Pipelines internal coatings

In some instances it is necessary to coat pipelines internally, and materials widely used are red lead, hot-applied enamels, concrete and epoxy resins. 

It is more difficult to combat internal corrosion than external corrosion. Internal corrosion can be mitigated by having an internal coating or lining. For internal coating repair, the pipeline will have to be completely out of service. The pipeline need not be out of service for external recoating operations. 

It is usefiil to consider the case of an installation of a subsea gathering system for a natural gas production field. The pipeline design for a new gas production facility consisted of 20 cm diameter subsea gathering lines (flow lines) emptying into a 19 km, 50 cm diameter subsea transmission gas pipeline. The pipeline was to bring wet gas from an offshore producing area to a dehydration facility on shore. The internal corrosion was estimated to be 300-400 mpy. The corrosion mitigation options considered were (i) carbon steel treated with a corrosion inhibitor (ii) internally coated carbon steel with a supplemental corrosion inhibitor (iii) 22% Cr duplex stainless steel (iv) 625 corrosion-resistant alloy (CRA). The chance for success was estimated from known field histories of each technique, as well as the analysis of the corrosivity of the system and the level of sophistication required for successful implementation (Table 4.42). 

FIGURE 8.1 Schematic of pipeline internal and external corrosion, showing that a combination of inhibitors, coatings, and cathodic protection helps in controlling the corrosion of undergronnd CCPs. 

As can be seen, pipelines suffer from both internal and external corrosion. Internal corrosion is due to presence of sulfur-bearing gases such as hydrogen sulfide, carbon dioxide, and moisture, which is entrapped as brine from the sea. The usual rule of thumb is that the internal coatings are applied only if the crude/gas is sour in nature (its hydrogen sulfide concentration is more than 500 ppm). Otherwise, the internal corrosion is usually tackled by addition of inhibitors, either continuously or in a batch process. 

The test section must be sealed off from the start and end shafts by sealing cushions. These are inserted into smooth pipes with an internal coating of epoxy resin. The same applies to the sealing of house connection pipelines in inspection shafts as well as road drains with adaptor pipes. After sealing off, the pipeline is evacuated by means of a vacuum pump that works on the principle of the water jet and is connected to a compressor. 

T. Yamamoto 2000. Filed test on the corrosion behavior of internally coated oil pipelines located in Daqing and Sichuan oil field in China. Zairyo to Kankyo/Corrosion Engineering 49 (4), 209-214. 

After 1860 in the United States, water mains were only occasionally given coatings of tar. About 18% the activities of Engii.sh undertakings were extended to America, where chiefly bare metal pipelines had previously been laid. Water supply pipes were coated internally with bitumen in America after 1912. Vical (1837) in France and J. Bull (1843) in America introduced the widely known cement mortar as a protective material for water pipes 16]. 

A similar danger of corrosion lies in cell formation in steel-concrete foundations (see Section 4.3). Such steel-concrete cells are today the most frequent cause of the increasing amount of premature damage at defects in the coating of new steel pipelines. The incidence of this type of cell formation is increased by the connection of potential-equalizing conductors in internal gas pipelines and domestic water pipelines [25], as well as by the increased use of reinforcing steel in concrete foundations for grounding electrical installations [26]. 

The use of electrochemical protection in the chemical industry started about 20 years ago, which is somewhat recent, compared with its use for buried pipelines 40 years ago. Adoption was slow because the internal protection has to be tailored to the individual plant, which is not the case with the external protection of buried objects. Interest in internal protection came from the increasing need for greater safety for operating plants, increased demands for corrosion resistance, and larger plant components. While questions of its economy cannot generally be answered (see Section 22.6), the costs of electrochemical protection are generally less than the cost of equivalent and reliable coatings or corrosion-resistant materials. 

The normal industrial practice for controlling the internal corrosion of petroleum pipelines is to use coatings, nonmetallic pipeline materials, or corrosion inhibitors. Corrosion inhibitors, which are used for the protection of oil pipelines, are often complex mixtures. 

