Table Of ContentCorrosion- and Erosion-Based Materials Selection for the M242
Autocannon Barrel in a Marine Operating Environment
by
Kathryn Sherrick
A Project Submitted to the Graduate
Faculty of Rensselaer Polytechnic Institute
in Partial Fulfillment of the
Requirements for the degree of
Master of Science
Major Subject: Engineering Science
Approved:
_________________________________________
Ernesto Gutierrez-Miravete, Project Adviser
Rensselaer Polytechnic Institute
Hartford, Connecticut
April, 2012
(For Graduation May 2012)
© Copyright 2012
by
Kathryn Sherrick
All Rights Reserved
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CONTENTS
LIST OF TABLES ............................................................................................................. v
LIST OF FIGURES .......................................................................................................... vi
ACKNOWLEDGMENT ................................................................................................. vii
ABSTRACT ................................................................................................................... viii
1. Introduction .................................................................................................................. 1
1.1 Interior Ballistics ................................................................................................ 2
1.1.1 Rifling .................................................................................................... 3
1.2 Ammunition ....................................................................................................... 3
1.2.1 Propellant ............................................................................................... 4
1.2.2 Cartridge Case ........................................................................................ 6
1.2.3 Projectile ................................................................................................ 6
2. Selected Firearm .......................................................................................................... 8
2.1 The M242 Autocannon ...................................................................................... 8
2.1.1 Naval Variant ......................................................................................... 9
2.2 Base Metal ........................................................................................................ 10
2.3 Ammunition ..................................................................................................... 11
3. Corrosion and Corrosion Protection .......................................................................... 12
3.1 Corrosion .......................................................................................................... 12
3.1.1 Introduction .......................................................................................... 12
3.1.2 Atmospheric Corrosion ........................................................................ 14
3.2 Corrosion Protection ........................................................................................ 16
3.2.1 Conversion Coatings ............................................................................ 17
3.2.2 Ceramic Coatings ................................................................................. 18
3.2.3 Paint/Organic Coatings ........................................................................ 19
3.2.4 Rust-preventative compounds .............................................................. 20
4. Erosion Mechanisms and Erosion Control ................................................................ 21
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4.1 Erosion Mechanisms ........................................................................................ 21
4.1.1 Chemical Erosion ................................................................................. 22
4.1.2 Thermal Erosion ................................................................................... 23
4.1.3 Mechanical Erosion .............................................................................. 25
4.1.4 Combined Effects ................................................................................. 26
4.2 Erosion Protection ............................................................................................ 26
4.2.1 Propellant Modifications ...................................................................... 26
4.2.2 Projectile Modifications ....................................................................... 27
4.2.3 Gun Bore Materials .............................................................................. 27
4.2.4 Metals and Alloys ................................................................................ 29
4.2.5 Ceramics ............................................................................................... 37
5. Results and Discussion .............................................................................................. 39
5.1 Scope ................................................................................................................ 39
5.1.1 Base Metal ............................................................................................ 39
5.1.2 Projectile and Propellant ...................................................................... 40
5.1.3 Operating Environment ........................................................................ 42
5.2 Corrosion Protection of the M242.................................................................... 42
5.2.1 System Selection .................................................................................. 42
5.3 Bore Erosion Protection of the M242 .............................................................. 43
5.3.1 Material Selection ................................................................................ 44
5.3.2 Coating Solution .................................................................................. 45
5.3.3 Liner Solution ...................................................................................... 47
6. Summary .................................................................................................................... 49
7. References .................................................................................................................. 50
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LIST OF TABLES
Table 1 - Alloy composition of gun steels (12) ............................................................... 11
Table 2 - Common Chemical Erosion Reactions in Gun Bores ...................................... 23
Table 3 - Properties of Ideal Gun Bore Materials (2) ...................................................... 29
Table 4 - Ceramic material property analysis (47) .......................................................... 37
Table 5 - Properties of 25 mm ammunition for the M242 (48) (49) ............................... 40
Table 6 - Composition of WC 890 propellant (50) .......................................................... 41
Table 7 - Analysis of Metals and Alloys for Gun Bores ................................................. 45
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LIST OF FIGURES
Figure 1 - Stave-construction cannon ca. 1545 AD (1) ..................................................... 1
Figure 2 - Gun Barrel Internal Geometry (4) ..................................................................... 3
Figure 3 - Schematic of a typical medium caliber high explosive round (5) .................... 4
Figure 4 – Schematic of M242 Autocannon (6) ................................................................ 8
Figure 5 - Mk 38 Mod 2 "Typhoon" naval weapon system (8) ....................................... 10
Figure 6 – Salt Concentration Cell (14) ........................................................................... 13
Figure 7 - Differential Aeration Cell (14) ........................................................................ 13
Figure 8 – Cross sections of as-produced and eroded gun bore rifling (27) .................... 21
Figure 9 - Modeled wall temperatures for M242/M791 (10) .......................................... 31
Figure 10 - Chrome Electroplating Process Schematic ................................................... 32
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ACKNOWLEDGMENT
I would like to express appreciation to my employer, the United Technologies
Corporation, for financing my graduate education. I would also like to thank my advisor
Dr. Ernesto Gutierrez-Miravete for his guidance.
