The preparers of this course are indebted to the various researchers and authors

of papers, presentations and books in the area of corrosion, electrochemical

methods, NDE, monitoring methods and the strategies of maintenance to understand

and to control corrosion.

The material presented in this course is not original work by the course

preparers, but rather an integration from source material from the best

engineers, scientists and managers in this area.

Welcome to the introduction to the Corrosion 202 course.

This course will include the following four segments:

1. Corrosion Management including the topics: Inspectability, and Inspection

Strategies

2. Corrosion monitoring

3. Inspection techniques, and

4. Failure analysis.

If you have not completed the Corrosion 201 course, which covered the basic

science of corrosion, you may want to review that material now.

The goals of this lesson are to provide a basic understanding of corrosion and

approaches to corrosion management. Having this exposure to the impact of

corrosion on the performance and safety of DoD equipment, you will be better

prepared for design, material selection, inspection, monitoring and maintenance

of DoD weapon systems, support equipment and infrastructure.

Welcome to the Corrosion 202A course, which discusses the various approaches of

Corrosion Management.

First, we will briefly review several of the fundamental concepts of corrosion.

Second, we will examine Corrosion detectability and establish the basis for

assessing corrosion flaws, failures and their consequences.

The concepts of Probability of Failure (POF) and Probability of Detection (POD)

will be introduced. The forms of corrosion will be covered with special focus on

the detectability of various types of corrosion damage. We will see how the

various forms of corrosion may be classified in several different ways as an aid

in identifying the form of corrosion present.

Questions to be answered are:

1. Is the form of corrosion uniform or localized?

2. Can the form of corrosion be identified macroscopically or must we use

microscopic or other analytical tools?

3. Corrosion inspection and monitoring are maintenance tasks, that should be

designed to provide information to general management and those managers

responsible for the operation of systems.

We will review the maintenance strategies as they are evolving from corrective to

predictive and increasingly focus on risk-based assessments. The concepts of life

cycle assessment and asset management will be reviewed in relation to inspection

strategies and key performance indicators. The principles of risk-based

inspection and various risk assessment methodologies will be discussed.

Corrosion is the destruction or deterioration

of a material due to its reaction within its

environment. The definition is not limited to

metals and includes: ceramics, plastics,

rubber, composites and other non-metallic

materials. For example, deterioration of

paint or other non-metallic compounds by

sunlight or chemicals.

There are at least four reasons to study corrosion.

First reason is Safety A number of weapon system and infrastructure mishaps

have been attributed to the effects of corrosion. For example, corroded

electrical contacts on F-16s caused uncommanded fuel valve closures with

subsequent loss of aircraft. Corrosion-related cracking of F/A-18 landing gears

resulted in failures (collapses) during carrier operations.

The failure of the Silver Bridge across the Ohio

River on Route 35 in 1967 was attributed to pre-

existing cracks from manufacture that propagated

to failure due to the combination of corrosion

fatigue and stress corrosion cracking

mechanisms.

Readiness is the second reason - Weapons System

and infrastructure support activities are

routinely out of commission due to corrosion deficiencies. For example, corrosion

has been identified as the reason for more than 50% of the maintenance needed on

KC-135 aircraft to date.

Third reason: Finance and Resources- The cost of corrosion to the DoD has been

determined to be greater than $20 billion annually.

And finally, the fourth reason: Performance- Approximately 70% of the cost of

corrosion is a result of decisions made in the design phase in the product

development cycle. The Boeing 737 lost a portion of its upper fuselage in Hawaii

in 1988 due to the interaction of a corrosive media, salty air, and fatigue. The

over-all mechanism is corrosion fatigue.

We will discuss this more later in the course.

The four fundamental requirements for corrosion are: an anode, a cathode, an

electrolyte, and current must flow. Corrosion occurs at the anode. Corrosion

control is based on the various ways to remove or hinder one of the four

requirements for corrosion cited above. For example, a paint may act to keep the

corrosive environment from coming in contact with the metal. We may store

aircraft in Arizona to benefit from the low humidity and essentially stop

atmospheric corrosion. We might wash our automobiles to reduce or to eliminate

dirt or mud. The dirt or mud can serve as media to produce differential aeration

cells that can result in pitting and crevice corrosion under the dirt. We zinc

plate the inside layer of steel door panels to extend the length of time before

perforation. We will be aware of the differences between using zinc versus

chromium plating as ways to protect steel from corrosion. A scratch in the

coating can result in an unfavorable area effect if the plating is noble to the

metal.

