Using carbon steel wire brushes or grinding wheels contaminated with carbon steel will result in rust. The mechanism for the red rust formation is simple:.
Rust can cause crevice corrosion or pitting of the stainless steel under the red oxide, therefore it must be removed. This is why passivation is necessary, not only to increase the chromium to iron ratio on the surface, but also to remove any iron contamination.
Both treated and untreated waters can rouge, even softened water. The culprit is what is in the water, primarily ferrous bicarbonate. Softening does not remove anions like carbonate, bicarbonate, sulfates, chlorides, etc. Unlike ferric carbonate, ferrous bicarbonate is completely soluble, but is easily oxidized to ferric carbonate.
Ferric carbonate is insoluble and reddish brown in color. It can be dissolved in strong acids. Treated or potable drinking water normally is clarified to remove suspended solids, filtered to remove fines and disinfected with chlorine or chlorine dioxide to destroy most bacteria. This process has little or no effect on the bicarbonate ion as long as it is in equilibrium with the carbon steel piping and the oxygen content is low. Once the water is in an inert environment, like stainless steel or porcelain, the bicarbonate begins to oxidize:.
Ferric oxide, Fe 2 O 3. H 2 O , is red and when it occurs in nature it is called hematite. In untreated water the chemical reaction is similar, except no chlorine is present and oxygen, dissolved in the water , is the active agent:.
Ferric carbonate will precipitate and the ferrous hydroxide forms a gelatinous compound that precipitates as ferric oxide. There is a slight difference in color because the ferrous hydroxide is yellow. In large tanks the reddest deposits are usually at the top and decrease toward the bottom. It is not unusual for the bottom of a large tank to be relatively clean.
Pure water and high purity water are typically used in industries where impurities can have a detrimental effect, such as pharmaceutical or semiconductor manufacturing. In the pharmaceutical industry it is called water for injection WFI. Typical treatments include filtration, softening, anion and cation ion exchange, reverse osmosis, ultraviolet and occasionally ozonation. Distillation may be used as final purification. The result is water with extremely low conductivity.
Type L stainless steel is the usual material of construction. Some of these systems remain clean, but others begin to rouge. In the presence of hot high purity steam these systems turn black, sometime glossy black, sometimes powdery black.
Sections of rouged stainless steel piping were obtained from a number of different pure water and steam systems. The SEM allows visual examination of the surface, EDS allows spot analyses of surface anomalies, and XPS allows layer-by-layer analyses of the rouge deposits and identification of the molecular species.
Comprehensive reports of the findings are given elsewhere 1,2. Class I rouge comes from an external source. Rouge particles are deposited on stainless steel surfaces, and in the early stages of deposition can be easily wiped clean. Surface composition of the stainless steel passive layer under the rouge is unchanged from that of the originally installed system. The rouge particles usually have the same composition as the material from which the particles came, certainly not that of corroding stainless steel.
Rouge concentration is heaviest near the source and decreases with distance from the source. Color of the rouge may change with distance from the source, being orange to red-orange near the source and changing to magenta some distance away.
When rouge is observed, the first worries often are that the system is degrading by corrosion or that the particles may have consequences on product purity. This situation has given us several opportunities to study rouging as part of field-inspections in pharmaceutical plants all over the world. Moreover, we have coordinated several projects on rouging involving laboratory tests and knowledge exchange 1,2. The intention of our paper is to present several cases of rouging and correlate the observations with published literature and experimental data.
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Article Navigation. This Site. Google Scholar. Published: March 11 NACE International. You can access this article if you purchase or spend a download. Sign in Don't already have an account? An example of chloride related rouging can be found in common denaturing agents and cleaners like guanidine hydrochloride sometimes referred to as guanidinium chloride. Guanidine hydrochloride is a powerful protein denaturing agent.
It works by disrupting the hydrogen bonding network in water which, in turn, will change the stability of the native state of other molecules that are in the water. In the biopharmaceutical world, it is often used as a cleaning agent in high concentrations. Concentrations of 6M or higher will cause most proteins to lose their ordered structure and turn into randomly coiled molecules. Unfortunately guanidine hydrochloride also corrodes stainless steel flow paths which can lead to pitting and rouging and promote areas susceptible to carryover and cross contamination in analytical and chemical process applications.
To test the effectiveness of Dursan as a rouging prevention option we compared coated and uncoated L stainless steel coupons.
When exposed to 6M guanidine hydrochloride, L stainless steel showed rouging while Dursan coated coupons were unaffected. You can test the performance of Dursan yourself by ordering free Dursan coated test coupons.
The coupons were evaluated at 1-week and 1-month intervals. Evaluations included:. Corrosion can be quantitatively calculated by the change in mass before and after exposure to the corrosive media.
In this study it was found that there was no mass change both after 1 week and 1 month of exposure for coated and uncoated coupons. Read more about how SilcoTek coatings can improve corrosion resistance in analytical flow paths. ASTM G31 immersion testing below shows that Dursan significantly reduces corrosion and rouging in common applications found in analytical testing.
After immersion in concentrated bleach, the Dursan coupon reduced corrosion by more than an order of magnitude. Visually, there was a difference between the coated and the uncoated coupons at the 1-week mark of the experiment as shown in Figure 1. The uncoated coupon on the left shows that there are pockets across the surface that are starting to show oxidation, rust, or rouging.
Figure 1 : A bare stainless steel coupon left and a Dursan coated stainless steel coupon right were exposed to guanidine hydrochloride for one week. The bare steel shows minor rusting across all faces of the coupon where the Dursan coated coupon appears to be unaffected. Since there was no loss of mass for the coupons after the 1-week trial scale accuracy is 0.
Again, there was no change in the mass of the coupons, but the minor rusting and rouging effects were much more dramatic as seen in Figure 2. Figure 2 : A bare stainless steel coupon left and a Dursan coated stainless steel coupon right were exposed to guanidine hydrochloride for one month. The rusting on the bare coupon is more severe than the 1-week exposure. As seen previously, the Dursan coated coupon is unaffected by the exposure.
In addition to the increased rouging , there was also a discoloration in the guanidine hydrochloride solution with the uncoated coupon immersed in it for one month. This color change, seen in Figure 3, was not seen at the one-week point of the experiment. The yellow tint to the liquid indicates that iron has leached out of the coupon and into the solution.
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