Corrosion Assessment of Piping Systems-The purpose of our service team is to detect corrosion risks in order to rank the structures in relation to their corrosion risks and identify options to, remove, mitigate or manage the risks. Several important factors often associated with corrosion of water related to aging pipe and tank structures are:
- Improper materials,
- Deficiencies in corrosion control,
- And not considering water/soil corrosivity.
In general, the following data should be considered in a corrosion assessment, mitigation and design of piping systems to prevent internal corrosion or external corrosion:
- Water Chemistry
- Corrosive Ions
- Buffering Capacity
- System Design and Materials/Coatings
- Stray Current
- Copper Grounding
These factors are of primary consideration when accelerated corrosion attack occurs. If identified early on, potential failures can often be prevented.Corrosion mitigation for piping includes materials selection, corrosion inhibitors, water treatment and corrosion monitoring, However one needs to know the primary cause and root cause by performing failure analysis investigation,
Corrosion of Iron Pipes in a water system can cause three distinct but related problems. First, pipe mass is lost through oxidization to soluble iron species or iron-bearing scale. Second, the scale can accumulate as large tubercles that increase head loss and decrease water capacity. Finally, the release of soluble or particulate iron corrosion-byproducts to the water decreases its aesthetic quality and often leads to user complaints of “red water” at the tap. The water industry must be concerned with all three of these aspects of corrosion.
Copper Tube Corrosion Assessment and Mitigation
Copper Water Tubes copper piping have been used to distribute drinking water within buildings for many years. The long life of copper when exposed to natural waters is a result of its’ thermodynamic stability, its’ high resistance to reacting with the environment, and the formation of insoluble corrosion products that insulate the metal from the environment. The corrosion rate of copper in most drinkable waters is less than 2.5 µm/year, at this rate a 15 mm tube with a wall thickness of 0.7 mm would last for about 280 years. In some soft waters the general corrosion rate may increase to 12.5 µm/year, but even at this rate it would take over 50 years to perforate the same tube.
If the general water speed or the degree of local turbulence in an installation is high, the protective film that would normally be formed on a copper tube as a result of slight initial corrosion, may be torn off the surface locally, permitting further corrosion to take place at that point. If this process continues it can produce deep localised attack of the type known as erosion-corrosion or impingement damage. The actual attack on the metal is by the corrosive action of the water to which it is exposed while the erosive factor is the mechanical removal of the corrosion product from the surface.
Erosion corrosion also known as impingement damage, is the combined effect of corrosion and erosion caused by rapid flowing turbulent water. It is probably the second most common cause of copper piping failures behind Type 1 pitting which is also known as cold water pitting of copper piping.attack produces highly characteristic water-swept pits, which are often horseshoe shaped, or it can produce broader areas of attack. The leading edge of the pit is frequently undercut by the swirling action of the water. Usually, the surface of the metal within the pits or areas of attack is smooth and carries no substantial corrosion product. Erosion-corrosion is known to occur in pumped-circulation hot water distribution systems, and even in cold water distribution systems, if the water velocities are too high. The factors influencing the attack include the chemical character of the water passing through the system, the temperature, the average water velocity in the system and the presence of any local features likely to induce turbulence in the water stream.
It is unusual for the general water velocity in a system to be so high that impingement attack occurs throughout the whole of the copper pipework. More commonly, the velocity is just sufficiently low for satisfactory protective films to be formed and to remain in position on most of the system, with impingement damage more likely to occur where there is an abrupt change in the direction of water flow giving rise to a high degree of turbulence, such as at tee pieces and elbow fittings. It is not generally realized how great an effect small obstructions can have on the flow pattern of water in a pipe-work system and the extent to which they can induce turbulence and cause corrosion-erosion. For example, it is most important, as far as possible, to ensure that copper tubes cut with a tube cutter are deburred before making the joint. Also a gap between the tube end and the stop in the fitting, due to the tube not having been cut to the correct length and fully inserted into the socket of the fitting, can also induce turbulence in the water stream.
Pump Corrosion and Stray Current Corrosion
Internal corrosion of water pumps will take place in corrosive water with high levels of chlorides, present of corrosive bacteria or upon exposure to reducing waters. Graphitization of cast iron components is a corrosion risk for aging pumps in corrosive water and should be considered in failure analysis investigations. Another important issue is stray current corrosion. Stray current corrosion is caused by the effects of a direct current that may be picked up on a pump or pipe that is not part of circuit of interest. This current flows to the structure(pipe or pump) and at some point discharges(leaves) the structure and travel back to the source. This cause stray current corrosion (sever) at the point of discharge, usually an external corrosion risk. The form of attack is usually very localized. electrochemical potential and current measurements are performed to identify stray current risk. We can also design and install cathodic protection system for your specific water application.
Our NACE Certified Corrosion team can inspect, assess and mitigate corrosion in your water systems. This will be done through an on-site investigation, laboratory failure analysis and water corrosivity testing. As an example, remedial measures for impingement attack include modifications to the system to reduce the average water velocity, e.g. by using larger diameter tubes or, if appropriate, to lower the pump speed, and/or to redesign the part of the installation concerned to eliminate the cause of local turbulence, e.g. by using slow or swept bends and tee fittings rather than elbows and square tees. It is important to consider the risk and the possibility of any local turbulence occurring by ensuring that the ends of tubes cut with a tube cutter are deburred and that the tubes are inserted fully to the stops in the fitting before the joints are made, as referred to earlier in this section. In some cases, where the above approaches are not possible, the length of copper tube affected can sometimes be replaced by materials more resistant to corrosion-erosion, e.g. 90/10 copper-nickel (BS Designation CN102) using appropriate fittings, or stainless steel.