We will now look at the detectability of the various forms of corrosion.

Corrosion damage, defects, and failures can have all sorts of consequences on the

operation of a system.

A universal representation, shown here, describes the interactions between

defects, faults, and failures of a system. The arrows in this figure imply that

quantifiable relations possibly exist between a defect, a fault, and a failure.

In materials, a defect is any microstructural feature representing a disruption

in the perfect arrangement of atoms in a crystalline material. There are

imperfections in all man-made material structures. Defects are not necessarily

flaws, but they can serve as initiation sites for actual faults and subsequent

failures. The growth of a defect into what becomes a fault depends on the form of

corrosion.

From fault tree analysis, the fault event is defined as a state transition from a

normal state to a faulty state. The state transition is irreversible. Corrosion

processes are irreversible. Corrosion faults in an electronic component, where a

small amount of surface corrosion can exist, dramatically alter the intended

behavior of the component.

Connector corrosion is well understood as an age-related problem that contributes

greatly to electrical wiring failures. Connector corrosion is also the prime

suspect in several military and commercial aircraft incidents and accidents.

Fretting corrosion in electronic components is the result of flaking of tin oxide

from a mated surface on tin-containing contacts. This problem has occurred more

often as tin replaced gold as a cheaper plating material. Fretting corrosion

between these very small contacts was implicated in at least six F-16 fighter

aircraft crashes when their main fuel shut-off valves closed uncommanded.

An example of a failed tin-plated pin corrosion of an F-16 actuator is shown

here.

A failure is an unsatisfactory condition or deviation from the original

condition. The determination that a condition is unsatisfactory depends on the

failure consequences in a given application. For maintenance purposes, failures

may be further classified as either functional or potential failures.

A potential failure is a detectable symptom or warning sign that indicates when a

functional failure is imminent. The fact that potential failures can be

identified is important to modern maintenance practice. It permits maximum use of

each system without the consequences associated with a functional failure. Units

can be removed or repaired prior to functional failure. An example of a potential

failure is the hard starting of your car on a cold morning due to a failing

battery.

A functional failure is the point where an asset fails to perform a required

function. The failure can be a complete or partial failure of any primary or

secondary system function. An example of a complete failure is a bearing seizing

in a fan motor and causing the fan to stop. An example of a partial failure is a

worn impeller in a pump that still pumps fluid but not to the required level.

In some cases, when safety is a concern, the functional failure may not be the

actual failure point but a predetermined point that should not be exceeded due to

the risk involved. An example of this would be a preset temperature in a car

engine. This temperature is lower than the point where the engine would sustain

critical damage. The actual failure would be the loss of the engine, but the

catastrophic nature of that failure requires an identification prior to the point

of shutdown. This becomes the functional failure of the engine.

The importance of the potential failure is to use an inspection to detect the

potential failure before the functional failure occurs.

A graphical representation of the Potential-Functional interval is displayed. The

vertical axis is the functional capability of the asset and the horizontal axis

is the operating age or number of cycles. As the life of the asset ages, it will

come to a point where the symptom appears. This is the potential failure. After

the symptom appears, there will be a period until the functional failure occurs.

This period is the P-F interval. As we look ahead to various inspection and

monitoring techniques, we will characterize the various forms of corrosion,

potential failure systems, as early as possible to maximize the P-F Interval.

And, we would like to understand the predictive technology that will let us

calculate the time from Potential to Functional failure.

The consequences of a failure may range from replacing a failed component to the

destruction of a piece of equipment and the loss of life. The consequence of

failure then determines the priority of maintenance or possible redesign to avoid

the failure. Corrosion has many serious economic, health, safety, and

technological consequences.

Safety Consequences - Safety is the first consideration in evaluating a failure

possibility. Does the failure cause a loss of function or secondary damage that

could have a direct adverse effect on operating safety? Corrosion of structures

can be a significant problem. Safety can be compromised by corrosion contributing

to failures of bridges, aircraft, automobiles, gas pipelines, etc. Different

corrosion mechanisms can produce different morphologies of damage. The difference

in the release rate created at a pinhole leak or a large rupture can be

significant.

Operational Consequences - an operational failure occurs when the need to correct

a failure disrupts planned operations. Operational consequences include the need

to abort an operation, the delay or cancellation to make unanticipated repairs,

or the need for operating restrictions until repairs can be made.

Nonoperational Consequences a functional failure that has no direct adverse

effect on operational capability. For example, the failure of a navigational unit

on an aircraft with a redundant navigation system. Since other units can provide

the lost function, the failed unit can be replaced at a later time.

