Rouging in pharmaceutical Water and Pure Steam Systems: Causes, Effects and Control Measures

In stainless steel pharmaceutical production facilities - particularly in water and steam systems - reddish-brown to black deposits occasionally appear on internal walls. These discolorations are referred to as rouge or rouging. Purified Water (PW) and Water for Injection (WFI) systems, as well as steam systems, are typically affected. Rouging also occurs in systems made of high-quality materials such as 316L. The causes are oxidation processes or changes in the passive layer, in which iron components from the stainless steel react to form iron oxides/hydroxides and deposit on the surface.

In practice, rouging typically manifests itself in water systems as a rub-off, reddish deposit. In hotter steam systems, however, dark to black, firmly adhering deposits are more commonly found (often described as 'blacking'). A common classification distinguishes between three classes: Class I as a wipeable, deposited corrosion deposit (without altering the underlying surface), Class II as an adherent corrosion product associated with insufficiently passivated surfaces, and Class III as a blue-black corrosion product (so-called magnetite), which typically forms in hot steam systems.

Why does rouging occur - and why, in particular, in hot systems?

The key factor is temperature: the hotter a system is operated or thermally sanitised, the more pronounced the rouging typically becomes. This is consistent with practical observations that rouging is virtually impossible to prevent entirely in systems stored at high temperatures or subjected to cyclic heating. At the same time, however, temperature control in water systems is a deliberate measure used to limit microbial growth. The reason for the increased formation of rouging at higher temperatures is the shift in the water equilibrium (autoprotolysis) towards the ionic side with H₃O⁺ and OH^- ions at higher temperatures, whereby the hydronium ions represent the actual aggressive agent.

In addition to temperature, the condition of the material plays a role. Technical articles indicate that rouging is linked to a destabilisation of the passive layer and that measures such as passivation and electropolishing can reduce the risk or delay its onset, without being able to prevent rouging entirely. Operating conditions such as flow/standstill and system operation can also influence the rate of rouging formation.

Is rouging 'merely visual' - or a quality risk?

Rouging is often first noticed visually and does not necessarily result in abnormal routine test parameters such as conductivity. Rouging can become a concern if particles are introduced into the system and thus potentially into the product, or if even very small amounts of metal ions can affect a product's stability. For this reason, rouging is regularly inspected in practice, assessed as part of maintenance and addressed on a risk-based basis (e.g. with particular attention in biotechnological applications involving particle-sensitive steps).

Levers for action in practice - what can be influenced?

The greatest scope for influence remains the operating and sanitisation temperature. Several sources discuss that thermal sanitisation does not necessarily have to be 'historically' set at 80 °C, but that - depending on design, 'cold spots' and validated efficacy - lower temperature ranges may be common and sufficient (frequently cited: temperatures above 65 °C). However, a reduction in temperature must be carefully planned and validated so as not to "trade off" rouging against microbiological risks (e.g. biofilm in colder areas). Other influencing factors include the condition of the passive layer (passivation/electropolishing); furthermore, an overlay with nitrogen in tanks can exacerbate rouging. Ozonisation, on the other hand, can counteract rouging. The reason for this is the oxidising effect of the oxygen, which contributes to the formation and maintenance of the passive layer.

In practice, rouging is removed using de-rouging processes; however, experience shows that it reoccurs after a certain period of time. What is crucial, therefore, is not so much a 'one-off' removal, but rather a lifecycle approach: monitoring, risk analysis, targeted measures and - if necessary - derouging plus restoration of a suitable surface condition (e.g. passivation). However, no chemical passivation processes are required to passivate stainless steel water distribution systems. If the stainless steel surfaces are clean, passivation is achieved within a few minutes due to the oxygen content of the water medium.

Conclusion: Rouging cannot generally be completely prevented in hot pharmaceutical water and pure steam systems. The most effective lever is temperature (operation/sanitisation) - though this is always in tension with microbiological control. Passivation and electropolishing can reduce the risk, but they are no substitute for risk-based monitoring and a robust maintenance/sanitisation concept.