Wear, Corrosion, and Reliability in Real Mechanical Systems
(Level 5)
Most mechanical systems do not fail suddenly—they degrade gradually through wear, corrosion, and environmental attack. Level 5 introduces the mechanisms of long term degradation and the principles of reliability engineering.
At this stage, the focus is on understanding how systems fail over time and how engineers design for maintainability, inspection, and predictable service life.
What You'll Learn
Wear: Surface Damage Mechanisms in Contact and Motion
Wear is the slow death of moving parts. Since everything mechanical with moving parts must be designed to have wear allowed in its design, the only option you have is to attempt to slowly wear out a part over time as opposed to having it catastrophically fail. In wear analysis it is important to understand the mechanisms by which the wear occurs and also which components are most at risk, since as time goes on all components with moving parts will wear down, regardless of initial dimensions.
There are 4 primary mechanisms of wear. Adhesive wear occurs when two surfaces adhere, welding together at microscopic points and then tearing apart. Abrasive wear occurs when hard, abrasive particles in a fluid or environment rub against a surface, wearing it away similar to sandpaper. Fatigue wear occurs when a part undergoes repeated cycles of stress and strain which, over time, can create surface cracks that develop into pits and spalls until a chunk breaks away completely. Fretting wear, typically considered to be insignificant, occurs when two surfaces are subjected to small oscillations relative to each other, leading to significant wear.
Wear progressively reduces the tolerance, increases running clearances and introduces vibration, ultimately leading to bearing failure. By establishing the predominant wear mechanism it is possible to select appropriate bearing materials and lubrication to reduce wear and its consequences. Making this simple selection reduces the chance of premature failure from wear, from years to months, using the same bearing design; with improved materials and lubrication.
Corrosion: Environmental and Electrochemical Degradation
Another environmental mechanism that eats at your parts is corrosion. Like wear, corrosion is a form of degradation that involves a chemical or electrochemical reaction; it is distinct from wear, however, in that it does not require any mechanical contact. Corrosion occurs because parts are exposed to the environment. Common corrosion agents include moisture, saltwater, acids and even normal oxygen in the air. Metal corrodes slowly from molecule to molecule. So, for example, you could have a stationary part with zero stress and it will still corrode away completely over time.
Corrosion is "Bad" in several different ways. Uniform corrosion is the easiest form of corrosion to be "bad" because it is easiest to identify and predict. However, this does not mean it is not important to protect against uniform corrosion. On the contrary, it is most cost effective to protect against uniform corrosion before it occurs. In contrast, Galvanic corrosion is generally the least predictable and most "fair" form of attack. However, once a galvanic current path is identified, the more electro-negative metal will attack faster than the more noble metal. This can be particularly problematic in steel bolts threaded into holes drilled in aluminum frames – in these cases, the aluminum is attacked preferentially. Two of the more insidious and severe types of attack are pitting corrosion and crevice corrosion. Pitting occurs when a small, isolated area of metal surface is attacked and a deep pit formed. This can go undetected for long periods of time, but can ultimately punch a hole right through the thickness of a part in a short period of time. Crevice corrosion occurs in tight gaps or recesses where oxygen cannot reach and creates an area for a concentrated corrosion cell to form. Another form of localised corrosion is stress corrosion cracking (SCC). This occurs when a part is subjected to tensile stress in a corrosive environment and will crack at a stress below the yield strength of the material.
"Dangers of corrosion are typically found in forms that are either highly localized or not readily visible. Pitting and Stress Corrosion Cracking (SCC) often result in a sudden, catastrophic failure with very little warning between the initiation and complete fracture. More than just a colored stain, pitting attack and SCC can break down metal within a few weeks. A cursory inspection may reveal a surface seemingly free of corrosion—only to suffer cracking and failure a few months later.
Tribocorrosion: When Wear and Corrosion Accelerate Each Other
However, in most cases, wear and corrosion are not applied in isolation and in fact, can mutually accelerate each other. Tribocorrosion (or erosion-corrosion) is a term commonly used in the literature to describe such simultaneous effects and it is generally agreed that the combined effect is far worse than either effect applied separately. Wear removes protective oxide layers from a surface, increasing the risk of attack by the environment on the underlying metal. Corrosion softens a metal, increasing its risk of wear. The two processes can interact in a negative feedback loop to progressively degrade a component.