Hidden Failure Consequences - a failure mode is defined as hidden when a

component is required to perform its function and the occurrence of the failure

is not evident to operating personnel. Hidden failures are typically failures of

one or more components aligned in parallel with no indication of failure for each

individual component. One of the two components could fail but since each one by

itself can satisfy the function, only when the second one fails will the

functional failure become evident.

The following failures have been selected as examples of documented corrosion-

related failures. These accidents could have been prevented if proper inspection

and maintenance had been carried out.

joints were exposed to atmospheric corrosion. Atmospheric corrosion can be

defined as the corrosion of materials exposed to

than immersed in a liquid.

air and its pollutants rather

F-16: The F-16 electrical connection failure caused by fretting corrosion and

subsequent loss of conductivity across connector pins that then closed the main

fuel shutoff valve.

Aloha Boeing 737 Incident: The structural failure on April 28, 1988 of a 19-year

old Boeing 737, operated by Aloha airlines.

Causes of failure. Multi-site damage: Fatigue cracks emanating from adjacent

fastener holes, crevice corrosion and pillowing.

Pillowing is the bowing out of the skin due to the volumetric expansion of the

corrosion products between the mating fuselage skins. Failure to detect the

corrosion damage present.

This accident may have been the defining event in creating awareness of aging

aircraft in both the public domain and in the aviation community and the effect

of corrosion on the crack growth rate in fatigue. The plane and the subject lap

Three classical patterns of failure are for failure rate versus time.

Pattern A is referred to as the bathtub curve. Region 1 infant mortality high

Probability of Failure (POF). Region 2 - constant and relatively low POF. Region

3 - wearout region with high POF.

Pattern B is a region of gradually increasing failure rate followed by a

pronounced wearout region.

Pattern C is a gradually increasing failure rate with no pronounced wearout

region.

Since corrosion is an aging process, it typically follows Pattern B. A

maintenance strategy with such a pattern of failure would be to remove and

replace the component before it went into the wearout stage. Inspection and

monitoring are to identify faults before they can progress into the wearout

region with a high probability of failure. A similar remove and replace strategy

for Pattern C without a pronounced wearout region might not be beneficial.

In systematic studies of failure pattern of airline components, a large fraction

of the items studied had no wear out zone. Therefore, their performance could not

be improved by removing and replacing those items after a specific time usage.

In corrosion-related failures, two factors must be considered: The first, what

are the forms of corrosion and their rates? The second, what is the possible

effectiveness of corrosion inspection or monitoring?

In the remainder of this course, we will be exploring the various techniques to

inspect and monitor corrosion. We will look at physical, electrochemical and

surface techniques.

The POD Concept is well accepted for Nondestructive Evaluation (NDE). Applying

the POD concept to corrosion is more difficult because of the various forms of

corrosion and their effects: such as thinning, pitting, crack formation,

embrittlement, etc.

Probability of Detection (POD) Curve for cracks not corrosion.

This curve shows the results of inspection of an aluminum alloy stringer-

stiffened panel using hand ultrasonic technique to measure crack depth. The

probability of detection is plotted versus the actual crack depth. As the actual

crack depths are deeper, the POD is higher. The crack depth, which has a 90%

probability with a 95% confidence, is 0.12 cm.

We would also like to have a POD for corrosion; however, corrosion may appear in

various forms depending on the alloy, product form, environment, general

conditions, and unacceptable residual stresses. These factors complicate the

metrics of corrosion and therefore also complicate the quantification of

detection reliability.

To measure the degree of pitting (depth, density, shapes) would be considerably

different than for uniform corrosion (thinning) and the other forms of corrosion.

In a following section, we will review the various forms of corrosion and their

respective defining characteristics, which will challenge us to inspect, monitor

and to interpret the results. We will then look at Maintenance, Management and

Inspection Strategies.

Mars G. Fontana and Norbert D. Greene at Ohio State University in their book

Corrosion Engineering first classified corrosion into eight forms. For each form

of corrosion, they showed the appearance, discussed the mechanism and identified

approaches to control that form of corrosion. Others have expanded the number of

forms of corrosion and regrouped by the ease of recognition.

Classification is based on identifying the forms of corrosion by visual

observation with either the naked eye or magnification. The morphology of the

attack is the basis for classification.

Twelve forms of corrosion are identified in the figure. An appreciation of the

appearance of the various forms of corrosion will be helpful as we consider

various approaches to inspect for and to monitor corrosion.

