Engineering & Technology · Material Selection, Degradation & Failure

Corrosion, wear and environmental degradation

Two strips lose the same amount of material. One becomes slightly thinner everywhere. The other keeps most of its thickness but develops a deep pit. An average mass-loss result can make them look equivalent, even though the pit may govern the second strip’s function.

Material Structure & Properties and Engineering Design Methods & Trade-offs supply stress, strain, modulus and requirements. This chapter recalls each relationship before using it. Calculations use multiplication, division and simple algebra; the tie example can be read first as a worked substitution. All candidate properties, component histories, profiles and impact values are original teaching assumptions, not supplier data, experiments, safe-life assessments or production specifications.

Back to the chapter

After reading, you can

Explain how environment and contact create local degradation; use loss records and surface evidence to compare corrosion and wear hypotheses.

Look at where material is lost

Corrosion is deterioration through chemical or electrochemical interaction with the environment. Rusting steel is familiar, but other metals and non-metallic materials also degrade. A polymer may swell, soften or crack in an incompatible chemical environment. Temperature, moisture, oxygen, dissolved substances and time can change the behaviour; “corrosion resistant” always needs a context.

Uniform loss is spread comparatively broadly. Pitting and crevice attack concentrate damage in particular places. A surface can retain most of its mass while a local feature becomes critical. The figure gives two exact, invented two-dimensional loss profiles with equal removed cross-sectional area but different maximum depth. The depth direction is magnified to make the distinction readable.

A thin adherent surface film can slow further reaction without making the underlying material chemically inert. Local breakdown can expose a small active region while the surrounding surface still looks intact. A narrow wet crevice also exchanges dissolved species with its surroundings differently from an open face. Its local chemistry can therefore depart from the bulk liquid. A smooth face or a bulk-liquid reading cannot stand in for the hidden interface.

Uniform area removed = 10 mm × 0.04 mm = 0.40 mm². Triangular pit area = ½ × 2 mm × 0.40 mm = 0.40 mm². Maximum depth differs by factor 10. Horizontal scale is 25 SVG units/mm and vertical depth scale 300 SVG units/mm; depth is magnified. This is a profile illustration, not a claim about an actual coupon.

The assembly can create a new environment

Galvanic corrosion can arise when dissimilar materials are electrically connected and share an appropriate electrolyte. One region becomes anodic and may corrode faster than it would alone; the cathodic region supports the corresponding reduction reaction. Electrical contact and ionic conduction through the electrolyte complete different parts of the path.

The outcome depends on the particular environment, surface state and exposed areas. A small anodic area connected to a large cathodic area can concentrate loss. A ranking measured in seawater is not a universal ranking for every liquid. A trapped wet crevice, fastener contact or damaged surface film can therefore change the performance of a material that behaved well as an isolated coupon.

The area effect follows a useful bookkeeping idea. If a galvanic current I is supplied, its mean anodic current density is j=I/A_anode, where A_anode is the exposed anodic area. The same current through less dissolving area concentrates reaction per unit area. In a real coupled system I also changes with surface reactions, geometry and electrolyte resistance, so halving area is not by itself a prediction of twice the corrosion depth. The simple quotient explains the concern without replacing an electrochemical model.

Include motion and contact

Wear removes or deforms material at contacting surfaces. Under load, a hard protrusion or particle can indent a softer surface; sliding can cut or displace material into grooves. Loose particles can also roll and make repeated indentations. Describe their movement and constraint rather than assuming every loose particle cuts. Abrasion can begin when contaminants enter an otherwise smooth contact.

Adhesive wear starts with contact at microscopic high spots, called asperities. Locally joined regions can deform and rupture as the surfaces slide; material can transfer or detach as debris. An adhesive junction need not produce a particle on every encounter. Surface films, interfacial strength and the size of the contacting regions affect the process. This is contact adhesion, not the deliberate addition of glue.

Small repeated relative motions at a joint can cause fretting, even when there is little overall travel. Debris can remain trapped and become oxidised. Particle erosion instead involves particles striking a surface from a moving fluid; their impact and subsequent deformation or fracture remove material. A joint with a small oscillation and a sand-laden jet therefore ask different contact questions.

The pair of surfaces, force, motion, lubrication, debris and temperature belong in a wear comparison. A hardness value alone does not determine bearing life. Wear can remove a protective film and expose fresh material, while corrosion products can become abrasive debris. Study the combined service conditions rather than adding unrelated laboratory rankings.

Measure resistance to motion and material loss separately

For steady sliding, a simple measured friction coefficient is μ=F_t/F_n: tangential resisting force divided by normal force. It is dimensionless. A wear result may instead be a mass or volume loss over a stated sliding distance. These measurements answer different questions. Lower μ can reduce the frictional force while a changed interface still loses more material.

