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Corrosion basics

Corrosion treatment and prevention in mining

Pitting corrosion of a 13Cr production tubular

The cost of corrosion

Pitting corrosion of a 13Cr production tubular.

Corrosion represents a significant cost burden to the mining industry. Recent reviews indicate that the true cost of corrosion is as high as 3% of the gross domestic product (GDP), which represents a figure of $60 billion/year. It can be shown that through the direct application of today’s knowledge for the treatment and prevention of corrosion, the industry sector can save $3 billion/year. Decades of research and field experience has lead to the development effective chemical treatment and prevention practices to control corrosion in many applications from the transportation of oil and gas to the processing of refractory ores. Despite what is known, there still exists a void of information for new applications in more challenging environments and the development of novel materials and treatment practices.

stress corrosion
Stress corrosion cracking in the heat affected zone

Prevention – thoughtful design and materials selection

In many applications corrosion can be prevented through thoughtful design considerations and materials selection. With the current understanding of the factors that affect corrosion we can often reduce the severity and rate of corrosion to an acceptable limit for the expected lifetime of the operation. Such factors may include the flow velocity and regime, temperature, fluid composition and particle loading. These factors may be predicted using process simulations together with background information on the process fluids and treatments. Adequate measures can then be taken in the design of the process to reduce the severity and impact of these factors.

There is a significant inventory of corrosion resistant alloys available for application in corrosive environments. However, this inventory has always represented a dilemma in the selection of the right materials for the application with the appropriate corrosion performance and capital costs. Higher alloyed steels undoubtedly represent better corrosion performance with the trade off of significant increases in the capital costs. Today there is a significant amount of R&D being conducted on the performance of existing and novel materials in a range of environments and flow conditions to quantify their practical limits for application in new developments. Understanding the environmental limits helps to narrow down the materials selection, though often parameters that are specific to a particular application need to be qualified. Due to the broad range of fluid compositions and process conditions such qualifications are often not easily transferred to other application without further testing.

Complementary design considerations that have been in use for decades include the use of internal and external coatings, cladding and cathodic protection.

Schematic of a typical organic corrosion inhibitor used in the inhibition of carbon dioxide corrosion

Treatment – chemical inhibition

In some instances it is simply not cost effective to prevent corrosion through design and materials selection and the most feasible option may be the use of lower alloyed steels in combination with an appropriate corrosion control practice. Corrosion inhibition using continuous and batch chemical treatments has long been used for the cost effective control of corrosion of pipelines and equipment. Corrosion inhibitors are chemicals that when treated at relatively low concentrations affect a reduction in the rate of corrosion. These chemicals, which may be inorganic (nitrite) or organic (imidazolines, quaternary ammonium salts, alkyl phosphonates ect), are formulated for the specific application and mode of corrosion. Decades of research has lead to significant advances in the formulation and application of corrosion inhibitors.

corrosion failure
Top of the line corrosion failure caused by the condensation of water on the top of the pipe in the presence of carbon dioxide and volatile organic acids (acetic and formic acid)

Nevertheless there still remain some practical limitations to overcome through innovative R&D. One particular limitation is the effective treatment of top of the line corrosion associated with the production of hot-wet gas. The orientation of this form of corrosion limits the transport of sufficient corrosion inhibitor to the top of the pipe for effective corrosion control. This problem represents a significant industrial and scientific challenge.

Good industry practice in the treatment and prevention of corrosion (corrosion education)

Corrosion education is key to the treatment and prevention of corrosion in the mining industry. An appreciation of corrosion relevant to the industry application will in many cases establish potential for corrosion to be removed during the design stages of the project or a successful treatment program to be implemented. Furthermore an appropriate level of understanding is required for the ongoing monitoring and assessment of asset integrity and management.