The forms of corrosion can also be classified by whether the corrosion is uniform

or localized. The various forms of localized corrosion can be further classified

by the magnification at which they are viewed.

Macroscopic forms of corrosion affect greater areas of corroded metal and are

generally observable with the naked eye or with a magnifying lens. In the

microscopic case, the amount of metal dissolved may be small and yet considerable

damage can occur before it is visible to the naked eye.

The degree of localization is an important aspect of any form of corrosion.

Corrosion severity usually increases with the degree of localization.

Detectability of corrosion defects decreases with the degree of localization.

The pie chart displays the failure statistics of the various forms of corrosion

in a chemical plant. Other industries, such as, aerospace, airlines,

infrastructure, military (AF, Navy, Army, Marines, and Coast Guard) may have

different distributions of the forms of corrosion experienced.

On the next chart, the various forms of corrosion will be briefly reviewed with

regards to inspectability and monitoring issues. This chart will also serve as

index to introduce some supplementary ideas or concepts for several of the forms

of corrosion.

Uniform Least threatening type of attack. An example is the uniform thinning of

a sheet.

Pitting often associated with crack initiation and other forms of corrosion:

intergranular, crevice, exfoliation. Pitting can be assessed by various methods:

visual exam of corroded component or localized NDE methods. Reason to reject an

alloy during the design phase for pressure vessels if the alloy has a propensity

for pitting.

Crevice - Crevice corrosion forms between mating surfaces, lap joints and loose-

fitting washers and gaskets.

Galvanic - the relative tendencies of metals to corrode tend to remain about the

same in many of the environments in which they are likely to be used.

Consequently, their relative positions in a galvanic series may be about the same

in many environments. Since more observations of potentials and galvanic behavior

have been made in seawater than in any other single environment, an arrangement

of metals in a galvanic series based on observations in seawater, is frequently

used as a first approximation of the probable direction of the galvanic effects

in other environments.

Erosion-corrosion - as the flow of a liquid phase becomes turbulent the liquid

impinges on the surface to remove the naturally formed thin protective film.

Fretting- abrasive oxide particles can form as asperities are scrubbed off.

Intergranular - classical cases of intergranular corrosion of 18-8 stainless

steel called Weld Decay and Knife Like Attack have occurred as a result of

welding and improper post-weld heat treatments. Intergranular corrosion in some

stainless steels occurs as a result of localized depletion of chromium due to the

formation of carbides.

Dealloying - localized corrosion with selective removal of one of the elements of

an alloy.

The schematic S-N curves represent the cyclic loading of a test sample to failure

at various stress levels. The dashed line shows the stress versus the number of

cycles when the test is conducted in air. The higher the stress; the fewer number

of cycles to failure. At a stress of 120 MPa tested in air, the sample would in

principle not fail. If the corrosion fatigue test was conducted in tap water at a

stress of 120 MPa, the sample would fail at less than 10^6 load cycles.

(a) is the schematic plot of the logarithm of the cyclic crack growth rate,

da/dN, as a function of the logarithm of the stress intensity range, K, showing

the threshold stress intensity factor Kth for fatigue and the critical stress

intensity, KIc,for fast fracture in an inert environment.

K is equal to Kmax minus Kmin.

At a stress intensity below the threshold stress, Kth, no crack growth occurs.

At a stress intensity greater than Kth, the crack growth is described by the

Paris Equation, da/dN = C(K)^n. At a stress intensity greater than KIc,

fracture toughness, fracture occurs.

(b) Shows the schematic plot of the logarithm of the cyclic crack growth rate,

da/dN, as a function of the logarithm of the maximum stress intensity, Kmax,

showing the effect of corrosion environment on corrosion fatigue performance. For

stress intensity above the threshold stress, Kth, the cyclic crack grow rate

would be considerably higher in corrosive versus an inert environment; therefore,

corrosion fatigue occurs.

The three groups of major factors affecting SCC susceptibility are shown in the

graphic.

Material variables are important in determining SCC resistance. Microstructural

modifications introduced by heat treatment can alter the SCC resistance, such as,

sensitization of stainless steels.

Stress the time to failure caused by the propagation of cracks under constant

load tends to increase with decreasing stress until a threshold stress is

reached. The stress can be due to applied stresses and/or residual stresses

caused by manufacturing processes such as welding, machining, and heat treatment.

Environment major factors of the environment include such variables as chemical

composition, pH, temperature, flow rate, etc.

The most important parameter for monitoring SCC is the crack growth rate.