Use a controlled comparison to connect the observations: keep the mating material, loading, motion, distance and environment specified, and examine where transferred material and debris originate. A groove supports an abrasion hypothesis but does not identify the particle source; a transferred patch supports material exchange but does not prove that one mechanism explains the entire history.

Both contacts have normal force 100 N and distance 200 m. A: tangential force 20 N, μ .20, volume loss 3 mm³ and loss/distance .015 mm³/m. B: 10 N, μ .10, volume loss 5 mm³ and loss/distance .025 mm³/m. Left y baseline 330, scale 960 drawing units per μ; right y baseline 330, scale 7200 per mm³/m. This is a constructed counterexample, not a law linking friction and wear.

Keep averages, current concentration and contact evidence distinct

An invented coupon loses 1.8 g. Take density as an assumed 7.5 g/cm³ and exposed area as 60 cm². Lost volume is 1.8/7.5 = 0.24 cm³. Dividing by area gives equivalent average thickness loss 0.004 cm = 0.04 mm.

That calculation treats the measured loss as if it were uniformly spread across the stated area. It does not measure the deepest pit. A local depth of 0.40 mm would be ten times the equivalent average.

Record area definition, cleaning method, density assumption and exposure conditions. To assess a thin section, obtain relevant local thickness or damage information as well. Changing “average” to “minimum remaining thickness” without new evidence would change the claim, not improve it.

For a separate galvanic illustration, supply a total current of 2 mA. Spread over a 4 cm² anodic area, j=0.5 mA/cm²; over 0.5 cm², j=4 mA/cm². The second quotient is eight times larger. Here the current is held fixed solely to expose the area relationship. No material, dissolution rate or exposure duration has been supplied, so the quotients cannot give a pit depth.

Now construct two sliding comparisons with the same normal force 100 N and distance 200 m. Contact A resists with 20 N and loses 3 mm³; B resists with 10 N and loses 5 mm³. Their μ values are 0.20 and 0.10. Loss per distance is 0.015 and 0.025 mm³/m. B has half the friction coefficient but greater measured loss in this invented record. Those quotients are interval results, not laws that predict another load or distance.

Suppose A has grooves and hard debris while B has transferred patches. Investigate particle cutting or indentation for A and adhesive material exchange for B; examine both faces and the debris composition. Removing external grit in a matched comparison could challenge the first hypothesis. If the grooves continue, debris generated within the contact is still possible. The observation leads to a discriminating comparison, not a diagnosis from appearance alone.

Try the reasoning

A second coupon loses 0.9 g at the same assumed density 7.5 g/cm³ over 20 cm². Compare its equivalent mean loss with the original 0.04 mm. Which deepest pit is larger?
Show the answer and reasoning
Volume is 0.9/7.5=0.12 cm³; mean loss is 0.12/20=0.006 cm=0.06 mm, or 1.5 times the first mean. Neither mass/area record gives the deepest pit; local depth evidence is still needed.
At a supplied fixed galvanic current, exposed anodic area falls from 2 to 0.5 cm². How does mean j change, and why is that not automatically the change in real corrosion rate?
Show the answer and reasoning
I/A increases by 2/0.5=4. Real current can also change when area, reaction conditions and electrolyte paths change. The fixed-current comparison explains concentration but does not forecast a coupled system.
A dry metal pair is electrically connected. Wetting it bridges both surfaces; later a nonconducting break removes every metal-to-metal connection. Which paths change?
Show the answer and reasoning
Wetting can establish an ionic path; an electronic path is also needed for the described galvanic coupling. Completely breaking that electronic path interrupts the coupled cell. Each surface can still undergo its own corrosion, so isolation is not a universal cure.
A contact with lower measured μ has more wear volume. Is the result contradictory, and which quantities should be checked?
Show the answer and reasoning
No. μ measures a force ratio, whereas loss per distance measures removal. Check force, sliding distance, both surfaces, debris, environment and comparable loss measurement before seeking a mechanism. The constructed A/B record demonstrates both outcomes together.
A clamp shows fine debris after small oscillations; a channel shows impact damage where a particle stream strikes. What processes and evidence questions follow?
Show the answer and reasoning
Investigate fretting at the clamp and particle erosion at the channel. Establish relative motion and trapped debris for the clamp, and particle/flow/impact conditions for the channel. Damage appearance alone does not exclude corrosion or another interacting process.

What to carry forward

Degradation belongs to a material in a specific system and environment. Track the local damage that matters to function, not only average loss.

References

Teaching resources and research supporting this lesson’s explanations and examples.