Corrosion Management Plan and Recommendations for Design-Stage Corrosion Control

Transcription

1 METRIS Plus project Metal part Corrosion Management Plan and Recommendations for Design-Stage Corrosion Control Report author: Sanja Martinez corrosion expert of the Regional Agency for the Development of Small and Medium Size Enterprises, Alma Mons Ltd., Novi Sad Zagreb, April 2013

2 2 1. Material 1.1. Stainless steel parts must be fabricated of steel number according to EN (UNS31603, Type 316 L). Compliance with the chemical composition, metallurgical state and mechanical properties will be checked by Metris. Stainless steel specimen and the manufacturer s specification will be provided by the fabricator. 2. Wall thickness 2.1. Wall thickness is to be chosen according to drawings for construction of stainless steel parts provided in the Tender and approved by corrosion expert and the Contracting Authority. 3. Surface finish 3.1. Surface finish of the stainless steel must conform to finish codes 1D, 2D or 2B given in EN Proper design 4.1. Stainless steel parts the fabricator must take into account recommendations given in Eurocode 3 EN and ENV as much as possible. 5. Good handling practice 5.1. Fabrication practices must aim to maintain the surface integrity of the stainless steel being processed, avoiding any contamination or surface irregularities that include: dust and dirt, loose iron particles, embedded iron, scratches, heat tint and other oxidation, rust areas, rough grinding and rough machining, welding arc strike marks, weld spatter, welding slag, welds defects, oil and grease, residual adhesives, paint, chalk and crayon markings Stainless steel parts must be stored in such a way as to prevent scratching of the surface and must be protected from abrasion and iron contamination by equipment used for handling, such as the forks of lift trucks or steel handling slings. Non-metal spacers must be interposed between stainless steel material and the bearers of support racks. Vertically stored sheets or plates must not contact one another and must be handled carefully to avoid abrasion Ideally, all stainless steel materials must be stored under cover to keep it clean. Ingress of dirt into pipework must be prevented by end caps. Processing activities (such as grinding steel near to the stainless fabrication) must be separated from those for carbon steels in order to avoid iron contamination. A cutting procedure appropriate for cutting stainless steel must be used. For mechanical cleaning, all abrasive media must be iron-free and must not have previously been used on carbon steel. Wire brushes must be made of a suitable grade of stainless steel wire.

3 All markings must be avoided and, if present, must be removed prior to welding to avoid their decomposition and the risk of carbon pick-up. Non-chlorinated solvents must be used for that purpose In order to assure corrosion resistance the contractor must at all times enable the representative of the Contracting Authority and the corrosion expert to inspect stainless steel surface cleanness during fabrication. In particular, the inspection methods may include Ferroxyl Test For Free Iron according to ASTM A380, test for dust according to ISO :1992 and visual and microscopic examination of the stainless steel surface at the workshop. 6. Good welding practice 6.1. Full penetration but welds must be used without excessive projection of the root bead. Intermittent fillet welds and intermittent and partial penetration butt welds must be avoided. The circumferential welds, especially those in areas exposed to the handling of sludge (the outside of the aeration piping and the inside of the digester and sludge piping) must be free of crevices and fully penetrated Compatible welding consumables must be used, such that the corrosion resistance of welds is equal to that of the parent material. Welding consumables must be chosen according to the European standards EN 1600, EN and EN Welding deficiencies such as undercut, lack of penetration, weld spatter, slag and stray arc strikes must thus be minimised. If present, these defects must be removed. Mechanical removal of defects must not reduce the thickness of the steel below the minimum specified in the design and the restored surface must be smooth and unblemished. Glass bead, garnet or walnut shell blasting is effective and results in satisfactory surface finish locally or over large areas. Wire brushing can produce a superficially bright and clean result but can also cause distortion of the metal surface. In particular, power brushing can lower corrosion resistance. However, grinding with aluminium oxide discs or flapper wheels or non-woven abrasive finishing products is a convenient manual operation that must be carried out progressively to a fine finish (at least 180 grit or finer). Mechanical cleaning methods must be supplemented by a passivation procedure. Avesta Pickling Solution 601 or an equivalent product may be used for that purpose When pipe is welded, attention must be focused on the condition of the weld root, since it will be exposed to the process fluid flowing through the system when in service. This is significant for the fabrication of stainless steel pipework where access to pickle/passivate internal welds is not possible. An inert backing gas must be used to protect the penetration bead of single-sided, full-penetration welds and must be used where possible to protect root runs from oxidation. Argon is normally used as a shielding gas to form the arc plasma and protect the weld pool from oxidation and air entrapment. Argon or a 90% nitrogen - 10% hydrogen gas mixture may be used. The combination of gas purity and purging system employed must ensure sufficiently

