Chromium

Zirconium

Molybdenum, Boron, Tin, Barium and

Lead

Cobalt

Silver

5-50%

3-30

1-10

0.1-1

0.05-.5 each

0.03-.3 each

0 . 02-.2

0.01-.1 each

0.005-.05

0 . 001-.01

Titanium 0.0005-.005 each

0. 0003- . 003

0 . 0001-.001

0 . 00001-.0001

8
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b.

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Figure

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to

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side

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be

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greater

in

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than

in

Section

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also

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present

in

both

views

at

heated

fire-side

and

heated

water-side

areas.

Unaffected

Structure

Figure 4. Photomicrographs of Structures Adjacent to Rupture in Tube Wall.

Transverse. Etched with 1% Nital

X 500

a. Microstructures adjacent to water-side. Upper half shows

unaffected structure consisting of pearlite (black) in a ferrite (white) matrix. Some decomposition of the pearlite

occurred at the metal-scale interface.

b. Microstructures adjacent to fire-side. Shows a penetration

of oxides (grey) and decomposed pearlite (black) in metal

adjacent to metal-scale interface. Steel has also been

decarburized

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pitting

reduced

the

tube

wall

thickness

to

approximately

0.024

inch

from

0.094

inch,

the

approximate

installation

thickness.

Rupture

Surface

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Profile

of

Rupture

in

Tube

Wall

Adjacent

to

Heated

Fire-Side

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with

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heated

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of

rupture

surface

resulted

from

corrosive

attack

penetrating

from

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fire-side.

Rupture Face

Water-Side at

Tube Wall

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JS-114A (REV. 7-73)

U.S. DEPT. OF COMM.

bibliographic data

SHEET

1. PUBLIC ATION OR REPORT NO.

NBSIR 76-1118

2. Gov’t Accession

No.

3. Recipient’s Accession No.

title and subtitle

EXAMINATION OF RUPTURED TUBING FROM A HOT WATER GENERATOR

AT BOLLING AIR FORCE BASE

5. Publication Date July, 1976

6. Performing Organization C ode

AUTHOR(S)

I. J. Feinberg

8. Performing Organ. Report No. NBSIR 76-1118

PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 3120586

11. C.ontract/Grant No.

.Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Naval Facility Engineering Command, Dept, of the Navy

Washington Navy Yard

Washington, D.C. 20374

13. Type of Report & Period

Covered

Failure Analysis

14. Sponsoring Agency Code

j. SUPPLEMENTARY NOTES

|. ABSTRACT (A 200-word or less factual summary of most si gni ficant information. If document includes a significant

bibliography or literature survey, mention it here.)

A failure analysis was carried out on a ruptured hot water generator tube.

Scale deposits were found on the inside and outside of the tube. Apparently, these

deposits promoted crevice corrosion and pitting corrosion, which resulted in wall

thinning. Furthermore, pitting attack created a stress-raising notch. Rupture

evidently occurred when generator operating pressure exceeded the remaining

strength of the deteriorated tube.

. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Boiler; corrosion; crevice; failure; notch; pitting; rupture scale; stress; tube.

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