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Mechanical testing in corrosive environments

Mastery of mechanical testing combined with control of corrosive conditions to reproduce in-service conditions.

In real-world industrial conditions, mechanical stresses and aggressive environments act simultaneously. Their combination can lead to unexpected failure modes, well below the limits established by conventional testing in air.
Stress corrosion cracking, hydrogen embrittlement, corrosion fatigue, environmentally assisted cracking: these phenomena affect a broad spectrum of materials including high-strength steels, aluminium alloys, titanium alloys and nickel-based superalloys. They concern sectors as diverse as automotive, aerospace, energy and the oil and gas industry.
The French Corrosion Institute has the equipment and expertise to reproduce these conditions while simultaneously controlling mechanical loading and the exposure environment: liquid or atmospheric corrosive media, temperature, humidity. Tests are carried out in accordance with the international standards in force or to bespoke specifications and cover the full range of approaches: from constant-load tests under immersion-emersion conditions through to crack propagation on pre-cracked specimens in corrosive environments.

Stress corrosion cracking tests

The French Corrosion Institute owns and develops test rigs to perform stress corrosion cracking tests in:
– Static mode: 3 or 4-point bending fixtures, U-bend specimens, C-rings, dynamometric rings, dead-weight tensile machines (10T)
– Dynamic mode: slow strain rate
These tests comply with ASTM G30, ASTM G36, ASTM G38, ASTM G39 and ASTM G40 standards.

In particular, the FCI carries out constant-load immersion-emersion tests in accordance with ISO 7539-4, ISO 11130, ASTM G44 and ASTM G47 on stainless steels and aluminium alloys. Tests can also be carried out in climatic or corrosion chambers, controlled in temperature and humidity, with or without saline pollution, for example to perform EAC tests on high-strength 7000-series aluminium alloys.

Corrosion fatigue tests in aggressive environments

At the Saint-Etienne site, the French Corrosion Institute has equipment to carry out corrosion fatigue tests in highly corrosive environments such as H2S. The following equipment is available:

– Cantilever

– Rotating bending

– 4-point bending (pressure 1-100 bar).

Corrosion fatigue tests under immersion

In Brest, the French Corrosion Institute has 3 servo-hydraulic machines with a capacity of +/- 25 kN, capable of operating in tension-compression. These rigs can be fitted with custom-made glass or plastic corrosion cells to perform corrosion fatigue tests in corrosive solutions. These machines can also be equipped with 4-point bending rigs with a capacity of 10 kN inside a titanium corrosion cell to run seawater fatigue tests at various temperatures (from 5 to 80°C). Cathodic protection can be applied and the oxygen concentration can also be controlled if required. Other solutions can also be considered. Fatigue tests are carried out under force control in accordance with ASTM E466 or ISO 1099.

Corrosion fatigue tests in corrosive atmospheres

A unique tool was developed during the European AUTOFATCOR project, enabling fatigue cycles to be performed inside a corrosion chamber meeting most of the accelerated test standards used in the automotive and aerospace industries. Testing capacity has been expanded over the following years to reach a total of 21 independent stations that can be operated in parallel.

The rigs offer force performance ranging from 5 to 15 kN and test frequencies from 0 to 4 Hz. They make it possible to test metallic alloys or assemblies (bolted, bonded, riveted, welded, etc.). The latest developments, in collaboration with RISE, focus on 4-point bending fatigue rigs with a maximum capacity of 10 kN.

Hydrogen embrittlement tests for the automotive industry

Ultra-high-strength steels, widely used in the automotive industry, are particularly vulnerable to hydrogen embrittlement, a damage mechanism that more or less severely reduces the mechanical properties of high-strength materials (steels, nickel-based alloys, aluminium alloys, etc.). The French Corrosion Institute’s testing capabilities cover the full range of standardised approaches to characterise this risk: constant-load loading, incremental loading or pre-stressed specimens, tested in corrosive environments (atmospheric exposure or immersion) or under direct cathodic hydrogen charging. Measurement of the absorbed hydrogen concentration and determination of its activity in the material complete the analysis, providing a comprehensive view of the material’s behaviour in the face of this phenomenon. Tests can be carried out in accordance with the VDA 238-201, 238-202 and 238-203 standards, key references in the automotive sector for the qualification of high-strength formed steels.

Fatigue cracking tests in corrosive environments

The French Corrosion Institute has 9 fatigue rigs for carrying out fatigue crack growth rate characterisation tests under immersion in a corrosive environment in accordance with ASTM E647 or ISO 12108. This test consists in subjecting a notched and pre-cracked metallic specimen to a controlled cyclic load. Crack length is measured continuously (by compliance using a COD-type extensometer) as a function of the number of cycles. The propagation rate da/dN is then derived as a function of the stress intensity factor ΔK, allowing the Paris curve to be plotted. The influence of the corrosive environment is quantified by calculating the increase in cracking rate compared with tests performed in air or in a neutral environment. The critical stress amplitude leading to propagation of the pre-crack with or without a corrosive environment can also be evaluated.