On a personal note I would like to thank my brother, GM2 Andrew Rauss, U.S.
Coast Guard, for serving as a technical resource throughout this project. Thanks also go
to my parents, Steve and Diane Rauss, and my father-in-law, Ken Sherrick, for their help
with babysitting when there were conflicts with work and school schedules.
Finally, I’d like to thank my husband Mark and our daughter Lily for their constant
love, support, and inspiration. You are the best family I could have ever hoped to have.
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ABSTRACT
Firearms operated by the military are often exposed to aggressive environments, and
improvements in propellant and projectile design has led to increased rates of gun bore
erosion. This project explores the atmospheric corrosion and thermo-chemical-
mechanical erosion that firearms are subjected to when operated in a marine
environment, and presents different methods of protecting against those mechanisms.
Using a mass-produced medium caliber autocannon, the M242 Bushmaster, as an
example, suitable materials, systems, and manufacturing methods are proposed with the
goal to provide atmospheric corrosion protection for the barrel as a whole and to offer
extended bore service life beyond what is provided by the current chromium coating.
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1. Introduction
Firearms have been used in warfare for hundreds of years and from the beginning they
have presented unique materials engineering challenges. The earliest cannons were
made from cast bronze and could throw a stone a few hundred yards using rudimentary
gunpowder as the propellant. In later efforts to increase range, cannons were
manufactured by arranging wrought iron rods in a cylinder and then welding or soldering
them together, and then wrought iron rings were driven or shrink-fit over the weldment
to provide it with additional structural support. The appearance of such cannon was
reminiscent of the staves and ring of a wooden barrel and this is actually the origin of the
term “gun barrel”. A picture of this type of cannon, raised from the 16th century wreck
of the British warship Mary Rose, is shown in Figure 1.
Figure 1 - Stave-construction cannon ca. 1545 AD (1)
As metallurgy progressed and new manufacturing techniques were developed in the 18th
and 19th centuries, guns began to more closely resemble today’s designs with the use of
elongated projectiles (rather than round cannon balls), progressive-burning propellant,
effective rifling to stabilize projectiles, and the use of cast steel for gun barrel
construction. The increased muzzle velocities and chamber pressures that enabled
greater range also led to the first observations of gun bore erosion in the late 19th century
(2). The materials used, cast iron and steel, were also susceptible to the effects of
atmospheric corrosion particularly when used in a marine environment, such as in a
coastal fort or on a naval ship.
1
The first half of the 20th century brought two world wars and associated
improvements in gun steel and gun and projectile design, new smokeless powder
propellants, and different types of gun bore liners and coatings. With increases in
projectile and gun muzzle velocities came a need for propellants with higher burning
temperatures which cause greater erosion. To the present day erosion and corrosion of
guns continues to pose challenges and opportunities in materials science and metallurgy.
The scope of this project is limited to a materials optimization of the barrel of a
commonly-used type of medium caliber military cannon, the M242 Bushmaster, when
applied in a marine operating environment. It begins with a brief overview of interior
ballistics and gun bore and projectile design, followed by a discussion of the M242.
The mechanisms of corrosion and bore erosion of firearms are explored in depth, and
materials and manufacturing processes to mitigate and protect against those mechanisms
are detailed. Finally, an optimized solution for the M242 is proposed.
1.1 Interior Ballistics
Interior ballistics is the study of the interaction of the gun, projectile, and propelling
charge before emergence of the projectile from the muzzle of the gun. It includes the
ignition and combustion of the propellant in the chamber, pressurization of the chamber
and first movement of the projectile down the bore, obturation of the round and
engraving of the rotating band, in-bore dynamics of the projectile, and tube dynamics
during the firing cycle (3).
A gun barrel is a tubular, thick walled pressure vessel which is intended to contain
the combustion of propellant and to provide a projectile with directional impulse and
stability during launch. The barrel itself consists of longitudinally coaxial combustion
chamber and bore, with the transition between the wider-diameter combustion chamber
and the narrower bore identified as the forcing cone. The forcing cone is the location
where rifling (if present) begins.
A general schematic of a gun barrel’s internal geometry is shown in Figure 2 below.
2
