Water Line Corrosion

<p><strong>Introduction</strong></p><p>Seal water blades &ldquo;SS304&rdquo; which surround the reheating furnace have been inspected by the technical team due to failure happening to those blades. This system was changed in 2016 with new SS304 blades. Effected Blades were showing a combination of deposits &amp; corrosion on the surface of the metal, and it comes in a linear &amp; straight shape.</p><p>A sample of that deposit and corrosion product was collected and analyzed in Veolia laboratory to give a clear picture of the nature of that deposit, and corrosion. An XRF spectrometer analysis was carried out, and the results confirmed high CaO, and SO3 compounds in those deposits. In addition to that, Ferric oxide, and Ferric Chloride compositions also existed, and this mainly came as a result of corrosion.</p><p>After further investigation done at the site and in our laboratory, we figure out that the failure happened due to mainly two reasons:</p><ul><li>Deposit build up on the water line due to dryness and high temperature conditions at the waterline.</li><li>Under deposit corrosion due to chloride ions and sulphate ions migration under deposit layer in absence of oxygen beneath this layer, this type of corrosion is called waterline corrosion.</li></ul><p><strong>Waterline corrosion on seal water blade</strong></p><p><strong><img src="https://kradminasset.s3.ap-south-1.amazonaws.com/ExpertViews/Water+corosion.png" alt="" width="308" height="201" /></strong></p><p><strong>What is the waterline corrosion?</strong></p><p>Waterline corrosion is a type of oxidation process that can happen to materials in contact with water. Waterline corrosion occurs when one portion of a base material is submersed in the water and another portion is in contact with the air. This creates a differential of the amount of oxygen in contact with the material's surface above and below the waterline and results in a corrosive reaction</p><p>Waterline corrosion is a concern in several different industries. Tanks that are used to store liquids such as water are often prone to waterline corrosion. Marine structures can also fall victim to waterline corrosion; this can lead to a complete structural failure. Ships that are left in the water for extended periods of time can be at risk of waterline corrosion as well.</p><p><strong>Where waterline Deposition/corrosion is usually found?</strong></p><p>This type of corrosion normally exists in wet areas, and specifically when water and air on the same borders. This type of corrosion happened to guttering, pipes, tanks, and metal sheets.</p><p><strong>What are the best conditions for waterline corrosion to grow?</strong></p><p>When water is stagnant or has a low flow in a steel tank or channel, the oxygen level above the water surface is greater than that under the surface of water. This generates an Oxygen concentration cell,</p><p><img src="https://kradminasset.s3.ap-south-1.amazonaws.com/ExpertViews/Water+corosion1.png" alt="" width="314" height="172" /></p><p>the metal above the water is cathodic with respect to the metal under the water (Anodic). Differential aeration Level is considered to be factor number one that boosts this type of corrosion.</p><p><strong>What is the mechanism of waterline corrosion?</strong></p><p>Waterline corrosion occurs when differential aeration is present in a neutral medium. It is usually found in steel guttering, pipes and tanks that are poorly maintained or damaged. The protected cathodic part is above the waterline and the area starved of oxygen is just below, which is where corrosion develops.</p><p><strong>What are the reactions of corrosion related to waterline corrosion?</strong></p><p>Waterline corrosion takes place when there is an uneven supply of oxygen to areas of the same metal component. It is a type of electrochemical corrosion that affects metals such as steel and iron. When a poorly oxygenated area is adjacent to an area with a good supply of oxygen, an anodic/cathodic reaction occurs. Oxygen screening, crevice corrosion and poultice action are other terms for this type of corrosion.</p><p>In the diagram below you can see how the cathode action reduces oxygen from the air to produce hydroxide ions. When the ions between the cathodic and anodic areas meet iron hydroxide forms, precipitates and oxidised to form the corrosive material or rust. The diagram shows an instance of waterline corrosion.</p><p><img src="https://kradminasset.s3.ap-south-1.amazonaws.com/ExpertViews/Water+corosion3.png" /></p><p>O2 + 2H2O + 4e&ndash; &rarr; 4OH&ndash; (cathode) and Fe2+ + 2e&ndash; &rarr; Fe&ndash; (anode).<br />&nbsp;<br /><strong>What does the corrosion related to the waterline look like?</strong></p><p><img src="https://kradminasset.s3.ap-south-1.amazonaws.com/ExpertViews/Water+corosion2.PNG" /></p><p><br /><strong>What are the indications of presence of Waterline deposition/corrosion?</strong></p><p>● &nbsp; &nbsp;Metal Discolouration near water surface<br />● &nbsp; &nbsp;Deposits or scale at the water surface<br />● &nbsp; &nbsp;Metal loss in the water line</p><p><strong>What can we do to stop under deposit/waterline corrosion?</strong></p><p>Waterline corrosion can be combated by several different methods. One way is to apply coating to the material with special paint or epoxy for the area exposed or submerged in the water. The coating keeps oxygen exposure across the substrate to a uniform amount and to a minimum. Another method is to use materials that are not as prone to oxidation as steel. Stainless steels (SS316L) and aluminum may be better alternatives to deter waterline corrosion.</p><p>&nbsp;</p>
KR Expert - Rajesh Kumar Damarla

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