Materials Blog
Letter to wise people Print E-mail
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Dear madam, Dear sir,


Since long time people and companies worldwide struggle with an anomaly that still survives in most languages. French people call it “acier inoxydable”, in German the same material is called “rostfreier Stahl” and in Dutch dictionaries “roestvrij staal” is considered to be an appropriate terminology; in English it’s called “Stainless steel”. So what’s the problem? All these expressions give the impression as if this type of steel will never rust: A major misunderstanding !

It already starts with a simple parapet in a coastal area. Several flat owners guarantee me that their parapet possesses an infinite life, just because their supplier told them that stainless steel never rusts. Yet, several of these flat owners are suddenly confronted with reality; their precious stainless steel nevertheless starts corroding. What has happened in several cases is that the parapet has been made of the basic stainless steel AISI 304. This type of material is not resistant to the chlorine rich sea climate. And when you weld this metal without precautions, the final result could be intergranular corrosion and stress corrosion cracking as well; these types of corrosion deliver nice pictures indeed, yet many worries as well. Would the use of 316L stainless steel be a solution? For many applications the failure risk would be reduced indeed, yet also 316L can fail fast and catastrophically. Using stainless steel is not just a matter of common sense, yet also knowledge on ‘what’s possible and what’s not’ and on ‘what influences what’ is a matter of life to stainless steel.

RVS_klepSo, is the use of stainless steel always a source of worry? Certainly not. Stainless steel can indeed be the miracle solution for many applications, yet for other applications a conventional steel covered with an appropriate coating could be as suitable as well. Materials selection is a matter of choosing the material “in function of the application, the way of construction, the environment to which the material will be exposed, the process conditions and the proper aftercare”. Only by taking all these aspects into account, it will be possible to determine whether the use of stainless steel is adequate and, if so, which type of stainless steel is most suitable.

The previous list of aspects ‘to be taken into account’ looks straightforward for most people, until they reach the word ‘aftercare’. An adequate aftercare of stainless steel components and constructions is of utmost importance indeed. Do you for example realise that it is not very wise to cut or grind conventional steel or copper when stainless steel is near? Also removing rust from stainless steel surfaces by means of a classic steel brush is a ‘not done’. Another, astonishing rule: never write with a pencil on stainless steel; and finally … Never believe the fairytale that stainless steel does not need passivation. Passivation of welded stainless steel is a stringent “MUST” when you want to guarantee the safe future of the stainless steel constructions in your company.

So, to finalise this plee: dear editors, please remove from your dictionaries all terminology that suggests that there exists something like a steel that never rusts. In Dutch wise people use ‘roestvast staal’, and this terminology is already in many dictionaries indeed. So take up the challenge … enrich your language.


Sincerely yours,

Dr. ir. Frans Vos

General Manager Materials Consult bvba



Green Energy through an engineer's eyes Print E-mail
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A blog originally written in Dutch for, a Dutch website on bulk handling operations.


Do you have solar panels on your roof? It’s a question that’s launched to me on every network event. In modern times ‘thinking green’ is apparently a requirement to be honoured  as a ‘cool’ person. Yet, there are many ways to be ‘green’ …

In first order I’m a firm believer of the RUE-principle. RUE stands for “Rational Use of Energy”. It invites everyone, individuals and companies, to consume as less energy as possible and to use the consumed energy in a responsible way. Lowering the thermostat by one degree. Regularly cleaning pump filters in order to prevent the pump from overload. Switching from oil- and coal-firing to … Good question indeed: to what?

No panic. I will not detail on all pro’s and contra’s of all possible alternatives. I just want to point out that any alternative will have its positive and negative influences as well. An example: Windmills transform wind energy into electrical energy, by which the wind looses part of its energy. The consequence: behind the windmill the wind has lost velocity. Therefore it’s likely that the microclimate behind a windmill park has changed in comparison to the situation when no windmills were there. And as there are lot’s of windmill parks in the world, isn’t it reasonable to state that all windmill parks together influence climate as well?

