Understanding The Differences Between Derouging And Passivation

Stainless steel is the material of choice for a lot of industrial equipment, due to its durability and strong resistance to corrosion. However, to maintain these properties and sustain surface quality, many stainless steel applications require regular derouging or passivation. Both processes have overlapping goals – to improve surface quality, but each serves a distinct purpose that is critical for sustaining the optimal performance of stainless steel equipment. In this article, we’ll look at the main differences between derouging and passivation.

Purpose and Process:

Derouging Stainless Steel

Derouging stainless steel involves removing the iron oxide deposits (rouge, or rust) that can form on the surface of stainless steel. These deposits often arise from oxidation processes where stainless steel equipment is exposed to high temperatures, acidic conditions, or other reactive substances. Rouge can be detrimental to stainless steel structures, leading to contamination and reducing efficiency.

There are two ways to remove these iron oxide deposits.

1. 1. Mechanical derouging employs physical cleaning methods such as brushing, scrubbing, abrasive blasting, or electro-chemical cleaning to remove rouge deposits. This method is sometimes used for heavily contaminated services, but can be labour-intensive and also risks damaging delicate surfaces.

   2. Chemical derouging uses (mostly) acidic compounds, such as citric acid, nitric acid, or purpose-designed derouging agents to dissolve surface-based iron oxides (some specialised non-acidic treatments are also available). This is a more thorough and cost-effective approach, with the ability to clean intricate and delicate services without causing scratches or damage.

What Is Passivation Of Stainless Steel?

Passivation treats stainless steel surfaces to encourage the development of a protective oxide layer to prevent corrosion and enhance durability. The treatment removes free iron from the surface and allows the spontaneous formation of a stable and inert oxide layer. This protective film reduces the further development of rouge.

In terms of process, the stainless steel is usually exposed to an acid, which dissolves free iron on the surface and leaves behind a chromium-rich layer. This chromium layer reacts with atmospheric oxygen to form a thin, or ‘passive’ oxide film that imparts superior corrosion resistance to the metal. Several steps are often involved in passivation, including precleaning and acid treatment, followed by thorough rinsing to remove residual acids.

Derouging and Passivation: Cure and Prevention

The main difference between derouging and passivation is, therefore, one of cure and prevention, respectively. Derouging is an ‘active’ process that removes tightly adhered iron oxides, and passivation is a ‘passive’ treatment that that removes free (or loose) iron, leaving the surface passive and significantly reduces the formation of rouge. Derouging cleans your equipment, removes surface impurities, and makes it easier to maintain strict hygiene standards, while passivation helps keep your equipment rust free and maintains its longevity.

Applications

Derouging and passivation often go hand-in-hand as a treatment for stainless steel equipment, and are widely used in the pharmaceutical, food and beverage processing, brewing, biotechnology, and electronics manufacturing sectors, where stainless steel equipment has to demonstrate strict purity and contamination control standards.

Safety and Maintenance

Historically, one of the downsides of derouging is that the strong acids traditionally used in the process can etch the steel surface and reduce the lifespan of equipment. Derouging with state-of-the-art pH-neutral products, on the other hand, avoids damage to delicate stainless steel surfaces, and thereby helps minimise maintenance and operational costs.

Find Out More

To find out more about derouging and passivation and how it can support your operation, please contact our team for expert advice today.