Analysing steel corrosion
Thermal analysis can play a role in investigating corrosion of steel, argue Dr Gabriele Kaiser, Dr Alexander Schindler, Dr Ekkehard Post and Peter Davies from Netzsch-Gerätebau.
Corrosion is a natural phenomenon that occurs everywhere. A well-known example is the formation of rust caused by iron oxidation in the presence of humidity. The World Corrosion Organisation (WCO) estimates the global annual cost of this (electro)chemical process to be more than 3% of the world’s GDP, which corresponds to more than US$2.2tln each year. However, corrosion is not only a reason for significant economic losses – it can also endanger public safety and the environment, making it important to study corrosion mechanisms.
The term corrosion describes the deterioration of the properties of a material – usually a metal – due to its interaction with the environment. Many structural alloys corrode just by being exposed to air, with its natural humidity content, but the reaction can be strongly affected by more aggressive media, such as acids, bases and halogens, by microorganisms or by high temperatures. Controlled corrosion treatments such as passivation have a positive effect on the material´s resistance. For instance, stainless steel does not stain, corrode or rust as easily as ordinary carbon or alloy steel.
Stainless steel typically contains (along with varying amounts of carbon, silicon and manganese) a minimum of 10.5 weight % of chromium, resulting in a passive layer of chromium oxide along the grain boundaries and on the surface, which prevents further corrosion in oxidising atmospheres. Additional elements, such as nickel or molybdenum, may be added to enhance formability or to create an even more improved corrosion resistance. In general, the steel properties depend strongly on the material´s composition.
Determining mass change
Thermogravimetric analysis (TGA) under defined temperature, gas and humidity conditions is a helpful tool to simulate and investigate corrosion processes. According to standards DIN 51005 (Germany) or ASTM E473 (USA), TGA is a technique in which the mass of a substance is monitored as a function of temperature or time, while the substance is subjected to a controlled temperature programme in a specified atmosphere. In other words, it is a technique to measure if, and by how much, the mass of a material changes during heating, cooling or an isothermal dwell time at elevated temperature. Contrary to thermal treatments accompanied with stepwise measurements of changes in weight, TGA allows for a continuous determination of the mass change.
The instrument using this technique is called a thermobalance. Various balance principles are conceivable – a classical hang-down system (also named bottom-loading), a horizontal arrangement of the balance beams or – today’s most-used principle in everyday life – a top-loading system, where the sample is positioned above the balance.
An investigation of the corrosion behaviour under humid atmospheres can be realised with special water vapour furnaces for the TGA system. The water vapour furnace, chosen for the following examples, allows for water vapour concentrations up to 100% in the sample chamber. A humidity generator or a water vapour generator, depending on the required level, generates the water vapour itself.
As corrosion is the result of interaction between the specimen surface and the surrounding atmosphere, it is important to maximise the gas access for the highest corrosion rates. In the case of metal sheets or bulky samples, which can be hung, a sample holder for suspended samples (see Figure 2, left and middle position) is a good choice. Alternatively, slip-on plates (solid or net-shaped) on which the test pieces are deposited can be used.
Long-term corrosion studies
An experiment on ordinary steel was carried out in a humid air atmosphere with a water vapour proportion of almost 50% (total purge gas flow – 160 ml/min). The sample was placed on a TGA sample carrier with an Al2O3 slip-on plate, heated to 800°C and kept isothermal for 24 hours. During this time, the sample mass increased by a total of 1.31% or around 4mg, respectively. Of this, 0.21% is attributed to the heating phase and 1.10% to the isothermal segment.
After 30 minutes (while still heated at about 360°C), the sample mass started to increase due to oxidation (corrosion) of the steel. This process continues during the dwell time and seems to be not yet completed at the end of the measurement. Rust (a mixture of iron(II) oxide, iron(III) oxide and water) is porous and, therefore, does not protect the surface from further reaction.
A similar test run was conducted on a sample-designated stainless steel. This time, the sample mass was about 500mg and the isothermal temperature was 900°C. Despite the higher temperature and the longer duration of the measurement (48 hours isothermal), the total mass increase was less than in previous test – it amounted to just 0.5% (or 2.5mg). The graphic, however, again exhibits a continuous oxidation.
In contrast, the sample designated iron-chromium alloy in the third test showed a mass gain of only 0.07% (corresponding to around 0.8mg) in an atmosphere containing 50% air and 50% water vapour. The TGA curve reached a plateau after around 3.5 hours. This behaviour indicates that this alloy is only oxidised at the surface.
TGA is a convenient and easily applied method for investigating the corrosion behaviour of metals and alloys under different atmospheric conditions. Using a stable thermogravimetric balance system with low-drift behaviour, combined with a water vapour furnace and water vapour generator, measurements under different concentrations of water vapour can be performed. Differences in the corrosion behaviour can therefore be studied not only in terms of humidity, but also in terms of composition of the metal or alloy.