And what about biomass? Several studies have proven that firing biomass – whether or not co-combusted with coal or natural gas – has resulted in additional, sometimes expensive challenges for bulk handling and energy production. The corrosion of e.g. boiler tubes already increased considerably when low-NOx combustion was introduced, and nowadays firing biomass increases corrosion of boiler tubes even further. As to the bulk handling of the feedstock, it has been shown that several breakers had to be modified or replaced by other models in order to be able to process certain types of biomass. Thorough cleaning of most biomass is required as well, as miscellaneous fractions could hamper the processing and/or firing of the fuel. Certain types of biomass require complete drying in order to be able to process it to burnable fuel etc.  All these additional operations … they require additional energy consumption as well. Yet .. isn’t that against the RUE-principle?

Biomass_energyWhat’s important in the end is the Total ‘Cost of Ownership’. What’s the total cost in order to operate an installation with as less downtime as possible? The lower, the better. The energy bill is a considerable part of this cost. Also the bulk handlers play a vital role in reducing the cost of ownership of energy production. Bulk handlers not only have to act themselves according to the RUE-principles, yet they are also well placed to advise their clients on RUE-optimisations of their bulk handling operations. This way the client’s energy bill will reduce and the client’s gratitude will be beneficiary to the bulk handler. That’s the ultimate goal in everything we do: a WIN-WIN situation.

And no, I don’t have solar panels on my roof.


Dr. ir. Frans Vos

2011, February 14th

Creepy Creep Print E-mail
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Mankind has always been attracted to secrets of different kind, hidden places and escape tunnels, documents that are not intended for everyone’s eyes. Some people want to hush them up, some people want to make them public or even start to scandalise and others …. want to search solutions in an efficient and objective way.

Scientists and engineers are definitely of the latter kind. They refrain from controversies and media appetite that some other people use to get attention. An adequate, efficient and objective search for solutions to a diversity of challenges, that’s what real scientists and technologists strive for.

Also the technical world is confronted with several types of non-visible defects. In materials phenomena can appear that are not visible to the human eye, yet can result in extensive damage. In most cases they evolve quite slow, but continuously and without being noticed; creepy thought indeed. As to metals hydrogen embrittlement, Hyrodgen Induced Cracking (HIC), relaxation and creep are certainly the most creepy failure phenomena. These creepy degradation modes of metals will be explained in a new blog series, starting today with the creep phenomenon.

messing_gegotenIn order to understand creep, it’s first of all necessary to get acquainted with the structure of metals. The picture on the right gives e.g. an idea of how the internal structure of a copper alloy looks like. A metal consists of grains, comparable to a snowflake that is build up by many ice crystals. Grain borders are the transition zones from one grain to another. At very high temperatures, such as in a steam boiler or jet engine, and when the metal is subjected to stress, very small cavities could develop at the grain borders. Such cavities develop after a certain period of time due to local weakening of the grain border, that is subsequently disrupted over a very small area. Development of such small cavities is the start of the creep phenomenon. No serious problems are to be expected as long as creep is limited to the presence of some individual voids on the grain borders. Only when the voids start to coalesce and become a line-shaped void along the grain border, corrective actions are necessary. The right part of the picture below this blog e.g. shows the birth of creep, i.e. some individual voids, whereas on the left part some creep voids are ready to coalesce.

When creep occurs, the action that has to be taken and when it should be taken, is dependent on the type of installation, the chance that the component would rupture and the consequences of such a rupture. Measures to be taken could vary from a slight reduction in temperature, a periodic inspection of the creep evolution, a replacement of the component, up to a preventive replacement of the component prior to any creep appearance. The maintenance of jet engines requires e.g. a preventive replacement of the turbine blades, hence before any creep has occurred. As to boiler tubes in a power station a limited number of creep voids is tolerated as long as they do not coalesce etc.

What makes creep creepy is that it develops ‘in’ the metal, hence hidden for the human eye and, in its initial stages, also non-detectable by non-destructive inspection techniques (NDT). The evaluation of creep initiation and evolution requires a regular verification of the microstructure - particularly the grain boundaries - of metal components that could be susceptible to creep. For metal components that should possibly continue operation if the creep level is acceptable, microstructure verification is performed with “replica”. In a subsequent blog this technology will be explained into more detail, yet it could be compared to making a plaster cast on microscopic level. This technique allows to detect the initiation of creep in a timely manner, to follow its evolution and to decide on replacement when necessary.