Schematic plot of crack propagation only occurs when the stress intensity is

above the threshold stress intensity value Kiscc. Crack growth rate increases

rapidly with increasing KI above KIscc value (Stage 1) until a plateau is reached

and the crack growth rate becomes independent of KI (Stage 2). The critical

stress intensity, KIc , for fast fracture in air is also indicated.

SSCCCC:: EEnnvviirroonnmmeennttaall--AAllllooyy CCoommbbiinnaattiioonnss

This chart lists some environments in which SCC has been observed for some of the

alloys for the systems listed.

Hydrogen damage is close to SCC, but is caused only by hydrogen atoms, molecules,

and tensile stress. It includes: hydrogen induced cracking, stress oriented

hydrogen induced cracking & sulfide stress cracking.

Cracking occurring because of hydrogen is, also called hydrogen embrittlement or

hydrogen cracking.

The sources of hydrogen are cleaning with acid, thermal dissociation of water in

metallurgical processes, decomposition of gases, cathodic protection, galvanic

plating, and some other corrosion reactions.

All types of hydrogen damage occur in stages:

1. Formation of hydrogen atoms and their adsorption on metal surfaces.

2. Diffusion of adsorbed hydrogen atoms into the metallic lattice.

3. Accumulation of hydrogen atoms inside metals

4. leading to increased internal pressure, and thus to blistering or cracks.

Hydrogen blistering (HIC) is the accumulation of hydrogen molecules, H2, inside

metals and the formation of blisters because of large hydrogen pressure.

Stepwise cracking (SWC) is a form of blistering in which laminating-type fissures

parallel to the metal surface link in the through surface direction.

Stress-oriented HIC is a variation of HIC, where the laminations are arranged in

parallel arrays perpendicular to the surface of the metal.

SSC is a form of hydrogen embrittlement that occurs in the presence of hydrogen

sulfide. In some cases, an alloy embrittled with hydrogen can be restored by

removing the hydrogen through baking.

HHyyddrrooggeenn EEmmbbrriittttlleemmeenntt EEffffeecctt ooff TThheerrmmaall TTrreeaattmmeenntt

For 4340, a low alloy high strength steel, the time-temperature combination would

be selected to remove the hydrogen without excessive tempering of the alloy.

Environmental cracking (EC) is a very acute form of localized corrosion. Because

of the intrinsic complexity of the situations leading to different forms of EC,

the parameters leading to this class of problems have often been described in

qualitative terms such as those in this table.

The factors are listed that contribute to one of three forms of environmental

cracking: stress corrosion cracking (SCC), fatigue corrosion and hydrogen damage

(embrittlement).

Corrosion is one of the major limiters of the life of various DoD structures and

systems. The cost of corrosion annually for the DoD is tens of billions.

Corrosion can have an effect on a system throughout its life from design and

development, through acquisition, operations, sustainment and retirement.

Selection of materials is frequently driven by the need for greater strength,

lower strength-to-weight ratios, lower costs in production, and lower ownership

costs.

Corrosion may be included in trade studies but is not necessarily a primary

driver in design goals; therefore,

1. It is essential that an integrated set of precautions, evaluations and

provisions be included as appropriate in the life cycle management process

to manage the impact of corrosion.

2. A Corrosion Prevention and Control plan should be considered during the

Acquisition Phase.

3. The best opportunity to reduce the costs of corrosion is in the initial

design of the system or vehicle.

The importance of both acquisition and sustainment is depicted in this graphic.

65 to 80 percent of a systems life-cycle costs occur in the sustainment phase.

However, most of the decisions (for example, material selection, component

reliability, design maintainability) are determined during the acquisition phase.

So one of my first jobs working for the Air Force was to start worrying about

corrosion. I actually got assigned to a corrosion organization in the laboratory

here at Wright-Patterson Air Force base. It was kind of interesting though seeing

that the magnitude of the problems early on of my career; I didnt have a good

sense of all this but you really learn quickly the impact in terms of cost and

energy required to prevent corrosion for airplanes.

One of the first jobs I had was working on the C-5 airplane back when it was

still in the early design phases. In that time, there was a great awareness about

the importance of this problem and the impact it could have on system performance

in the long time.

We had written into a lot of those early specifications, the requirements to do

certain things to prevent aircraft corrosion in the future. And I think it is

very important.

