Rouging of Stainless Steel in High Purity Water Systems

Just as their name implies, high purity water systems have one purpose and one purpose only: produce and transport high purity water to where it is required. That means no minerals, no particulates, no bacteria, and no other organic material should be in the water stream. When an inspection of the system results in a wipe with a red or brown smudge on it, it’s an indication to  remove  the rouge  and efforts to slow it from occurring  again should be investigated.

A high purity water system has two basic parts: the initial purification train and the distribution system. The former removes most, if not all, of any contaminates carried within the raw feed water. Once the water is purified, it is carried by the distribution system to wherever it is needed for the manufacturing process.

Because of its resistance to chemical interactions, 316L stainless steel is the most commonly used alloy in these types of system. The high level of chromium at the surface of the metal is what gives stainless steel its resistance to corrosion. Stainless steel is typically chemically treated, or passivated, to increase the ratio of chromium to iron on the surface.

All stainless steel contains chromium – at least 10.5% of the alloy, in fact.  316L averages 17.5% chromium content with the balance consisting of primarily of iron plus 11.5% nickel and less than 3% molybdenum.

Sources of Rouge

Given the critical importance of ensuring that the water going into the product is of the highest purity, these systems are routinely inspected to ensure that rouge and biofilm are not present within the water. Typically, this is the case, but it is also frequently the case that more attention is paid to the purification train, with less given to the distribution system.

While stainless steel makes up all components of high purity water systems, not all of the system’s components are subjected to the same corrosive environment. Rather, some components, such as pumps, or stills contain conditions that induce more corrosion. For that reason, the source for the iron that forms rouge is often from within the system itself rather than from an external source. This type of rouging, where the material is deposited onto the surface is referred to as Type 1 rouge and is usually heavier near the source of the iron. Cavitation or erosion of the impeller within a pump is frequently one source of such deposits. Other sources and causes can be welding defects, heat tint or improperly cleaned areas as well as combinations of oxygen, chlorides and pH within the water.

Another type of rouging, Type 2, comes from the stainless steel structure itself. A rough, improperly passivated, or compromised surface can allow the elemental iron within the stainless steel to react and form ferric iron oxide crystals.

Regardless of the type, if left uncleaned, the area affected by rouge will continue to grow. It may also become a safe harbor for bacteria and other organisms which can form a biofilm. At some point both the rouge and the biomass can slough off material into the water stream. This can then enter the product, causing discoloration, bacterial contamination or unwanted chemical and biological reactions.

Controlling Rouge

The first step in preventing rouging to occur is to have a regular inspection and maintenance plan in place. Part of that involves regularly scheduled cleaning, which can include pre-emptive derouging and passivation treatments.

Even with those preventative measures, it is still possible for conditions to occur that allow rouge to form. Excessive temperature of the water for a prolonged period can cause the formation of rouge. If there are elevated levels of CO2 in the water, this will cause the pH to drop and can destabilize the passive layer. On the other end of the spectrum, excessive concentrations of ozone will increase the corrosivity of the water, especially if chlorides are present. High concentrations of chlorides or caustic solutions can also damage the passive layer. This latter situation happens when the cation bed of an ion exchanger is improperly cleaned, or when caustic cleaners, including bleach, are used in an attempt to sanitize the system.

When bioburden or elevated iron levels are detected in the water, or by a wipe test, a professional chemical cleaning process is needed. On systems which must be serviced in place, Astro Pak technicians bring in custom-designed pump skids which is connected to a system inlet through temporary hoses and is used to perform the entire procedure. The exact duration, chemistry and other aspects of the service will be based on a sample of the rouge and from analysis of organics taken from the system, but the general procedure is largely the same.

If there is evidence of biogrowth, chemicals specific to the detected contamination are circulated through the system with the temperature being closely controlled as well. A combination of alkaline degreasers and oxidizing solutions will break up the biofilm and allow it to be flushed out.  This is followed by, appropriately enough, a high purity water rinse to remove the contaminants and the cleaning solution. Next, a combination of citric acid and other organic compounds is used remove the built-up contaminates from the surface depending upon its composition. In addition to the ferric oxides that provide the red, orange, brown or even black coloration from ferrous oxides, silica and aluminum can also be found in the rouge. The derouging chemistry, heated to prescribed temperatures, dissolves the structure of the rouge and binds the iron so that it cannot precipitate out or chemically react before it too is flushed. After another high purity water rinse, the steel’s surface is ready to be repaired and prepared by chemical passivation. Passivation builds upon the stainless steel’s natural process of forming a chemically inert, “passive” layer. It does this by removing remaining surface iron, while also deterring the migration of further iron from the base metal to the surface. Once this has been completed, the system can be returned to service in an “as-new” condition.

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Daryl Roll

Astro Pak Consultant, Daryl serves as the primary senior technical advisor for corrosion, surface chemistry and stainless steel Passivation. With over 40 years of experience in chemical processing, Daryl has been published in MICRO, UltraPure Water Journal and Chemical Engineering for his papers on passivation and rouge control. He is a participant on the ASME BPE Subcommittees for Surface Finish and Materials of Construction requirements and a leading contributor for the Rouge and Passivation Task Groups. Daryl holds a B.A. in Chemistry and Earth Science from the California State University of Fullerton and a Professional Engineer's license from the State of California.

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