Degradation rates don't add up, they multiply. One process does not simply cause independent degradation - the result is always greater. When pump impellers operate in seawater, for instance, there is both cavitation erosion and saltwater corrosion happening simultaneously. Turbomachinery bearings wear mechanically but are also chemically attacked as they spin in offshore structures. The impact of waves and exposure to saltwater in these environments create complex situations with synergy-built degradation.
When designing for wear resistance and corrosion resistance, remember that wear alone or corrosion alone is not enough. The effects of wear and corrosion must also be considered and how they interact. If this is not taken into consideration the predicted service life will be very conservative and parts will fail prematurely.
Designing Against Wear With Lubrication, Materials, and Surfaces
You can't completely stop wear, but you can significantly slow it down. Lubrication - Keep metal surfaces away from each other with oil or grease to significantly drop friction and cause the wear rate to plummet. Boundary lubrication and hydrodynamic lubrication have different applications, but the mechanism is the same: keep metal surfaces away from each other to stop destructive friction. Hard coatings - Instead of having your base material wear away due to abrasive wear, coat the surface with something like ceramic, nitride or carbide that will wear away at a significantly slower rate.
The choice of material for parts in contact can greatly affect lifespan. Some common materials and their uses include tool steels for heavily loaded sliding or rolling parts, case hardened surfaces (such as zinc plated steel) for parts that must have a hard wear resistant surface, and wear resistant alloys (such as bronze or cast iron) for lightly loaded bearings. A polished surface finish can be very beneficial since it has a lower risk of adhesive wear due to the lower asperity heights compared to a rough surface. Designing for sacrificial wear can also be effective. Here, a cheap consumable part is designed to absorb wear damage instead of a more expensive part that wouldn't easily be replaced. A good example of this would be a wear plate or wear liner attached to the inside of a housing to protect a precision machined bushing.
In most real systems, wear is accepted and managed through scheduled replacement. The brake pads need to be replaced periodically, bearings have a rated life based on wear, seals need to be replaced before they leak. The challenge that the engineering faces is the maintenance of such wear and on time replacement of parts before they cause further damage.
Designing Against Corrosion With Materials, Coatings, and Drainage
The first step taken in protecting metal from corrosion is choosing a material with naturally greater corrosion resistance. Materials such as stainless steel, certain aluminum alloys, and even titanium offer a level of corrosion resistance not offered by plain carbon steel. However, in many applications, using a naturally corrosive resistant material may not be the most cost effective solution. In these cases, the metal can be coated with a layer of protective material such as paint, galvanizing (zinc coating), or anodizing for aluminum. These coatings tend to degrade before the base metal, and must be frequently recoated when the coating fails.
Implement Cathodic Protection - either by using sacrificial anodes (zinc or magnesium that sacrifice themselves to corrode first) or an impressed current system that makes the structure the cathode in the corrosion cell - Cathodic Protection is widely used on ships, pipelines and offshore platforms. Remove the possibility of moisture trapping by eliminating crevices, and allowing for drainage to prevent the accumulation of moisture. Avoiding standing water is also beneficial, as it can contain oxygen and become very corrosive. Environmental control - including dehumidification, sealed enclosures, dry cabinets and even the use of dry nitrogen to maintain a dry environment that prevents corrosion inducing moisture and other corrosives from coming into contact with the item under test.
Another corrodible piece of wisdom: corrosion protection is cheapest to design-in, and most expensive to retrofit. After design freezes, retrofitting corrosion protection is expensive, time consuming, and incomplete. Access becomes a problem, adhesion defects appear out of nowhere, and critical pieces can't be protected adequately. Design it correctly, or expect early wear.
Reliability Engineering: Predicting Failure Rates and Service Life
Reliability is the probability that a system performs its intended function without failure for a specified time under specified conditions. It is a statistical measurement expressed as a percentage or number that represents how reliable a system is, and translates to a specific probability of failure after a given period of time. For example, 95% reliability over 10,000 hours means that only 5% of the units will fail early, before 10,000 hours. It's quite predictable and manageable. What's more damaging are the failures that you didn't anticipate.
