Written by:
Marina Arguimbau, Technical Coordinator at Klinea Biotech & Pharma Engineering
In the pharmaceutical industry, stainless steel is a fundamental and ubiquitous material. It is found in process facilities (such as tanks, reactors, and centrifuges), in piping for processes and associated services (purified water, gases, heating and cooling systems), and in auxiliary room elements such as stairs, furniture, and cabinets.
A bit of history
The first stainless steels appeared in the 19th century. Although they were manufactured on a small scale, they already showed corrosion resistance. The reason for this resistance, however, was not yet fully understood.
By the early 20th century, thanks to the research of Guillet and Portevin, some types of stainless steel were already known. However, it was the wars of that century that ultimately gave a significant boost to the technological development of these materials. The First World War thus led to the simultaneous development of stainless steel in England and Germany.
In England, while studying how to improve the protection of gun barrels, metallurgist Brearley discovered that adding chromium to low-carbon steels increased their resistance to oxidation.
In Germany, meanwhile, doctors Maurer and Strauss patented two groups of low-carbon chromium-nickel stainless steels. Of these, one in particular, AISI 304 steel, has stood the test of time and become one of the most widely used materials thanks to its multiple applications.
The properties and compositions of these steels were kept secret during the First World War. Once the war ended, their use quickly spread to the manufacture of kitchen knives, tanks for transporting dairy products, sinks, and other items.
Main alloying elements in stainless steel
The main alloying elements in stainless steel are carbon (C), chromium (Cr) and nickel (Ni). Strictly speaking, corrosion protection is provided by chromium oxide.
Carbon has a limited content in stainless steels, especially in austenitic steels, as it tends to form chromium carbides. It is crucial that chromium forms chromium oxide to protect against corrosion, and not carbides.
Nickel provides stability at room temperature to austenitic steels.
Austenitic stainless steel surface after final polishing.
Source: https://www.struers.com/es-ES/Knowledge/Materials/Stainless-Steel#grinding
In addition, other elements are added to the alloy to improve its properties:
- Molybdenum (Mo): Improves corrosion resistance in chlorinated environments.
- Manganese (Mn) and nitrogen (N): Added as substitutes for nickel, acting as austenite-forming elements and, in the case of nitrogen, to increase mechanical strength.
- • Titanium (Ti), zirconium (Zr) and niobium (Nb): Prevent the precipitation of chromium carbides after heat treatment.
One of the most widely followed standards in the construction of pure fluid process and service systems is the ASME BPE Standard. This standard specifies the composition of AISI 316L stainless steel (1.4404 and 1.4435) as follows:
| Acero | C | Mn | N | Cr | Ni | Mo |
| 1.4404 | 0,03 | 2,0 | 0,1 | 16, 5 – 18, 5 | 10, 0 – 14, 5 | 2, 0 – 2, 5 |
| 1.4435 | 0,03 | 2,0 | 0,1 | 17, 0 – 19, 0 | 12, 5 – 15, 0 | 2, 5 – 3, 0 |
As shown in the table, the sulphur (S) content is not specified in the composition of AISI 316L steel.
It is also important to note that sulphur significantly improves the machinability of steel, but is detrimental to resilience and corrosion resistance.
Influence of sulphur content on welds
The sulphur content in the alloy must be kept within maximum and minimum values. During the development of ultra-pure steels, the extreme reduction in sulphur content led to weldability problems due to a lack of wetting.
Excess sulphur reduces corrosion resistance and worsens the quality of the weld bead. However, too low a sulphur content also reduces weldability. In fact, low sulphur content causes a change in the weld pool, as the liquid metal spreads more but penetrates less, resulting in weaker welds.
On the other hand, moderate sulphur content (0.03% to 0.05%) can promote the formation of microcracks during weld cooling. In addition, it is crucial to control the shielding gas, as insufficient supply can form sulphur dioxide that becomes trapped in the weld pool in the form of pores.
Special attention should be paid to welding components with significantly different sulphur contents, as this will cause the arc to deviate towards the part with the lower sulphur content.
In this regard, as has been demonstrated, it is essential to seek a balance in sulphur content. Orbital welding is very important because it allows precise control of welding parameters to achieve a uniform weld bead without sinking.
Sulphur and corrosion
Sulphur content plays a subtle but important role in the corrosion of stainless steels.
A high sulphur content will promote the formation of inclusions and microcracks at grain boundaries. These areas can be points of corrosion attack if the surface has not been properly passivated or if it is exposed to aggressive media. In addition, manganese-sulphur (MnS) inclusions are preferential sites for the initiation of pitting corrosion in stainless steel.