We had specifications written. We had lessons learned put together and we made

them all part of the contractual effort at that time. So it resulted in the

contractors paying attention to this early on. The other thing we did back then,

which I think is really important is we created something called Corrosion

Control Boards where we had, I would say subject matter experts maybe 8 to 10 of

them from the government, few of them from the contractors and we would review

every drawing release and look at it in terms of its corrosion. We would conduct

design reviews as part of the normal design review process. There are always

sessions dedicated to what are you doing to protect against corrosion. And I

always thought those boards were really important because they brought attention

to the designers, the guys who were making the day-to-day decisions on whether or

not you use this material or that material, or whether you use this coating or

that coating. They have people knowledgeable about the subject sitting down with

the designers at the right time early on because thats where all the main

decisions are made about what the long-term corrosion effects will gonna be.

I like to tell a story, though despite how good I thought we were doing at the

time. We did make some errors. And Ill never forget one time; we were sitting in

the designer room for the landing gear on the C-5. Now, things that you dont

know much about the C-5, theres something called the yoke of the landing gear.

Its a huge piece of aluminum. I mean its really huge. Its the main load-

bearing member of the landing gear for the C-5.

And the contractor because they were interested in saving weight, they want to

make it out of this, an alloy called 7075-T6 aluminum and that was probably the

highest strength, lowest weight producing alloy you can get at the time. But we

all knew it is also very prone to stress corrosion cracking.

That led to a lot of other things subsequently through the 70s and 80s. We start

putting a lot of emphasis on putting in the requirements in the specifications

and standards that the Air Force used at that time, requirements for corrosion

prevention and corrosion control.

Early on in the design process is the most important thing you could think

about. We think about the corrosion; material selection is important. Not only

what material you pick but how do you treat the material. You have painting; you

have plating, anodizing, all these techniques. Theyre all really important and

the only problem is - there is some of them who are expensive too. But I think my

experience is if youre willing to invest that kind of money upfront, the long-

term payoffs are there.

Theres been some change recently probably within the last year about how the

Air Force is gonna worry about the managing of systems in the future. We used to

have an organization here in Wright-Patterson called the Aeronautical Systems

Center. They bought all the airplanes for the United States Air Force. It was in

fact an acquisition center.

I was the Executive Director; I was the senior civilian for that organization

for 5 or 6 years and I learned a lot at that time. One of the things I learned is

unless these people who run these programs and they are all really good people

have very well documented requirements for them to follow, not much is gonna

happen. Theyre scored on how they beat the requirements in their program

management director and nothing else.

One of the things Im hoping starting to happen when they make this change from

an acquisition organization to what we call a life cycle management organization

is that they now have to start thinking broader. They cant just think about

well lets build this airplane cheap as fastest way we possible can, turn it

over to some other organization like a logistics center and let them worry about

the sustainment cost to it. You cant do that anymore. You got to think of the

whole life cycle of the airplane and I think thats gonna be a tremendous help.

So how do you do that? How do you worry about sustainment, at the front of the

airplane design and all through the life cycle? Thats gonna be a tough issue to

deal with?

I think the way theyre gonna do it and I dont know for sure yet but one of the

ways I think can be done is to building Instructional Management systems. We have

a very good instructional management system today for fatigue and life of

fatigue-based and structurally-based. We dont have a good life cycle management

system in place for corrosion. The hope is the two could be married somehow

or another, that you could in fact do structure monitoring, structure

maintenance, structure control and do corrosion control at the same time. That

remains to be seen but thats one of the hopes I think we have.

Four general types of maintenance philosophies or strategies can be identified,

namely corrective, preventive, predictive, and reliability centered maintenance.

Predictive maintenance is the most recent development. In practice, all of these

types are used in maintaining engineering systems. The challenge is to optimize

the balance between these types for maximum profitability. In general, corrective

maintenance is the least cost effective option when maintenance requirements are

high.

Preventive Maintenance. In preventive maintenance, equipment is repaired and

serviced before failure occurs. The frequency of maintenance activities is pre-

determined by schedules. Preventive maintenance aims to eliminate unnecessary

inspection and maintenance tasks, to implement additional maintenance tasks when

and where needed, and to focus efforts on the most critical items. The higher the

failure consequences, the greater the level of preventive maintenance that is

justified. This ultimately implies a trade-off between the cost of performing

preventive maintenance and the cost to run the equipment to failure.

Corrective maintenance refers to an action only taken when a system or component

failure has occurred. It is thus a retroactive strategy. The task of the

maintenance team in this scenario is usually to effect repairs as soon as

possible. Costs associated with corrective maintenance include repair costs and

loss of operational readiness. To minimize the effects of lost production and to

speed up repairs, actions such as increasing the size of maintenance teams, the

use of back-up systems, and implementation of emergency procedures can be

considered. Unfortunately, such measures are relatively costly and/or only

effective in the short-term.