4 4 low oxygen (< 50 ppm) and water contents at the weld location. Shielding must be maintained until the metal temperature has fallen below 250 C In order to assure and verify corrosion resistance, welded samples are to be produced by the Contractor and are to be subjected to the appropriate corrosion testing as defined by the drawings and the table given at the end of this document. Design connections must be produced by the procedures identical to those that would be used during the pilot fabrication. In case the corrosion failures are observed on specimen exposed to raw wastewater before the start of stainless steel parts fabrication, welding practice will be modified and the tests will be repeated until the satisfactory result is attained Detailed welding procedure and welding inspection specifications must be agreed between the representative of the Contracting Authority, the fabricator and the machining/welding engineer. These must take into account corrosion related welding aspects. 7. Mechanical joints 7.1. Galling may occur when stainless steel nuts and bolts are used together where the contact points are subjected to high tightening torques. The stainless steel oxide film breaks down (galling) as a result of direct metal contact leading to surface damage and seizing up. The load must be sufficient to disrupt the protective oxide layers covering surface asperities and permit metal to metal contact. Care is required to avoid any thread damage and to keep the threads clean and free from dirt, coarse grime or sand. Bolts must be tightened to the correct torque using a torque or wrench. Appropriate lubrication or anti-seize compounds prior to assembly should be applied. 8. Pickling/passivation 8.1. The extent and type of pickling/passivation procedure will be chosen based on the state of the stainless steel surface assessed by procedures given in 9.1 applied prior to pickling/passivation treatment Pickling/passivation treatment of welds and surrounding heat affected zones is obligatory. A simple chemical method for local treatment of weld areas is to apply a paste containing the acid mixture in a binding agent, by brush or roller. Alternatively, electrochemical cleaning of welds may be used Pipework or other contained parts of system can be treated chemically by circulation or filling in with the acid mixtures described in ASTM A380/06 and/or ASTM A967-05e2. This option is precluded on finished product due to the possibility of damaging sensitive measuring equipment and non-stainless steel parts. Alternatively, larger areas of fabrication (other than welds) can be sprayed with a passivation viscous solution or a gel.

5 Pickling/passivation may be done in workshop or on site by a contractor having the facilities for protection of personnel and the surroundings. Both, chemical and electrochemical processes have the advantage of requiring a minimum of equipment and skill. However, the application procedure must be followed correctly to avoid over-pickling and damage to the underlying material. In all cases, appropriate safety procedures must be followed and thorough rinsing with clean water (final rinse using water with a maximum TDS = 200 ppm) is essential, with appropriate disposal of the effluent according to the relevant regulations Procedure given in 9.1 must be applied in order to qualify the applied pickling/passivation treatment. Additionally, heat tint adjacent to the welds must be removed. Removal must be rated according to AWS D18.2(1999). A straw-coloured or yellow heat tint up to and including level Number 3 on the colour reference chart can be considered acceptable. 9. Passivation Procedure Qualification 9.1. Passivity of the surface will be tested at various stages of pilot production by local measurements of the surface potential according to ASME BPE Criteria of acceptance will be agreed between the Contracting Authority, the Contractor and the corrosion expert based on the measurements done on referent specimen (clean as-received and passivated surfaces). Additionally, other methods given in ASME BPE 2009 may be used to confirm the results of potential measurements. In case the passivation procedure fails to produce passive surface, additional pickling/passivation measures agreed between the Contracting Authority, the Contractor and the corrosion expert will be applied and passivation procedure qualification will be repeated until satisfactory result is attained. 10. Corrosion monitoring Corrosion monitoring must be done by corrosion probe capable of detecting both general corrosion and localized corrosion i.e. by recording corrosion rate and pitting factor in the form of 4-20 ma output signals. Probe electrodes must be fabricated from stainless steel (AISI 316L). The software for representation of the probe results must be included Corrosion monitoring probe must be installed in tank B3. The probe must be mounted according to the manufacturers instructions and located close to the bottom of the tank as it must at all times be immersed in the corrosive material. Corrosion probes must be integrated into the remote monitoring system of the pilot. Alternatively, corrosion data may be monitored periodically by connecting PC to the monitoring probe Multiple crevice assembly specimen constructed according to ASTM G48 and G78 and welded specimen (defined in the drawings and table at the end of this document) will be dipped into the various contained system parts and used for direct corrosion monitoring. The multiple crevice assembly nuts will be tightened with various torques. Mounting of the specimen will be done by the Contractor in concordance with the directions given by the Contracting Authority and the

