CHAPTER ONE: INTRODUCTION
1.1 Background of study
Cathodic protection (CP) is a method of controlling corrosion or a means of preventing corrosion of metaland can be applied to any buried and/or submerged metallic structures. It is normally used in conjunction with coatings and can be considered as a secondary corrosion control technique.
Cathodic protection can, in principle, be applied to any metallic structure in contact salty media(electrolyte). In practice its main use is to protect steel structures buried in soil or immersed in water. Structures commonly protected, includes:
Cross country pipelines
Exterior surfaces of pipelines immersed in water
In plant piping
Above ground storage tank bases
Buried tanks and vessels
Internal surfaces of tanks, vessels, condensers and pipes
Foundation piling, steel sheet-piling
Piling – tubular, sheet steel and foundation
Marine structures including jetties, wharfs, harbours, piers
Reinforcing steel in concrete
Corrosion is a very serious problem. Three areas in which corrosion are important are in economic, improved safety and conservation of resources. The leakage of hazardous materials from a transport pipeline represents not only the loss of natural resources but also the potential for serious and dangerous environmental impact, and human fatalities. While pipelines are designed and constructed to maintain their integrity, diverse factors (e.g., corrosion) make it difficult to avoid the occurrence of leakage in a pipeline system during its lifetime.
All metals needs energy to be transformed from their oxide (natural) state to a refined state. The process of taking this energy away from the metal is called corrosion. Metals tend to revert back to their natural state when reacting with the environment. This corrosion reaction that occurs is an oxidation-reduction reaction. The purpose of cathodic protection is to stop this corrosive process.
Cathodic protection is the most important of all approaches to corrosion control techniques. One of the types of cathodic protection is sacrificial anode or galvanic cathodic protection. Corrosion occurs through the loss of the metal ions at anodic area to the electrolyte. Cathodic areas are protected from corrosion because of the deposition of hydrogen or other ions that carry current (Sandoval, A., et.al 2001). By using the sacrificial anode technique, the steel pipe will be protected from corrosion but the other metal which is the anode will corrode. In designing this method we must analyze parameters such as factor affecting corrosion, the amount of anode and rate of corrosion, the current densities and the total resistance.
Corrosion is an electrochemical process in which a current leaves a structure at the anode site, passes through an electrolyte, and reenters the structure at the cathode site. Differences in potential at different points along the pipe begin to develop. For example, because it is in a soil with low resistivity compared to the rest of the line, current would leave the pipeline at that anode site, pass through the soil, and reenter the pipeline at a cathode site. These potentials generate corrosion currents which leave the pipe to enter the soil at certain selective locations.
1.2 Problem statement
Despite many corrosion control method and inhibition techniques, there are still many challenges relating to cost effective operational performance, asset integrity management and maintenance of piping and other process equipment in the oil and gas industries due to corrosion effect.
Since carbon steel is commonly used in industrial units and for any buried and/or submerged metallic structures because of it’s low cost and excellent mechanical properties, it suffers severe corrosion attack in service.
Aside the detrimental effect of corrosion to humanity, it reduces the optimum transportation and delivery of crude oil and other products in the Oil and Gas industry. When this material (pipeline) comes in contact with salty media, corrosion will occur due to the loss of metal ions at the anodic area to the electrolyte, hence there is a need for a secondary corrosion control technique that will effectively prevent external corrosion of pipelines buried in the soil or water and to reduce material degradation rate.
1.3Aim and objectives
The main goal or aim of this work is to design and install a cathodic protection system as a secondary corrosion control technique for pipeline buried underground, using zinc anode.
The objectives in carrying out this study is to
- investigate the effect of corrosion for underground steel pipeline
- Evaluate Factors affecting cathodic protection system underground
- The application of corrosion mitigation methods, particularly cathodicprotection
- Design acathodic protection system using sacrificial anode (zinc) for underground steel pipeline.
1.4 Significant of study
Mitigating corrosion is important from both an operational and cost standpoint. Among other consequences, ignoring corrosion can shorten structure life, increase equipment or system out-of-service time, and increase maintenance cost. In addition to the dollar cost, the loss of critical material and human effort are also quite important. Measures such as cathodic protection help diminish such losses. It can also help to protect these structures from damaging. The need for cathodic protection system for pipelines cannot be overemphasized. When corrosion damages a pipeline and the content leaks out, it can destroy aquatic life, and this can in turn affect us as humans.
It is estimated that somewhere between 3 and 5 % of the gross national product (GNP) of industrialized countries is attributed to corrosion damage. Corrosion of metals costs the USA economy almost $300 billion per year and it is estimated that one third of this value could be saved with better selection of corrosion prevention techniques, including cathodic protection.
1.5Scope of study
Due to its wide range of the application of cathodic protection system and variations both in the design and installation, I decided to narrow it;
- Sacrificial AnodeCathodicProtect (SACP) system will be used
- Using zinc as the anode of the system
- Applied in buried steel pipeline
- Designing the test rig at the site and sample preparation
- Analyze of the parameters; coating and type of anode that can be used forcathodic protection.