Inspection assumes a crucial role in preventive maintenance strategies.

Components are essentially inspected for corrosion and other damage at planned

intervals, in order to identify corrective action before failures actually occur.

Preventive maintenance performed at regular intervals will usually result in

reduced failure rates.

As significant costs are involved in performing preventive maintenance,

especially in the terms of scheduled downtime - good planning is vital.

Predictive or Condition-based Maintenance.

Predictive maintenance refers to maintenance based

on the actual condition of a component.

Maintenance is not performed according to fixed

preventive schedules but rather when a certain

change in characteristics is noted. Corrosion

sensors supplying diagnostic information on the

condition of a system or component play an

important role in this maintenance strategy.

A useful analogy can be made with automobile oil changes. Changing the oil every

5000 km to prolong engine life, irrespective of whether the oil change is really

needed or not, is a preventive maintenance strategy.

Predictive maintenance would entail changing the oil based on changes in its

properties, such as the buildup of wear debris. When a car is used exclusively

for long distance highway travel and driven in a very responsible manner, oil

analysis may indicate a longer critical service interval. Some of the resources

required to perform predictive maintenance will be available from the reduction

in breakdown maintenance and the increased utilization that results from pro-

active planning and scheduling.

Good record keeping is very important to identifying repetitive problems, and the

problem areas with the highest potential impact.

Reliability-Centered Maintenance (RCM) takes Condition-Based Maintenance (CBM)

to the next level and condition-based maintenance, you're in a more of a reactive

mode - you see a failure and youre going to correct that.

With reliability-centered maintenance, youre taking a look at the actual parts

and trying to do and find methods and processes to increase the reliability so

you do not have these failures again in the future. Its a process that started

back in the 60s and it takes a more in-depth look to try to find the methods and

processes to increase the reliability which has impacts, again, on the

availability and the over-all system performance.

One of the foundations of our Corrosion Prevention and Control (also known as

CPAC) is Identify, Correct and Maintain. Basically, we identify our different

category codes of corrosion from 1-5:

One being a pristine asset. Two, that it requires organizational corrosion

prevention and control which the corrosion service teams can accomplish. And

categories 3 and 4 - is beyond the organizational level or it has to be

completely blasted and repainted. And this capability, when a vehicle reaches

categories 3 or 4, it is sent to the corrosion repair facility. As a part of the

whole identify, correct or maintain, once weve identified the corrosion we work

to correct it via the corrosion service teams or the corrosion repair facility

which is the blast and paint facility. Either way, we have a pristine asset

thats now a category 1. And the final element is to maintain it, which we use

through our dehumidified shelters or Transhield covers leaving the assets ready

for the war fighter in a ready-to-roll condition.

Life cycle management aim is to maximize a return on the investment in assets by

providing comprehensive information about their condition and value throughout

their life. The emphasis is not on short-term costs of an asset but rather on the

total value (performance) throughout its entire life. Life cycle asset management

focuses on the application of three basic facility management tools: life cycle

costing, condition assessment, and prioritization.

Life Cycle Cost Analysis. A structured methodology to determine the total cost of

assets or systems over their service life. In practice, defining and controlling

life cycle costs may be difficult. The future behavior of materials is often

uncertain, as is the future use of most systems, and the environmental conditions

to which they may be exposed.

An effective life cycle cost analysis depends on having a reasonable range of

possible alternatives that are likely to deliver equally satisfactory service

over a given service life.

Condition assessment uses a systematic condition assessment survey to obtain

comprehensive data about the condition of an asset. The survey is used to

predict: maintenance requirements, remaining service life, long-term maintenance,

replacement strategies, and future usage. Condition assessment surveys provide

the data to permit financial resource allocation for maintenance and replacement

of assets.

Prioritization. Prioritizing maintenance activities employ a methodical approach

in contrast to a reactive short-term approach. Each maintenance action is

evaluated against pre-established values and attributes. This approach usually

involves a system to ensure that the most critical work receives priority

attention.

Inspection consists of the following actions: measurement and comparison with a

specification; judging conformance; classification of conforming and non-

conforming cases; and recording and reporting the data obtained. When different

inspection techniques can be used, the choice of a specific schedule will depend

on the accuracy, speed, and relative costs of the various inspection techniques.