6 6 corrosion expert. Subsequent corrosion analysis will be done by Metris and the corrosion expert in order to obtain correlation between waste-water chemistry and corrosion processes. 11. Transport and Installation While transported to site, the pilot needs to be protected from damage by suitable packing and to be carefully stored until installed. Temporary covers must be provided and any dust washed off the stainless steel before welding. Welds made in the field require the same degree of care as those made in the workshop. Requirements, such as internal purging are practically impossible to set up in field and should be substitute by bolted flange connections. 12. Commissioning Plant must be washed down with potable grade water at the end of installation operations If equipment is not going into service within 5 days after hydrotesting, the system must be drained thoroughly and dried within 24 to 48 hours of the test. Alternative is to flush the system every 2 days. 13. Working parameters Process stage involving the introduction hypochlorite and citric acid is detrimental to stainless steel. The Conractor must avoid design solutions that would include adding these compounds to the process stream in concentrated form so that they are directed at or down the side of stainless steel piping or equipment because that can result in localised attack and dangerous spillage of the chemicas. Injection and thorough mixing must be performed well upstream of the stainless steel parts. Equipment for the handling, storage and dispensing of such chemicals in their concentrated forms must not be made of stainless steel. 14. Documentation 14.1 Detailed documentation about the procedures used (cutting, cold working, welding, passivation etc.) during fabrication of the stainless steel samples and parts is to be provided at any time upon the request if the Contracting Authority. Prior to commissioning the complete documentation must be handed to the Contracting Authority.

7 7 15. Welded specimens 15.1 Specimens of different welded structural details should be prepared by the Contractor according to instructions above and in close agreement with corrosion experts delegated by Contracting Authority. Following pictures describe five different types of specimens to be produced. Figure 1 Weld over specimen surface Figure 2 Single side fillet weld Figure 3 Double side fillet weld

8 8 Figure 4 - Butt weld Figure 5 Piping circular weld

9 A total number number od specimens to be produced is 220, according to table below. Specimen Number of specimens Weld over specimen surface 44 Single side fillet weld 44 Double side fillet weld 44 Butt weld 44 Piping circular weld Specimens must be produced in the same way as the rest of the Pilot structure. Technology used should be the same whenever possible so that specimen material (base and weld) corrosion properties resembles as close as possible to Pilot corrosion properties. Production technology should be cleared and approved by corrosion expert delegated by Contracting Authority prior to production of specimens Specimens must be prepared for the application in the corrosive environment according to instruction of corrosion expert delegated by Contracting Authority. This includes mechanical and/or chemical passivation treatment of specimens resembling (but not exclusive) to passivation treatment of the Pilot Final remarks Corrosion expert delegated by Contracting Authority should be consulted in case of any doubts or troubles interpreting this document, in the case of suggestions for modification of design and technology and other similar reasons Close cooperation with corrosion expert delegated by Contracting Authority is required at all stages of Pilot and specimen production. Corrosion expert delegated by Contracting Authority will verify and must approve all activities (design, production, technology etc.) subjected to corrosion requirements described in this document.