Quantifying protection

Dr Nicola Symonds, operations director of nC2, Engineering and the Environment at the University of Southampton, discusses issues around material selection.

How protected are your components and systems from their environment? In most applications, the materials are selected for their intrinsic structural properties, such as ultimate tensile strength or hardness.

However, an equally important question is: how is the material going to survive its environment for the life of its application? This is where coatings and surface treatments provide solutions to protect the structural material.

Examples of protective coatings and surface treatments are diverse: from simple organic paints laid on with a brush to hot-dip galvanising to the vapour deposition of diamond-like-carbon coatings. Each coating is designed to meet a specific need. Coatings should be selected for their adherence to the substrate and their ability to protect it against the principal failure modes expected in the intended environment.

Before selecting a coating or a surface treatment, the immediate environment must be understood. This may include (but is not limited to): temperature, humidity, acidity, salinity, UV exposure, local abrasives, impinging media, and details of static or sliding contacting counter faces. Additionally, the stress state of the substrate must be understood; for example, will the substrate deform under cyclic load to which any coating present must also be subjected to?

The environment to which any modern rolling stock is subjected will vary depending on the haulage type, the country, the location and even the season. It is known that anti-corrosion paint is applied on train and the integrity of this coating is checked at maintenance inspections, and most of the repairs are completed at overhaul, which is approximately every 600,000 miles.

After defining the environment in which the substrate is to survive, it is fundamental to understand the possible failures affecting the area to be protected. For example:

  • Corrosion: High salinity and humidity or moisture;
  • Erosion: Impinging media such as stones or sand;
  • Wear: Relative movement and sliding contact;
  • Fatigue: Cyclic stress state.

Each of the above categories can result in many types of problems. For example, common modes of wear include two-body or three-body abrasive wear, sliding wear and fretting wear. Different coatings and surface treatments are optimised to survive different conditions.

The difficulty facing most design engineers is that data on the ability of a material to resist each possible failure mode is not provided. This is because these properties are not intrinsic. What is required is a dynamic environmental parameter such as a coefficient of friction, a corrosion rate or an erosion rate. These values can only be found through empirical testing under conditions similar to those expected in-service. The values and behavioural trends are dynamic and will change under different conditions; for example, from testing at nC2 we know that a coating that resists erosion at a high angle of attack may well behave poorly under glancing angles.

Coupon testing

Coupon or screening tests are often employed in the initial stages of environmental testing, as they offer a fast and cost-effective methodology for testing many potential coatings under a range of conditions. In this way, solutions can be identified and ranked, allowing the designer to take the best system forward towards large- or full-scale testing. Trials should be used to make any final selection and to define the operational envelopes.

The coupon testing facilities at the University of Southampton employ research grade equipment able to create many environments where temperature, humidity, speed, load (direction, quantity, frequency), media concentration and lubrication delivery are all controllable. 

Before testing can commence there must be agreed measures of performance. These measures will ideally be quantitative, such as volume loss or cycles completed. But qualitative assessments are also important, particularly to understand the mechanism of failure. For example, a high-powered microscope such as a Scanning Electron Microscope will be employed to examine a wear scar to determine if the coating has worn by micro-cracking or micro-cutting – the former often unwanted as it can leave initiation routes for secondary failures.

Two particular types of coupon testing that may be of interest for rolling stock manufacturers, owners and operators are those for corrosion and erosion resistance.

Corrosion resistance

Corrosion is a natural process where a refined metal degrades via an electrochemical reaction with its environment. It is important to remember that corrosion will occur not only over the whole of the surface but locally, and corrosion will only occur if water and oxygen are present. Some examples of localised corrosion are discussed below:

  • Differential aeration corrosion cells can occur due to the presence of water droplets or dirt particles on the material surface. They develop a corrosion pit surrounded by a ring of rust;
  • Bimetallic corrosion occurs when two dissimilar metals are in contact, causing the more anodic metal to corrode;
  • Pitting corrosion often occurs in materials which have protective films. Damage to the protective layer can lead to cavities or holes on the metal surface;
  • Crevice corrosion can occur between two joining surfaces and is indicated by high rates of corrosion in the crevice.

The effects of localised corrosion should be taken into account when designing structures, as well as when choosing the correct anti-corrosion system for a specific environment.

Materials selection for corrosion resistance commonly involves testing with either environmental chambers and salt spray cabinets, which can accelerate the effects of corrosion simulating decades of exposure in months; or electrochemical techniques. In electrochemical studies, a metallic sample is immersed with additional electrodes in a solution typical of the metal’s environment. Through a potentiostat the potential of the metal sample is changed and the resultant current measured. This method leads to a measure of uniform corrosion rate.

Erosion resistance

Erosion resistance is important in many sectors. Erosion is possible anywhere where a particulate medium carried by a fluid or gas impinges on the material. In the natural environment, we see this in action as waves crash against stone cliffs. In industrial sectors such as mining and wind turbines, erosion protection is paramount. Rail rolling stock will see potential erosion issues on the lower faces of the vehicle impacted by small particles and stones. These effects may well increase in the coming years as trains become faster.

The University of Southampton has designed and developed several specialist coupon test rigs, which simulate the effect of solid particle erosion on components and surfaces: 

  • Air-sand erosion is simulated with a fast-moving (up to 240 m/s, or 450 mph) pressurised source of dry air in which solid particles are introduced;
  • Slurry jet impingement erosion is a process by which a mixture of solid particles and a liquid is transported through a closed-loop pipe network and a nozzle prior to being blasted on the sample surface;
  • The high velocity impact test rig creates localised high-strains in the test surface and measures the resultant energy absorption. The rig uses pressurised air to fire a 9mm diameter ball bearing (up to 100 m/s) at a sample to create controllable Hertzian impact conditions.

Coatings and surface treatments offer solutions which are often not provided by a bulk material. Ranking is possible by coupon testing, but only given a prior understanding of the environment and the expected failure modes, which will indicate the appropriate dynamic parameter to be quantified.


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