When working with fluids containing chlorides, sulphides dissolve and destabilise the passive layer. A higher sulphur content means a higher amount of MnS, which makes the weld bead area more susceptible to pitting.
Influence of sulphur on installation
Clean, smooth surfaces are essential in the pharmaceutical industry. Rough areas and cracks can promote microbial growth and the accumulation of product residues and cleaning materials. The less rough the surfaces are, the easier it is to carry out CIP-SIP (Cleaning In Place and Sterilisation In Place) processes. Therefore, the quality of welds is particularly important.
The sulphur content affects the quality of the weld and its resistance to corrosion. A low sulphur content can cause a sunken weld bead or small cracks, promoting product accumulation and biofilm formation in the cavities.
On the other hand, an increase in sulphur can cause pores or microcracks, which are not always visible immediately after welding, but are revealed after further treatment such as electropolishing or pickling. MnS inclusions are dissolved by electropolishing, leaving small depressions on the surface.
All these defects increase roughness, make cleaning difficult and promote the proliferation of microorganisms. For these reasons, the industry has developed steels with limited sulphur content. Thus, the ASME BPE Standard, in its section MM-5.2.1.1 Austenitic Stainless Steel, indicates that the sulphur content must be between 0.005% and 0.017% by weight for welds without filler material.
Conclusion
Although sulphur content is not directly evaluated in an FDA or GMP inspection, weld quality is evaluated, looking for homogeneous welds without colour changes. Materials with controlled sulphur content promote weld reproducibility.
Therefore, special attention should be paid to the sulphur content of components during the material procurement process. Likewise, during the execution of the work, it is crucial to avoid welded joints between components with varying sulphur content.
Other factors influencing weld quality
There are other crucial factors that affect weld quality:
- Purging gas control and quality:
- Excess purging gas can cause uneven weld penetration along the entire bead.
- The absence of purge gas can generate sulphur oxides during welding, which escape in the form of gases, causing pores.
- Poor-quality purge gas can lead to areas of lack of fusion and oxides (both internal and external).
- When welding is performed in an environment with high relative humidity, irregular weld beads, areas of lack of fusion and reddish oxide layers appear.
- The loss of purge gas in the outer area of the weld causes the weld bead to sink in some areas.
- Welding machine configuration parameters:
- A high intensity value can cause loss of fusion in the bead or excessive penetration.
- If the speed is not correct, bead overlap may occur.
- Preparation of the elements to be welded:
- The preparation of the components to be welded has a direct impact on the quality of the weld. A mechanically deformed or bulging element will result in misalignments and lack of weld penetration.
- It is also important to chamfer the edges of the elements to be welded and to ensure that there is no grease, oil or metal contaminants.
Source: AWS D18.2 (1999) Heat tint levels on the inside of welded 316L austenitic stainless Steel tube
Source: ASME BPE-2019. All rights reserved by the American Society of Mechanical Engineers.
Source: ASME BPE-2019. All rights reserved by the American Society of Mechanical Engineers.
So what about plastic?
Until now, it seems that stainless steel only causes complications and requires a great deal of attention during installation. So why not opt for plastic materials?
Although there are materials such as polyvinylidene fluoride (PVDF) on the market with acceptable characteristics for pharmaceutical installations, such as:
- Suitable internal roughness
- Traceability in terms of certificates
- Biocompatibility
- Acceptance by standards such as ASME BPE
- Manufacture in clean room conditions
It is not widely used in the pharmaceutical industry due to certain limitations. For example, the number of sterilisation cycles it can undergo is limited. This rules out its use in certain cases, such as water loops for injectables maintained at 80 °C and sterilised periodically.
Furthermore, it is manufactured to order, which means that there is no stock stored as in the case of stainless steel components. This implies that the design and layout of an installation of this type must be determined well in advance of construction, which limits the possible modifications during construction. However, there are other industries where high-quality water is required and where PVDF piping is acceptable and used.
Acknowledgements:
- Anja Quattelbaum, Neumo Ehrenberg Group
- Jaume Urpí, Neumo Egmo Spain
- Santiago Fernandez, BWT Pharma & Biotech Ibérica
Fuentes:
- CESOL- Tema 2.15 Aceros Inoxidables, IWE Módulo 2. Asociación Española de Soldadura y Tecnologías de Unión.
- Propiedades y Soldabilidad de los Aceros Inoxidables- Angela Lazaro Martin.
- Art.: “Defects produced in orbital welding for pharmaceutical Process piping: case study and simulation”, por Jorge Domingo y Margarita Morquillas. Pharmaceutical Engineering, September-October 2016.