Spotlight – Testing and inspection

Materials World magazine
1 Sep 2017

Dr Joy Sumner, Lecturer in Energy Materials at Cranfield University, UK, explains hot corrosion testing.

What is hot corrosion?

Hot corrosion, unlike ‘normal’ or aqueous-based corrosion, occurs at high temperatures. While the exact definition of how high can be debated for different materials or corrosion systems, nickel-based superalloys (used in aerospace engines and power generation systems) have twin peaks of damage, reported at around 700°C (type II) and 900°C (type I). This is different to oxidation (or related mechanisms such as sulfidation and carburisation), where the severity of metal attack increases with temperature.

Why worry about hot corrosion?

The main problem with any corrosion is the loss of metal section. Often a component, such as a gas turbine blade, has been designed to operate with a certain component thickness. In addition to metal lost directly to the corrosion mechanism, type I hot corrosion has the added problem of introducing brittle internal precipitates that further reduce the load-bearing section, while type II creates pits capable of locally raising stresses.

What affects hot corrosion?

Hot corrosion is driven by the presence, and amount, of corrosive deposits (typically liquid) on a component’s surface. Their formation depends upon factors including gas composition, underlying component alloy composition, gas and component temperatures and any heat fluxes. Protective alumina or chromia scales formed on a component can help slow hot corrosion’s propagation.

What is the best way of assessing hot corrosion?

Potential new alloys, protective coatings or system operating parameters will change the extent of corrosion. As such, some prior testing is needed. However, the best test depends upon the data needed.

Engine testing

This most accurately simulates the conditions in operation. However, due to the lack of control of independent variables, it can be difficult to accurately interpret the data produced.

Burner rig testing

There are a large range of different ‘burner rig’ styles in laboratories and research stations around the world. These range from small, single sample set-ups with direct exposure to a flame firing a desired fuel, through to large, pressurised gas flow systems capable of exposing multiple components with component cooling and the expected increase in operational cost.  

Burner rig tests can offer an attractive compromise of control of key variables compared with engine testing, while still having the gas flow rates and heat fluxes missing from furnace testing. However, it’s worth noting that data from different burner rig set-ups can be measuring very different things.

Furnace corrosion testing

Laboratory-scale furnace testing is popular with scientists and engineers in R&D around the world. Set-ups are small, and therefore have controllable costs. The parameters – test temperature, gas composition, applied deposit and testing time – are well defined and the collection of data for comparison is simple. However, the technique’s weaknesses tend to be linked to that small-scale of the testing, which results in low gas flow rates and often a lack of heat flux. The former may inhibit the formation of scales/deposits representative of those seen in real-world operation, while the latter means that there is no driving force for condensation of corrosive deposits onto a sample. Laboratory testing tends to offset this by applying synthetic deposits, either using the ‘deposit recoat technique’ or the ‘Dean’s rig’ method.

Testing corrosion

Specific testing rigs have been developed to address some of the weaknesses found in conventional furnace testing. For instance, they may combine mechanical loading, thermal cycling, fatigue cycling or erosion in parallel with hot corrosion testing.

Furnace oxidation testing

Sometimes hot corrosion testing is not required. If modelling indicates corrosive compounds are unlikely to condense, conventional oxidation testing can suffice.

Bio: Dr Joy Sumner’s work is related to high-temperature degradation mechanisms. Joy is a member of the Institute of Physics and IOM3.

GB Inspection Systems launches LIMBOTOFD probe

A new ultrasonic transducer flanges probe has been released by GB Inspection Systems Ltd, and will be featured by the company at the Materials Testing 2017 conference. The ultrasonic probe has been developed to allow time-of-flight diffraction (TOFD) inspections in areas difficult to access.

The company says that ‘traditional TOFD probes have separate transducers and wedges that are far too tall to allow them to go under obstructions such as bolts on flanges. The LIMBOTOFD is only 10mm high due to its integral wedge design so it can inspect where other probes can’t reach’. 

The probe also has application in boiler tube inspection, and is available in various sizes to meet requirements.

New Olympus portable flaw detector 

Olympus, USA, has released its new handheld EPOCH 6LT flaw detector, an ultrasound specifically for rope access and high portability applications.

The product weighs 890 grammes with grip-oriented weight distribution, and can be held in a harness. The EPOCH employs a rotary knob and button design, allowing the operator to navigate the UI while wearing gloves.

EPOCH features functions that meet the requirements of nearly any conventional ultrasonic inspection application, including core functionalities of the EPOCH 650 flaw detector and EN12668-1:2010 compliance.

Creaform takes on aerospace

3D measurement technology company Creaform, Canada, is entering the aerospace sector with a beta surface inspection metrology solution to meet aviation maintenance requirements.

The company’s HandySCAN 3D scanning series has already been certified by Airbus, France, and has been added to Airbus’ technical equipment manual and applied to the A320, A330/ A340 and A300/310 models. Creaform claims that the use of its equipment in aerospace will shorten time to final report and reinforce decision-making.

M2M displays latest GEKKO

M2M will be showcasing its new flaw detector, GEKKO 2.0, which it says improves productivity and ergonomics while keeping up with innovations in PAUT and TFM (Total focusing Method), at Materials Testing 2017.

GEKKO 2.0 features a clickable scroll wheel to complement the touch screen, or a more sensitive, multi-touch screen. With this new release, standard TCG calibration features are also available with Total Focusing Method (TFM). TFM imaging enables operators to have high-quality resolution detection and characterisation for larger zone coverage, as well as for more tolerance to probe positioning. TFM is implement on GEKKO for circumferential and longitudinal weld-flange-bolt configurations to ease the diagnostic of the inspection. 

All techniques implemented on GEKKO are made compatible with the industry’s most popular probes and scanners. GEKKO can be used for high-resolution corrosion mapping and the detection and characterisation of early stages of HTHA, HIC and So-HIC defects.

Fischer’s new microhardness testers

Fischer Instruments Ltd, UK, has introduced its new range of microhardness testers, the FISCHERSCOPE HM2000 and HM2000 S. The testers are high-precision measurement systems for determining indentation hardness, modulus of indentation, Martens hardness, elastic characteristics and other material parameters.

The FISCHERSCOPEs can be used to determine the mechanical properties of materials ranging from soft coatings to very hard ceramic layers using a re-engineered measurement head that permits several parameters to be ascertained reliably, so a variety of material characteristics can be investigated with the same device.

Features include a programmable XY-stage and motorised Z-axis that allow semi-automated measurements on multiple samples with high throughput and straightforward handling, and an integrated microscope with three different magnification settings.