The Accelerated Weathering Laboratory were pleased to have been approached by a local college to provide an idiots guide to basic corrosion mechanisms, their causes and how to stop it or slow it down to manageable levels when using cheap materials like low carbon steel. The questions were also asked ‘What is Accelerated Weathering?’, and curiously ‘Do we still need Metallurgists?
The audience was to be primarily aimed at BTEC, HTEC students and undergraduates studying Materials Technology as a part of their Mechanical Engineering courses.
The problem for me is that corrosion is such a complex subject. It would be easy for me to send a student to sleep within the first few minutes if I started rattling on about the ‘Galvanic Series’ or ‘ Electrochemical reactions’……………….……so I won’t!
I did an interview back in the day for Granada TV (remember them?) when they were making a programme about the Forth Railway Bridge in Scotland and why it has survived the ravages of the weather for almost 130 years in a very exposed and rainy part of the world despite being made of steel?
Take a peek at my draft script for the interview below. It’s about as basic as it comes and particularly useful at 3am when you can’t sleep. So make yourself a hot milk, turn down the lights and read on……………….…..Night night!
Q. Why does metal corrode
A. Moisture and air. The interaction of hydrogen (from the moisture) and oxygen (from the air) starts a reaction at the surface of the steel known as the galvanic cathodic reaction.This produces a by-product called Iron Hydroxide which we all know as Red Rust.
The red rust formed is non-protective since its volume is greater than the metal from which it is formed, so it is full of holes and cracks which allow more moisture and more air to reach the fresh metal below so further corrosion takes place.
Stop the water (held in the air, rain and dew) or stop the oxygen (from the air) – and the anodic / cathodic reaction cannot take place and the corrosion will stop. Easier said than done!
Q. Why is corrosion a problem for bridges?
A. Primarily due to extreme exposure to the elements, but also due to their cyclic loading. Whether it is a footbridge, and road or railway bridge traffic will produce cyclic loading stresses within the structure. Corrosion will often form pits. These pits can act as initiation points for severe concerns such as fatigue cracking. Combine fatigue cracking and the corrosion, the structure is doomed to failure.
Q. What sort of things cause corrosion?
A. We have already mentioned that moisture and air are required. We must remember that moisture is present in air. Air can also carry any number of contaminants, namely salt (if your structure is near to a coastal area) or industrial contaminants like sulphur, nitrates and chlorine.
These air-bourne contaminants speed up that galvanic reaction which will increase the corrosion rate and significantly and reduce the service life of the structure or component.
Q. What do engineers do to stop corrosion?
A. In plain carbon steels, they need to do stop the moisture and/or the air from coming into contact with the exposed steel.
In other words, they need to stifle that galvanic reaction. A popular way to do this is to provide a barrier coating over the surface of the steel. These can be simple or complex paint systems, or more robust coatings such as plating or galvanising.
Another way to prevent corrosion would be to play around with the chemical composition of the steel.
For example, if the chromium content is raised typically around 13 to 18% (along with other elemental changes to maintain strength and ductility) a Cr rich oxide layer is naturally formed when exposed to the oxygen in the air.
This provides its own natural barrier coating protecting the steel from tarnishing and further corrosion – hence their term Stainless Steels.
The downside is, these materials are more expensive than plain carbon steels and are sometimes difficult to fabricate.
Q. Why is it important to understand metal and what it does over time?
A. Over time structures made from metal experience a wide variety of stresses, some of which are quite complex. They can include tension (pulling), compression (swashing), torsion (twisting), or shear (cutting).
These combined stresses can produce changes in the microstructure which can reduce its service life.
When choosing a material for a given application, the engineer and metallurgist must fully understand what stresses are involved and work within the limits of the material which will include a significant safety factor.
Q. What does Galvanising mean?
A. Galvanising means ‘dipping steel into molten Zinc’ to provide a barrier coating over the surface of the steel.
Q. What does Galvanizing do to the steel?
A. Galvanising does 2 things to the steel:
- Provides that barrier coating protecting the underlying steel from moisture and air.
- Provides a ‘sacrificial’ coating to the steel. At first, this sounds alarming, but the corrosion products of Zinc actually provide extra corrosion protection. This is how it works. Zinc (the primary component of galvanising) has a lower oxidation potential than Iron – i.e. it will corrode quicker than steel, but at a much slower rate protecting the underlying steel as the zinc corrosion cycle continues.
Q. What is a sacrificial coating?
A. Being a reactive metal, zinc readily forms a thin protective coating of Zinc Oxide when exposed to air. When exposed to moisture that Zinc oxide layer reacts to form Zinc Hydroxide.
Upon evaporation or drying of the moisture, the Zinc Hydroxide reacts with Carbon Dioxide in the air to form Zinc Carbonate on the surface of the Zinc layer.
These complex layers provide excellent protection to the surface of the Zinc, and in addition will bridge any scratches or damage to the zinc layer that may have exposed the steel.
Q. The Forth Bridge isn’t galvanised. How come it has lasted so long?
A. The Forth Railway Bridge has been operational since 1890. The reason it has lasted so long is because it has been continuously repainted so providing a barrier coating to the underlying steel. Due to the local climate, there are only 90 days available for painting. This high maintenance schedule has not been neglected over the years which explains its conservation up to the present day. I would imagine almost 50% of the total maintenance budget is consumed by the painting costs.
There is a saying that ‘You will never finish painting the Forth Bridge’ because as soon as you finish, it is time to start all over again!!
Q. Do we really need Metallurgist’s
A. The short answer is a definite yes. Metallurgy was often thought as an old fashioned subject no longer required by the modern engineering sector. Back in the 1980s, many UK companies replaced the Works Metallurgist with new shinny graduate design engineers that were skilful in the design aspect but sadly lacking in the understanding of metals and their dynamics on a molecular level. In my own work experience these so called ‘new wave’ design engineers were so wet behind the ears they couldn’t wear glasses! Sorry about that, I couldn’t resist! This loss of this skill set resulted in many failures in the field. Lessons have been learned and these days design engineers work closely with the Metallurgist at every level to provide greater confidence in quality assurance.
Q. Why do we use Accelerated Weathering Tests?
A. We are trying to mimic the effects of real time exposure tests that sometimes take years to complete. We achieve this by subjecting a part or test panel to aggressive environments in order to accelerate the natural corrosion or degradation process over a much shorter length of time for practical purposes. It’s important to understand that no test can guarantee the useful life of coatings, and there is no real-time correlation due to all the variables associated with product placement and natural weathering but Accelerated Weathering Tests can help us predict how long a coating will last in its service environment.
Q. What do these tests show you?
A. The tests can show us whether a coating is able to withstand certain environmental conditions or test criteria and still maintain their mechanical properties and performance requirements after the required exposure period.
These conditions can be a combination of extreme humidity, either wet or dry, salt fog and other complex electrolytes at various concentrations and temperatures.
We can also accelerate the effects of sunlight damage using special lamps to simulate either full or part of the sunlight spectrum. Degradation of materials due to sunlight is of major concern on its own, but can also accelerate the corrosivity of any material particularly paint systems and polymers.
‘Prefer counting sheep!’………………………………………………………..Zzzzzzzzzzzzzzzzzzzzzzz