We are often engaged in water related projects to identify deficiencies in the water piping systems and to render opinions on whether the failures are a matter of water corrosivity, design, material selection or installation. We may recommend any of the following tasks for mid size or large water systems:
1. A water treatment plan should be developed and implemented.
2. Alternative materials may be needed. For example, brass <14% Zinc UNS C87400, silicon red brass UNS C69400 and leaded silicon brass UNS C69700 are not recommended in corrosive waters due to dezincification and corrosion. Valves consisting of a cast bronze body, screwed-in body seat with reinforced nylon diaphragms, and built-in strainer are recommended for pressure regulating valves.
3. The chloride level in the water entering the building and the corrosivity of the treated water should be monitored by installing test coupons in large water systems. Wireless corrosion sensors are also available for corrosion monitoring.
We also provide on-site training upon the request of the client to ensure that the piping/tank/plumbing system within the building or plant is maintained properly by qualified trained personnel in order to mitigate corrosion risk.
Water Corrosivity Testing for Different Metals/Materials and Inhibitor Evaluation
The tendency for a material to corrode is normally determined by measuring its rate of corrosion and comparing it with the corrosion rates of other materials in the same water environment. Conversely, the relative corrosivity of water may be determined by comparing the corrosion rate of a material in the water with the corrosion rates of the same material in other waters. We use such tests , for example, for evaluating the effects of corrosion inhibitors on the corrosivity of water. Although this test methods is intended to determine the corrosivity of water, it is equally useful for determining corrosiveness and corrosion rate of materials. Examples of systems in which this method may be used include but are not limited to open recirculating cooling water and closed chilled and hydronic heating systems.
We can provide corrosion rate or loss in thickness ( thickness loss per year(mpy)) through electrochemical or immersion techniques per ASTM and NACE recommended tests.
Boiler Tube Failure Analysis
The occurrence of failed boiler tubes as a result of hydrogen attack, pitting corrosion, chelant corrosion, fatigue, creep, or overheating is a common occurrence. Oxide films may play a significant role in each of these failures. The interaction of oxide films and hydrogen can strongly influence, or even control, corrosion associated failures in boilers.
An oxide of iron, identified as magnetite, forms a protective barrier on the internal steel surface of the boiler component. The oxide provides a protective mechanism that resists corrosion attack. If the oxide film is damaged, it reforms immediately in certain environments. The oxide film forms on the surface as a result of the steel reacting with water when exposed to the boiler environment, or as a result of contact with passivating compounds. The oxide, magnetite, forms on the metal surface under normal boiler conditions as demonstrated in the following:
3Fe + 4H20—–>Fe304 + 4H2
The process of oxidation is governed by the reaction of the surface, as well as the transport of the materials through the oxide, which governs the rate at which the scale grows. The process of formation of the protective layer is parabolic, and the film may consist of two layers depending on the growth conditions.
Past experience has demonstrated that unless this protective film is damaged by physical change, mechanical or electrochemical action, no serious problems may arise. Cases of poor resistance to corrosion are often connected with the stability of this protective oxide film, notably in the presence of high oxygen, overheating, hydrogen, and chlorides that may induce localized corrosion and cracking. Upon your request, we can perform laboratory failure analysis on your failed boiler tubes and determine the primary cause for the failure.
Failure Analysis Root Cause Investigation-Our Approach
Once it has been determined that a failure has indeed occurred, the point of origin must be found, and a determination must be made as to whether the failure occurred as a result of design, method of manufacturing, service history and conditions, water chemistry excursions, or from a deficiency in the material. When the point of origin is located, the investigation may proceed to a study of how the failure occurred, possible causes or factors in the failure, and possible means of preventative measures.
Why a failure occurs is an important question in the method of evaluation. This question can be approached by breaking down the failure into “mode of failure” and “cause of failure.” Mode of failure is the process by which the failure occurred. Cause of failure is that which can be fixed or changed to prevent future failures. Each question provides important clues to the investigation, and although priorities may be quite different, each question must be addressed and resolved to determine why a failure may have occurred.
Several important factors often associated with component failures are corrosive environments, deficiency in design, fabrication, operating conditions, unsuitable materials selection, and expended useful life. Our failure analysis procedure, or methodology for evaluation, will be provided in a step by step approach. This includes justification for conducting a failure analysis investigation, developing a logical plan for the investigation to follow, collection of background information, sample removal techniques, on-site inspection, laboratory testing and analysis, and the formulation of a final report based on relevant data, analysis, and recommendations.
Your input is vital for our failure analysis root cause determination. By giving us a detailed description of the problem, you can be sure that all aspects of your water corrosion problems will be considered promptly. We’ll send you a proposal for your work/project, including methods, testing, specific recommendation and costs.
Please call Dr. Mirshams at (469)-964-4040 to discuss the investigation.