What to Inspect? The selection of the components, parts, or systems that should

be inspected is key. This selection should also be based on a thorough

understanding of the system, materials and processes used, awareness of the

mechanical and environmental stresses, and field experience or history.

Knowing where to expect a potential failure from mechanical predictive technology

improves the chances of finding the defect that may progress to failure.

Historical data gathered during previous inspection and repair cycles can be

quite useful to determine the particular locations where future maintenance

actions should be focused.

For example, more than 50 years of service data collected for the P-3 maritime

patrol and antisubmarine warfare aircraft has resulted in the identification of

`hot spots that should be monitored as part of a predictive maintenance regime.

The benefits of identifying hot spots result in improved probability of detection

of defects.

When to inspect requires a thorough understanding of the structure or system to

include operational environment, structural and corrosive intensity, inspection

reliability, and the preventative maintenance plan. Inspection of fracture-

critical parts will depend on predictive technologies such as structural

integrity programs using fracture mechanics and nondestructive inspection.

Historical data gathered during previous inspection and repair cycles can be

quite useful to determine when and where future maintenance actions should be

focused.

Corrosion Inspection or Monitoring? Corrosion inspection and monitoring are used

to determine the condition of a system and to determine how well corrosion

control and maintenance programs are performing.

Inspection. Corrosion Inspection refers to the process to determine the existence

or extent of corrosion of a component at one point in time compared to a standard

or predicted behavior. Inspection techniques may vary from simple visual

inspection to nondestructive inspections. Inspection techniques should have

sufficient precision to detect defects before failure or continued growth before

the next inspection.

Corrosion monitoring. In comparison to the short term testing in corrosion

inspection, corrosion monitoring occurs over a longer period of time and in some

cases, over the life of the system. Corrosion monitoring might be considered an

in-service corrosion tester. We would envision it somewhat differently if we are

monitoring a chemical plant, pipeline, off-shore tower, bridge or aeroplane.

For a chemical plant, we may monitor a process to: identify a corrosion problem,

monitor corrosion control inhibitor additions, exercise process control,

establish a maintenance schedule and provide data to our predictive models for

prediction of useful life.

For an aircraft, we might use a sensor to evaluate environment-severity of

various locations and missions to determine wash schedules, use of corrosion

preventative compounds or sheltering.

Risk-Based Inspection. Risk-based inspection refers to the application of risk

analysis principles to manage inspection programs for components or systems. The

goal of RBI is to develop a cost-effective inspection and maintenance program

that provides assurance of acceptable mechanical integrity and reliability.

Risk is defined as the combination of probability and consequences.

Risk based inspection procedures can be based on either qualitative or

quantitative methodologies.

Risk Based Inspection schemes are a planning tool used to develop the optimum

plan for the execution of inspection activities.

A risk-based approach to inspection planning is used to:

Ensure risk is reduced to as low as reasonably practical

Optimize the inspection schedule

Focus inspection effort onto the most critical areas

Identify and use the most appropriate methods.

Event tree analysis (ETA) is a logical representation of the various events that

may be triggered by an initiating event, such as a component failure.

Data required for developing a risk assessment program are often acquired during

the analysis of failed components and systems.

However, conducting a failure analysis is not an easy or straightforward task.

Early recognition of corrosion as a factor in a failure is critical, since much

important corrosion information can be lost if a failure scene is altered or

changed before appropriate observations and tests can be made. To avoid these

pitfalls, certain systematic procedures have been proposed to guide an

investigator through the failure analysis process.

The process of failure analysis will be presented in a later section of the

course.

FMEA and FMECA - are inductive failure analyses used in product development,

systems engineering, and operations management analysis of failure modes within a

system for classification by the severity and likelihood of the failures.

Fault Tree Analysis is a top down, deductive failure analysis in which an

undesirable state of a system is analyzed using Boolean logic to combine a series

of lower-level events. This analysis method is mainly used in the field of

Reliability Engineering to determine the probability of a safety accident or a

particular system level, or functional failure.

HAZOP - is a structured, systematic examination of a planned or existing process

or operation in order to identify and evaluate problems that may represent risks

to personnel or equipment, or prevent efficient operation.

Risk matrices provide a framework for an explicit examination of the frequency

and consequences of hazards. This may be used to rank them in order of

significance, screen out insignificant ones, or evaluate the need for risk

reduction of each hazard.

In summary, a basic understanding of corrosion and approaches to corrosion

management have been provided. Various forms of corrosion and their recognition

and inspectability have been explored; including different risk assessment

methods. Maintenance strategies in acquisition and sustainment phases of a system

life cycle were discussed.