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January 22 - January 26, 2018

What can be done to prevent rust bleed in crevices when applying a zinc/epoxy/urethane system?

Selected Answers

From Regis Doucette of Durable Solutions on January 26, 2018:
How do you prevent the “symptoms” of a problem from appearing, is what this question asks. Dealing only with symptoms seldom fixes real problems. This myopic approach to problem solving has many parallels, three of which are the emperor wears no clothes, not looking at a deep and “structure critical” wound on one’s own human body, and my favorite “parking lot search for a key,” which I will reference in the last sentence below. Placing 2 more coats of high quality coating or caulking on top of bleeding crevice-corroded connections where 3 coats of high quality expensive material did not work (hence the question), without changing something, does not make sense. Albert Einstein said that “the definition of insanity is doing the same thing over and over again and expecting a different end result.” Placing any coating without the correct and adequate surface preparation will assure the same problem developing again as it only hides from visual inspection or appreciation that there are details needing proper remediation. Rust mitigation chemistry on a structure critical corrosion problem would necessitate altering that acidic environment in the crevice-corroded joints, connections, and corrosion-frozen bearings. Chemically active corrosion cells when covered up have been found to actually increase the corrosion rate by 50%, so the very act of hiding a problem and not addressing it correctly INSTEAD INCREASES real world harm that victimizes both owners and users of those assets. This incomplete addressing of a problem or “other agenda” opens our industry to perceptions similar to churning done by certain financial advisors to generate commissions on financial transactions. In that industry the true needs of the customers are ignored while generating income for those doing the churning. Some enlightened professionals in our protective coatings industry have chosen to embrace “performance specifications” that directly change the focus from vanity or aesthetic painting of structures to managing the root cause of crevice corrosion and maintenance of joints, connections, and bearings. Private industry, motivated by accountability and profitability, has developed many specifications that include ion-specific assessment and near non-detectable thresholds of soluble salts, including non-visible sulfate and nitrate contaminants. To “manage” non-visible contaminants with visual treatment or aesthetically focused methods is at odds with and contradictory to a true corrosion abatement program. As we speak, a major bridge in the Northeast is half finished with major bleed-through on the finished half, which shows the owners/users of that asset are not being properly served. Pictures tell a thousand words and I can share those photos with interested parties. So the symptom is being addressed wrongly as the cause of the sickness goes unchecked. The final parallel illustrated by insistence on covering up a problem and not managing it, concerns the situation where two pals in a dimly lit parking lot are searching for a dropped key. One person, the owner of that key, looks where the key dropped and the other, along for the ride, goes to the other side of the parking structure with the justification that the light is better over there so he can see better. Clearly, we have to stop circumventing the proper answer to finding the “key” here—stop the chemically active corrosion cells with a chemically active solution that can make the corrosion cell chemically neutral, and stop pretending the corrosion has been mitigated.

From Harry Peters of CHLOR RID International, Inc. on January 23, 2018:
Certainly cannot state it much better than the way Wayne Senick has explained the issue. Oxygen deprived and occluded areas such as crevices result in the promotion of hydrolysis, or dissolution of metal to an acidic salt and promotes further pitting and metal loss. The corrosion cell is a dynamic location with multiple reactions occurring as the iron salt is converted to an iron oxide in an oxygen-rich environment. Yet, in stagnant or oxygen-deprived areas (crevices or corrosion cells covered over by a coating, for example), salt-induced corrosion can actually have much greater effect on metal loss because the balancing reaction to iron oxides is reduced. The root cause is the presence of a corrosion-inducing species such as a chloride, sulfate, nitrate or other soluble salt forming anionic species. Though soluble, these salts are not removed satisfactorily with either abrasive and/or water. The use of an acidic soluble salt remover has proven, with one pressure wash, to react with the corrosion-inducing salt species and remove the salt to extremely low or non-detectable levels. Once the salt is removed, the cell or crevice is deprived of the anionic salt species and the metal will react with any oxygen available to create a more stable oxide form.

From Anthony Koontz of SherwinWilliams on January 23, 2018:
Apply a 100% solids epoxy penetrating primer followed by the intermediate and finish coats. Lastly, apply a bead of clear moisture-cured caulking.

From Wayne Senick of Termarust Technologies on January 22, 2018:
Rust bleed is a symptom of a chemically active corrosion cell in the connection. Before you can attempt to solve the problem, we need to understand what causes the crevice corrosion in the first place. This type of corrosion takes place in  what is called a negative oxygen concentration cell. Metallic corrosion can produce very corrosive environments through the chemical change of water into acid, called hydrolysis. This phenomenon is particularly noticeable when the environment is confined, such as in most forms of localized corrosion (pitting, crevice, environmental cracking). Crevice corrosion is a localized form of corrosion, under the influence of crevice geometries. Stagnant solutions play an important role in creating highly corrosive micro-environments inside the crevices. Metallic materials tend to assume a more anodic character in the stagnant crevice solution compared with the bulk surface (exposed to the bulk environment) outside the crevice. The highly corrosive micro-environment of crevices tends to be similar to the micro-environment established at the base of corrosion pits. Crevice corrosion is usually a result of a differential oxygen concentration cell, in which the mouth of the crevice is richer in oxygen than the metal within the crevice, which therefore becomes anodic and dissolves. Subsequent pH shifts within the crevice may lead to even more intensified attack associated with the induction (initiation) and propagation phases of the corrosion cycle. The chemical change in question is true of most metals since the metallic ions produced by the corrosion processes are not soluble in their ionic forms. Therefore, these ions will then react and form more stable species such as oxides and hydroxides. In aerated environments iron oxidizes to ferric ions that subsequently react with water. In the final stage of development of crevice corrosion, a few more accelerating factors fully develop: 1. The metal ions produced by the anodic corrosion reaction readily hydrolyze, giving off protons and forming corrosion products. The pH in a crevice can reach very high acidic values, sometimes equivalent to pure acids. 2. The acidification of the local environment can produce a serious increase in the corrosion rate of most metals. 3. The corrosion products seal and further accelerate corrosion in the crevice environment. 4. The accumulation of a positive charge in the crevice becomes a strong attractor to negative ions in the environment, such as chlorides and sulfates, that can be corrosive in their own right. In summary, to simply the above technical explanation, there is an active acid-base corrosion process taking place in a sealed up environment, so trying to stop it by sealing or covering it with a thick barrier coatings may keep it out of sight for a time,  but the corrosion is still taking place. The expansion caused by the corrosion by product (FE3 04) will continue to expanded the crevice, crack the coating and then the rust bleeds out, causing the rust stain. The only way to stop this type of corrosion is to chemically neutralize the corrosion-causing chemistry before you seal the connection up. This process and chemical treatment must first of all open up the corrosion cell, remove the loose debris and flush out any salts. Then you must blow the crevice dry to get out any remaining moisture and debris. After this process the crevice must be flooded with a low viscosity chemistry capable of displacing any residual moisture at the steel interface, neutralizing any remaining acid and scavenging out the oxygen. Then, once the crevice is chemically neutral, it must be sealed up with a chemically compatible flexible coating capable of staying flexible so it works with the joint to maintain a positive seal. This will process will stop the rust bleed problem. The rule of thumb here is "You cannot cover up a chemically active corrosion cell."

From Paul Tsourous of Jupiter on January 22, 2018:
An application of 100% solids penetrating epoxy sealer after the zinc application has been effective for us on bridge projects.

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Tagged categories: Epoxy; Polyurethane; Quality Control; Rust; Urethane; Zinc

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