Uncontrolled corrosion can cause release of hazardous substances and components or can reduce both the performance and reliability of equipment until their failure. This latter situation can put at risk the safety and well-being of both plant employees and the general public as well as lead to severe damage of process units, and in some cases shutdown of refinery operations. Notably, of the 137 major refinery accidents reported by EU countries to the EU’s eMARS database since 1984, around 20% indicated corrosion failure as an important contributing factor. Moreover, this remains the average percentage of the total accidents reported even in recent years.

Corrosion represents a particularly relevant risk to petroleum refineries because refineries typically have several high risk factors because of the type of substances and processes involved in refinery operations. Other local conditions may also contribute to an acceleration in the corrosion rate, including physical location of equipment and the climate. Moreover, certain operating conditions in a refinery, both normal and abnormal, by their nature are particularly likely to present favourable opportunities for a
corrosion failure to initiate a chain of events leading to a major accident.

Types of corrosion
Corrosion can appear as either uniform corrosion or localized corrosion. The American Petroleum Institute Recommended Practice 571 (API 571) lists over 25 common corrosion damage mechanisms to industrial activity plus 11 addition types that are specific to refineries. In addition, studies of aging facilities may classify corrosion effects into different groupings on the basis of characteristics such as failure mechanisms (e.g., wall thinning, cracking and fracture, physical deformation), common causal factors (e.g.,
stress-driven damage, metallurgical/ environmental damage) or other commonalities.

Uniform corrosion is also known as general corrosion and is the classic form of corrosion in which the entire surface area, or a large fraction of the total area, is affected by a general thinning of the metal. In chemical processing uniform corrosion is considered the least dangerous form because it is easily visible long before it is degraded enough to fail. Nonetheless, uniform corrosion may sometimes be a cause of accidents, for example, in pipelines that are in remote locations, underground, or otherwise, not viewed frequently, general corrosion may continue for a long time undetected.

Conversely, there are numerous types of localized corrosion that are far more difficult to detect without targeted effort. Thus, consequences of

localized corrosion can be more severe than uniform corrosion as failure occurs without warning and often after only a short period of use or exposure. Typically, localized corrosion occurs between joints (crevice corrosion) or under a paint coating or insulation. Stress corrosion cracking and hydrogen-assisted stress corrosion are also forms of localized corrosion. They are often grouped together with hydrogen embrittlement and stress embrittlement, even though these are not corrosion phenomena, because the conditions and the resulting failure mechanism (cracks in the metal) are remarkably similar. As such, it is not necessarily easy to determine which phenomenon caused such a failure following an accident; hence, by necessity, analyses of accidents involving corrosion-related failures may refer to both phenomena.

Process Conditions
A fundamental ingredient of corrosion is exposure to a corrosive agent via a refinery process, that is, a substance that under certain processing conditions acts upon the metal and weakens it. These corrosive agents are in effect oxidizing substances, which may include water, a variety of acid compounds introduced or generated in the process as well as the crude oil and final and interim products, such as coke and kerosene. Some substances have unique corrosion “signatures”, that is, the corrosion produced is characterized by a particular specific visual or textural pattern, reacts with specific metal compounds, and frequently occurs in the same types of locations. As noted in Figure 1, substances cited most commonly in relation to corrosion failures were sulphur and sulphur compounds and water (14 cases each) followed by hydrogen sulphide (11 cases). The substances identified in Figure 1 are normally present in the highest volumes and in a variety of processes throughout a typical refinery site.

The Importance of Implementing Safety Management Systems to Address Corrosion Risks
Neglecting to identify or manage corrosion hazards also continues to be a problem on some refinery sites. Accident reports studied by JRC-MAHB were quite clear that the lesson learned was less about the technical challenge of managing corrosion but simply about having an effective risk management program. In fact, many of the reports studied by JRC-MAHB (60%) contained detail that suggested that a risk assessment should have occurred at a particular point in the life cycle, and that at the time it was either not performed or it was insufficient in identifying the corrosion hazard and/or its associated risk potential.

The study found that these inadequacies could be grouped into
four different categories according to their occurrence in the safety
management process, as follows:
• Inadequate risk analysis at design and construction stage
• Inadequate risk analysis prior to change, which is essentially a lack of or failure in the management of change process
• Failure to identify or address process risks in planning inspections
• Inadequate identification of hazards and risks for other purposes, such as safe performance of repairs and establishment of detection and mitigation systems.

In addition, one of the most important challenges in managing refinery corrosion is the element of change. Already changes to process design and equipment pose a challenge and need a certain competency to identify if a new corrosion risk has been introduced.

However, other changes that can affect corrosion rates may go unrecognized and thus not be evaluated for an elevated risk. Particular changes of this nature could be a change in the source of crude oil or an increase in production rate, particularly if they are considered to be somewhat temporary. Inconspicuous changes can also create risk and in this regard, the refinery’s greatest risk may be change over time. Loss of experienced personnel, lack of knowledge of the original process and equipment design (some-times decades ago), and aging equipment all fall in this category. Strategies such as risk-based inspections, life-cycle management, and safety performance indicators, to name a few, are all good practices that can support risk management for this somewhat insidious changes that can greatly influence the level of risk. Corporate leadership and safety culture, areas of renewed emphasis following the accident at BP Texas City in March 2005, also offer promising conceptual frameworks for organizations to reinforce and sustain efforts at the operational level.