Coatings, cathodic protection, and chemical additives are used extensively to prevent internal and external pipeline corrosion. The excessive use of incompatible chemical additives has caused severe problems in gas-transporting systems. Costs arising from these problems often exceed the costs of the chemicals themselves. The careful evaluation and selection of chemical additives can minimize these problems and reduce operating costs [I860]. 

T. Arai and M. Ohkita. Application of polypropylene coating system to pipeline for high temperature service. In Proceedings Volume, pages 189-201. 8th Bhra Internal External Protect of Pipes Int Conf (Florence, Italy, 10/24-10/26), 1989. 

Resistant coatings High build solventless Range of colours Corrosion resistance Chemical resistance Excellent adhesion Potable water tanks Food industry Internal pipelining, etc. 

Andres B Peratta, John M W Baynham, and Robert A. Adey. A Computational Approach for Assessing Coating Performance in Cathodically Protected Transmission Pipelines. CORROSION 2009, Paper 6595 Atlanta, Georgia. NACE International 2009. 

Corrosion occurs either internally or externally. External corrosion can occur on both surface and buried portions of the pipeline. Aboveground, piping is protected by painting or specialized coatings. Below-groimd, piping is protected by specialized coatings and... 

Rehabilitation of internally corroded pipelines is somewhat more difficult to manage than external corrosion. Internally corroded pipes require cutting out and replacing the affected sections of the pipeline. Other methods of internal rehabilitation consist of pulled liners and epoxy flood coating. Cost estimates for these options can vary greatly and predominantly dependent on the extent of cleaning required to prepare the internal surface for coating. 

A large percentage (57%) of mains and services (46%) is metal (steel, cast iron or copper), and corrosion is a major issue. For distribution pipe, external corrosion is of primary importance, although internal corrosion has been noted in some cases. The methods of monitoring corrosion on cathodically protected pipe are similar to those in the transmission pipeline sector, including pipe-to-soil potential and coating surveys. One difference is that in distribution systems, leak detection is an acceptable method of monitoring for these pipelines without CP (nearly 15% of the steel mains). 

ASTM G8-96, Standard test methods for cathodic disbondmnet of pipeline coatings, in Annual Book of ASTM Standards, vol. 06.02, ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA, 1996. 

FIGURE 4.50 (See color insert.) Subsystems of a buried pipeline as a system (system = pipe, internal subsystems including flnid and lining conditions, external subsystem including the coating and soil conditions). 

FIGURE 4.51 Danger and control lists for internal corrosion components in acase study. (From Javaherdashti R., Corrosion assessment for buried, coated metallic pipelines with cathodic protection Proposing an algorithm, Paper No. 03154, Corrosion 2003, NACE, Houston, TX, 2003. With permission.)... 

The main problem is the external problem, mainly due to the corrosive nature of the soil. Across the pipeline route, the soil can be soft mud, hard mud, rocks, sand, sand with minerals, moisture, and other environments such as marshy lands, brackish water, seawater, and so forth. Ordinary steel with several compositions, shown in Table 8.1, shows severe corrosion, general corrosion, pitting, and so forth. One kind of serious corrosion problem that takes place is microbiological corrosion, which occurs due to presence of sulfate-reducing bacteria (SRB). They convert sulfate in soil to sulfide, which attacks steel, causing severe pits. Thus, external corrosion is taken care of by a combination of a good coating and cathodic protection. Let us now deal with internal and external corrosion in more detail. 

A.S. Khanna and N. Kumar. Coatings for underground pipelines. Proceedings of the Global 2000 Corrosion Meet, NACE International India Section, Bombay, India, Nov. 20-23, 2000, Vol. II, 335. 

R. Norsworthy. Polyguard Products, Inc. Causes of external corrosion on coated and cathodicaUy protected pipelines. Proceedings of Corrosion 2009, March 22—26, 2009, Atlanta, Georgia, NACE International. 

J. J. W.B. Cox. Three layer HDPE exterior pipelines coatings job reference and case histories. Proceedings of the 14th International Conference on Pipeline Protection, OcL 29-31, 2001, Barcelona, Spain. 

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