What is a Corrosion Rate

Corrosion is one of the most significant causes of deterioration in drainage, sewerage and wastewater infrastructure. Every year, water companies, local authorities and private asset owners spend millions of pounds repairing or replacing pipes, manholes, pumping stations and treatment equipment damaged by corrosion. While corrosion itself refers to the gradual destruction of a material through chemical, electrochemical or biological processes, the corrosion rate describes how quickly that deterioration occurs over a given period of time.

Understanding corrosion rate is essential for engineers, asset managers and maintenance contractors because it allows them to assess the condition of infrastructure, predict service life and implement preventative maintenance strategies before failures occur. In modern drainage systems, corrosion affects a wide range of materials including cast iron, steel, ductile iron, concrete, reinforced concrete and even certain plastics when exposed to aggressive chemicals.

Within sewer networks, corrosion is particularly important because assets are constantly exposed to moisture, wastewater, dissolved gases, biological activity and changing environmental conditions. Some materials may deteriorate very slowly over decades, while others can suffer severe damage within a relatively short period if exposed to highly corrosive conditions. Measuring and understanding corrosion rates helps infrastructure owners make informed decisions about inspection frequencies, repair programmes and asset replacement planning.

As drainage infrastructure across the UK continues to age, corrosion monitoring has become increasingly important. Many sewer networks contain assets that have been in service for several decades, making accurate assessment of deterioration rates critical for maintaining reliability and avoiding costly failures.

What Is Corrosion Rate?

Corrosion rate refers to the speed at which a material loses thickness, mass or structural integrity due to corrosion. It is typically expressed as a measurable amount of material loss over a specific period of time.

In engineering applications, corrosion rate is often measured in millimetres per year (mm/year), micrometres per year (μm/year) or mils per year (MPY). These measurements provide an indication of how rapidly a material is deteriorating and allow engineers to estimate its remaining service life.

For example, a steel pipe experiencing a corrosion rate of 0.1 mm per year would theoretically lose 1 mm of wall thickness over a ten-year period. Although this may appear insignificant, even relatively low corrosion rates can eventually compromise structural integrity when assets are expected to remain operational for many decades.

Corrosion rates are rarely constant throughout an asset’s life. Environmental conditions, flow characteristics, chemical exposure and biological activity can all influence the speed of deterioration. As a result, engineers often use corrosion rate assessments alongside physical inspections and condition surveys to develop a more accurate understanding of asset health.

In drainage and wastewater systems, corrosion rate analysis plays an important role in asset management because it helps identify areas where deterioration is accelerating and intervention may be required before serious damage occurs.

How Corrosion Develops in Drainage and Sewer Infrastructure

To understand corrosion rate, it is first necessary to understand how corrosion occurs within drainage and wastewater environments. Sewer systems create unique conditions that can be highly aggressive towards many construction materials.

One of the most common forms of deterioration is electrochemical corrosion. This process occurs when metals react with moisture and oxygen, resulting in the gradual breakdown of the material. Buried metallic pipework can be particularly vulnerable because soil conditions often support electrochemical reactions.

In wastewater systems, microbiologically induced corrosion is another major concern. Certain bacteria produce compounds that contribute directly to corrosion processes. Sulphate-reducing bacteria generate hydrogen sulphide gas, which is frequently found in sewers, pumping stations and treatment facilities.

When hydrogen sulphide rises above wastewater levels, it can be converted into sulphuric acid by naturally occurring bacteria on moist surfaces. This acid aggressively attacks concrete, mortar and metal surfaces, often causing severe deterioration within sewer structures.

Corrosion can also occur due to chemical exposure. Industrial discharges, cleaning chemicals and certain wastewater constituents may accelerate deterioration rates by creating highly aggressive conditions within pipes and treatment equipment.

Environmental factors including temperature, humidity, oxygen concentration and flow velocity all influence the rate at which corrosion develops. As these conditions vary throughout a drainage network, corrosion rates can differ significantly between apparently similar assets.

Factors That Influence Corrosion Rate

Corrosion rate is affected by numerous variables, many of which interact with one another. Understanding these factors is essential for predicting infrastructure performance and selecting appropriate materials for drainage applications.

Material composition is one of the most important considerations. Different materials exhibit varying levels of resistance to corrosion. Stainless steel, for example, generally corrodes much more slowly than unprotected carbon steel under similar conditions.

Environmental conditions also have a major influence. High moisture levels, elevated temperatures and acidic environments typically accelerate corrosion processes. In sewer systems, warm and humid conditions often promote biological activity that contributes to corrosion.

Flow conditions can significantly affect deterioration rates. High flow velocities may remove protective surface layers, exposing fresh material to corrosive attack. Conversely, stagnant conditions can encourage the accumulation of corrosive substances and microbial growth.

Common factors influencing corrosion rate include:

• Material type and composition
• pH levels
• Temperature
• Moisture availability
• Oxygen concentration
• Hydrogen sulphide exposure
• Biological activity
• Flow characteristics
• Chemical contamination
• Protective coating condition

The presence of protective coatings can dramatically reduce corrosion rates. However, damage to coatings often creates localised areas where corrosion accelerates, potentially leading to concentrated defects such as pitting.

Soil conditions also play a crucial role for buried infrastructure. Soil resistivity, moisture content and chemical composition all affect underground corrosion behaviour and can influence the lifespan of pipes and other assets.

Measuring Corrosion Rate in Wastewater Assets

Accurately measuring corrosion rate is an important aspect of infrastructure management. Various techniques are used to assess material deterioration and monitor changes over time.