Preventive replacement is an even more stringent measure: replacement is performed before any creep void can initiate. Based on calculations and experience it’s possible to determine when a part will start to creep, hence replacement can be planned ‘before’ any creep void will develop. This is obviously the most safe approach.

'Safety First’, that’s what we strive for, day in, day out; for you, together and thanks to you. And “You”, that are our suppliers, our clients and all our great supporters.

Many Thanks !



Dr. ir. Frans Vos

General Manager Materials Consult

2011, January 26th

An engraving in my brain Print E-mail
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jet_engine“Emergency landing of Airbus 380”, that news message of 2010, November 4th earned a prominent position in my memory. I empathize with the passengers and crew that had to overcome frightening moments. Yet anxiety must also have peaked in the workshops and offices of the companies that supplied to the new Airbus. Photographs pass through the world at an incredible speed. I really feel pity for the employees of the engine manufacturer. And my engineering reflex wonders: what went wrong?

My thoughts stray off to the world of thermal spraying. Many jet engine blades are covered with a so called “thermal barrier coating”, commonly also indicated as a TBC. TBC’s prevent the underlying superalloy from oxidation and other types of degradation. Did something go wrong with the TBC’s?

Thermal spraying is a superb, yet complex technology to provide surfaces with coatings that can enhance a diversity of properties. Protection from wear, corrosion and thermal effects, taking care of lubrication and fire resistance, it are just some examples of property enhancement where thermal spraying can lead to unexpected solutions. Yet, the previous are only the results. And again my thoughts stray off, this time towards my beloved student time.

Plasma what? Plasma who? A gigantic plasma spray installation in a much too small room at the university of Leuven (Belgium) is my first memory to the world of thermal spraying. First there was a theoretical project on whether and how that installation could be modified in order to deposit diamond coatings. And than, came the real work: the production of self-lubricating coatings, applicable in pumps and automotive applications. Blood, sweat and tears, that’s what that installation has brought to me for four years. Choice of base powders, production of a composite powder and than getting the composite powder “hopped”, that were my first challenges. For those who like beers: “hopped” has nothing to do with the famous base ingredient of this marvelous thirst extinguisher. It refers to two small silos that contained the powder to be sprayed and that brought me lots of sleepless nights; it seemed quite impossible to find a method to fluently pass my self made powder from the ‘hoppers’ to the spray gun. Yet, after many attempts and modifications, success came along.

After the hop issue was solved, the next problem arrived at the injection point of the powder into the plasma flame. Up till to that moment, the plasma gun was only used to spray single phased powders. Now it was confronted with double phased powders, where one phase had such a low melting point that the powder particles were readily sticking together. As soon as the composite powder - transported from the hoppers to the gun through a narrow tube - came near to the plasma flame, its stickiness blocked the injection point. Adaptation of the nozzle and the injection positions of the powder, a slight change in plasma parameters, and that problem was solved too. Yet, the next challenge appeared soon.

The first composite powders were produced by means of spray drying. Apparently the thus created composite particles were not sufficiently strong to withstand the turbulent flows within the plasma flame. Back to square one. The problem was solved by sintering the ceramic powders and mechanical alloying of the metallic powders. Again the spray parameters had to be optimized, and so the story continued.

‘Instructive’ and ‘challenging’ are the two words defining my first steps in the world of thermal spraying. Though it is complex, fantastic things can be realised by using this wonderful technology. Yet, despite all optimisations we perform and all preventive measures we take, nothing is infallible; the unavoidable interventions of mankind will always remain the weakest link in our highly technological world.

And to return to where we started. Has the failure of the Airbus 380 engine been caused by failing TBC’s? Is it possible that, despite all optimisations and precautions taken by thermal sprayers, such a failure would occur? Only detailed research can tell. I only wanted to launch an appeal, an appeal to permanent attention, to even more passion for everything we do inside our companies. No blind passion of the student that’s exploring the world, yet the passion of daily reality, a reality that will never fail to astonish mankind.


Dr. ir. Frans Vos

General Manager Materials Consult bvba


This column was originally posted in Dutch on

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