Mean Time Between Failures (MTBF): Average life of a repairable item in hours or whatever time measurement is used. Failure Rate: Number of failures per unit time, usually expressed as failures per million hours. The bathtub curve which describes three distinct periods of device failure: infant mortality during the early life of a product due to faulty manufacturing; random failures during the life of a product where failure rate is at its lowest; and wear out failures at the end of a product's life where the failure rate increases again. This curve is useful for burn in, reliable life and scheduled replacement decisions.
Using redundancy is a powerful way to increase reliability. Many systems use backup systems, or multiple parallel components. Designing a system to fail safe is also common. For example, on an aircraft, the flight control system uses three separate hydraulic pathways to move the flight surfaces. Each valve is controlled by three separate actuators. The data center where I store my virtual machines has redundant power supplies for each rack. Valves used in safety-critical applications are often designed with a redundant backup actuator in case the primary actuator fails open or fails closed. Reliability engineering uses statistical models to estimate the probability of failure for complex systems. It is used to design schedules for maintenance activities, aimed at preventing unexpected failures by replacing components before they reach their wear out life. Crack growth predictions can be used to develop a schedule for inspections based on the time required to grow a crack to failure, rather than an arbitrary time interval between calendar years.
Maintainability: Designing for Inspection, Access, and Replacement
Things fail not because they were poorly designed, but because they could not be inspected or maintained. Accessibility: Critical components which take three hours to remove panels in order to inspect, are never inspected. If left standing in racks, pipes, or other exposed locations, they can be inspected as the inspector "sweeps by" even though only a few components need to be checked. Design must also anticipate the tools and the accessibility of internal locations for people of all physical abilities. Put in quick access ports and panels. Modularity: Designing systems and components to be replaceable sub-assemblies which can be swapped out and commissioned rapidly. Turbomachinery cartridges (e.g. pump and motor combinations that can be lifted out as a single unit and replaced with a new one within a matter of minutes) are good examples. Turbomachinery can be replaced in their completely assembled housing in days. Motor modules and distribution units, that can be bolted in and removed within hours are very effective as well. These elements have dramatically reduced downtime and errors during maintenance operations.
Inspection access - allow access through ports, windows or a removable section of the product. Wear indicators - allow the end user to sense wear before brake pad failure occurs. Examples of wear indicators include brake pad wear indicator magnets, oil debris sensors and vibration monitors. Standardization - use of standard fastener sizes, standardized interfaces and the use of readily available replacement parts. Custom proprietary components are maintenance nightmares.
Best systems employ condition based maintenance (monitor condition, replace/repair/overhaul when condition is no longer adequate) rather than time based maintenance. This is more reliable and more cost effective than replacing parts with life left when they are still good, rather than over running them. Design for maintenance (DF&M) should be a consideration during initial design in order to allow adequate maintenance of critical components.
Why Reliability Means Designing for Degradation, Not Perfection
When you start to consider wear, corrosion and reliability changes your approach to engineering completely. The objective of engineering isn't to design something which is flawless. In fact, everything wears, corrodes and fails. Your job is to design something that fails safely and predictably, i.e. when you want it to fail, in a location you choose and in a manner which you have planned for. Anything less is a risk which could result in dangerous or catastrophic failure.
This level of understanding wear and corrosion enables you to choose appropriate materials and apply wear protection that actually works. You will learn how to design for inspection and maintenance. The best design in the world is worthless unless it can be effectively and affordably maintained. Apply reliability principles to predict life, establish maintenance intervals and make wise decisions when parts are good to replace.
Know this: experience shows that systems designed with longevity and maintainability in mind can more successfully survive the rigors of the real world over their design life. You've completed How Things Fail, so now you know the major failure modes that affect mechanical systems: stress, material failure, fatigue, creep, buckling, thermal effects, wear, and corrosion. Use what you now know to design safe, reliable systems that fail in a safe and predictable manner, allowing intervention to prevent catastrophic failure.