Visual inspections remain one of the simplest methods. Experienced inspectors can identify signs of corrosion, surface degradation, pitting and material loss during routine surveys. While visual inspections do not directly measure corrosion rate, they provide valuable information about deterioration patterns.

Thickness measurement is commonly used for metallic structures and pipelines. Ultrasonic testing allows engineers to determine wall thickness without damaging the asset. By comparing measurements taken over time, corrosion rates can be calculated with reasonable accuracy.

Concrete structures often require different assessment methods. Engineers may evaluate surface loss, structural degradation and chemical attack using specialised testing techniques designed for wastewater environments.

Corrosion coupons are frequently employed in treatment plants and industrial facilities. These small samples of material are exposed to operating conditions for a specified period before being analysed to determine material loss.

Advanced monitoring technologies have become increasingly common within modern wastewater infrastructure. Electronic sensors can provide continuous data regarding environmental conditions, corrosion activity and asset performance.

Some facilities now utilise remote monitoring systems that allow operators to track corrosion indicators in real time. This approach supports proactive maintenance strategies and helps identify emerging problems before significant damage occurs.

The Impact of Corrosion Rate on Asset Lifespan

Corrosion rate has a direct influence on how long infrastructure assets can remain in service. Even relatively small increases in deterioration rate can significantly reduce asset lifespan and increase maintenance costs.

For example, if a concrete sewer chamber experiences a corrosion rate that doubles due to increased hydrogen sulphide exposure, the expected service life of the structure may be substantially reduced. This can result in earlier repair requirements, higher capital expenditure and increased operational disruption.

Pipe failures often occur when corrosion reduces wall thickness below safe design limits. In pressurised systems, this can lead to bursts or leaks. In gravity drainage systems, corrosion may cause structural weakness, infiltration, exfiltration or eventual collapse.

The economic implications can be considerable. Large sewer networks contain thousands of individual assets, each with different deterioration characteristics. Understanding corrosion rates allows asset managers to prioritise investment and target resources where they will have the greatest impact.

Corrosion rate data also supports lifecycle cost analysis. By predicting future deterioration, engineers can compare repair, rehabilitation and replacement options more effectively and select the most cost-efficient solutions.

In many cases, proactive intervention based on corrosion monitoring proves far less expensive than responding to unexpected failures after significant deterioration has already occurred.

Corrosion Control and Prevention Strategies

Because corrosion can never be completely eliminated, infrastructure owners focus on controlling corrosion rates and reducing their impact on asset performance. Effective corrosion management begins during the design stage and continues throughout the operational life of the asset.

Material selection is one of the most important preventative measures. Choosing corrosion-resistant materials appropriate for the expected operating environment can significantly extend service life and reduce maintenance requirements.

Protective coatings are widely used throughout drainage and wastewater infrastructure. Epoxy coatings, polyurethane linings and specialised chemical-resistant materials help isolate structural components from corrosive environments.

Cathodic protection systems provide another effective method for controlling corrosion in metallic assets. By altering electrochemical conditions, these systems reduce the rate at which metal deterioration occurs.

Additional corrosion control measures include:

• Improved ventilation in sewer structures
• Hydrogen sulphide management
• Chemical dosing programmes
• Regular cleaning and maintenance
• Protective liners and rehabilitation systems
• Moisture control where practical

Within wastewater treatment facilities, process optimisation can also help reduce corrosive conditions. Controlling sulphide generation, maintaining appropriate pH levels and improving flow conditions can all contribute to lower corrosion rates.

Routine inspections remain essential because even the most effective protective measures can deteriorate over time. Early identification of defects allows repairs to be carried out before corrosion accelerates significantly.

Corrosion Rate and Modern Asset Management

Modern drainage infrastructure management increasingly relies on data-driven approaches, and corrosion rate analysis has become an important component of this strategy. Water companies and infrastructure operators now utilise sophisticated asset management systems that incorporate condition assessment data, inspection records and predictive modelling.

Corrosion rate information helps prioritise inspections, identify high-risk assets and optimise maintenance schedules. Rather than relying solely on asset age, operators can make decisions based on actual deterioration trends.

Predictive models are becoming increasingly accurate as monitoring technologies improve. By combining corrosion rate measurements with environmental data and operational history, engineers can forecast future asset performance and plan interventions more effectively.

The growing adoption of digital twins and smart infrastructure technologies is likely to further enhance corrosion management capabilities. Real-time monitoring systems can provide continuous insight into asset condition, allowing more proactive responses to emerging deterioration.

As climate change, urbanisation and ageing infrastructure place increasing pressure on drainage networks, understanding corrosion rates will remain a critical aspect of maintaining reliable and resilient wastewater systems.

Conclusion

Corrosion rate is the measure of how quickly a material deteriorates due to corrosion and is one of the most important indicators of infrastructure condition within drainage, sewerage and wastewater systems. By quantifying material loss over time, engineers can assess asset health, predict service life and develop effective maintenance strategies.

In wastewater environments, corrosion is influenced by a combination of chemical, biological and environmental factors. Hydrogen sulphide, sulphuric acid formation, moisture, temperature and material characteristics all contribute to the rate at which deterioration occurs.

Accurate measurement and monitoring of corrosion rates allow infrastructure owners to make informed decisions regarding inspections, repairs and asset replacement. Through effective corrosion management, protective measures and ongoing monitoring, the impact of corrosion can be reduced, helping to extend asset lifespan and maintain the long-term reliability of drainage and sewerage infrastructure.

As wastewater networks continue to age and environmental expectations become more demanding, corrosion rate assessment will remain a fundamental tool for ensuring the safe, efficient and sustainable operation of critical underground infrastructure.