Kingsley Lau, Ph.D.
The physical civil infrastructure contains buried structural elements in support of highway, bridge, and building structures, as well as drinking-water, storm-water, and sewer utilities. Common construction materials used in these systems, including steel and concrete, are susceptible to degradation in aggressive environments.
Why does corrosion develop?
Corrosion is a material degradation process that develops from the unwanted oxidation of a metal. The degradation can result in loss of strength and shortening of service life of the structural element. This causes increase in costs for maintenance and repair, and in severe conditions can result in catastrophic failures.
Corrosion mechanisms vary by material, design details, exposure environment, and other service conditions. The wide variety of factors contribute to the development of corrosion, but the general process is electrochemical.
Metals exposed to moisture develop an electrochemical cell where the complementary oxidation of the metal and reduction typically of available oxygen (but other cathodic reactions can develop) allow for the wastage of the metal and the accumulation of rust. The metal loss can cause a reduction of mechanical strength including decrease in tensile and compressive resistance to loads, localization of fractures, and inability to hold pressure and convey liquids in distribution systems. In reinforced and prestressed concrete systems, the accumulation of rust allows for the subsequent degradation of the concrete in the form of cracks, delamination, and spalls.
What is the role of salt?
Oxygen is a primary cause of corrosion of metals used in the physical civil infrastructure. So why are salts from marine environments, de-icing salts, industrial applications, and soil/ground water often associated with corrosion? While not a part of the redox reaction, salts are involved in the corrosion mechanism in different ways.
Depassivation
Some metals in certain environments can develop a protective layer comprised of its oxides and hydroxides. In some cases, the protective layer is a thin passivation layer that allows the rate of metal oxidation to be significantly lowered to levels that are not detrimental. In other cases, a protective patina develops but is categorically different from a passive film. In this case, the efficacy of the layer in part depends on how well and how uniform the patina forms on the metal. Salts are detrimental by that they can prevent these protective layers from forming, disrupt the uniformity of the layer, or destabilize the layer even when in environments that would otherwise promote passivity.
Ionic Conductivity
As an electrochemical process, transport of both electrical and ionic charges needs to be facilitated to allow corrosion to occur. Ionic charges move through the electrolyte (that may be the solution that the metal is immersed in, concrete or soil ground water, or even small amounts of moisture on the metal surface from the atmosphere at elevated relative humidities). If there is a large resistance to the transport of charges, corrosion rates will be relatively diminished even in corrosive environments. Conversely, conductive electrolyte such as those containing salts can facilitate corrosion reactions.
Deliquescence
In a similar vein, corrosion requires sufficient moisture presence. In dry conditions, corrosion rates are low because of the poor coupling to facilitate ionic transport. In atmospheric exposure environments, corrosion rates are elevated at relative humidities above ~65%. However, if the metal surface is contaminated with salts, the hygroscopic nature of the salts allows for the absorption of moisture so that elevated corrosion rates can develop even when the environmental relative humidity is low and at levels that would otherwise would not facilitate corrosion.
Crevice Corrosion
The presence of salts for metal structures with complicated geometries that create tight occluded spaces can exacerbate the development of crevice corrosion. The oxidation of metals in these conditions mostly occurs within the crevice environment and the reduction reaction (such as oxygen reduction) can develop throughout and elsewhere. The metal oxidation reaction within the occluded space allows for the accumulation of metal cations that cannot be easily transported out of the crevice. When salts such as those of chlorides are present, the small anions can transport into the crevice to maintain charge neutrality of the solution. The increased concentration of those anions increases the corrosion rates within the crevice in part due to water and metal-ion hydrolysis, where the cycle continues autocatalytically causing fast localized corrosion within the crevice.
Corrosion of Metals in Soil
Structural elements buried underground can have complex and diverse exposure environments that can range from the soil/fill material type and ground water condition (its chemistry and hydrogeology) that is complicated by spatial, temporal, and seasonal variations. Furthermore, the wide variety of construction material types, structural design/detail, loading conditions, service conditions, and even microbial ecology further diversify possible material degradation and corrosion mechanisms.
Soils and groundwater contain some level of aggressive ionic chemical species, such as chloride and sulfate ions, that can facilitate the corrosion process. The salt may come into direct contact with buried metal infrastructure or diffuse through reinforced/prestressed concrete elements to initiate corrosion of metal components. Soils and groundwater with higher salinity can also facilitate corrosion by increasing the conductivity of the medium.
Buried structural elements may be laid across spatially different soil/fill materials with different levels of compaction and moisture contents. The differential physical, chemical, and electrical characteristics of the exposure environment can facilitate galvanic coupling of metal components to promote macrocell corrosion as well as corrosion due to differential aeration. The soil and fill material can sometimes interact with the ground water and cause unintended changes to the water chemistry such as pH, hardness, and alkalinity that can promote corrosion. Various bacteria in the microbial ecology of the soils can promote corrosion in various ways depending on the metabolic behavior of the microbes. Sulfate-reducing bacteria has been ascribed to corrosion of buried pipes in anaerobic sulfate environments.
For a given subsurface geology or engineered substructural system, environmental parameters of interest include:
- Soil pH
- Soil resistivity
- Ground water hardness
- Moisture content
- Time of wetness
- Chloride and sulfate content
- Aeration
- Microbiological activity
- Corrosion rate
Corrosion of metal components of buried structures can lead to structural failure, increased maintenance and repair costs, shortened service life, and reduced operation and public service. Mitigation strategies vary and can include selection of corrosion resistant metals, concrete and fill material, fill compaction, use of protective coatings, implementation of cathodic protection systems. Design of structural systems often allow for wastage of materials in consideration of uniform corrosion at an expected corrosion rate. Despite these efforts, severe degradation does occur with direct impact to the public. Examples include disruption of water and sewer service and failure of bridge piers.
Interaction with Environmental Changes, Water Usage/Management, and Salt-Water Intrusion
Material selection and element design will consider a variety of service conditions such as loads, exposure, serviceability, and durability. In terms of possible corrosion, the aggressivity of the soil is often categorized by environmental classifications and from initial soil investigation prior to construction and service. Environmental classifications depend on parameters such as soil pH, electrical resistivity, chloride and sulfate ion content. Some guidance in corrosion allowance from various industries are provided for some environmental classifications. For example, FHWA provides guidance for 0.003 inch/year corrosion loss for steel piles in aggressive non-marine environments. Some transportation agencies provide guidance on minimum required sacrificial wastage depending on environmental classifications for a given design service life. The variation of allowances based on environmental classification identified prior to construction would suggest that spatial and temporal variations due to salt-water intrusion and recurrent ground water flooding could cause possible corrosion vulnerability.
Monitoring of spatial and temporal changes in soil and groundwater chemical and electrical characteristics is needed to fill the gap in information for current risk assessment in light of environmental changes associated with water management and salt-water intrusion. Further examination and verification of corrosion rates and electrochemical behavior is needed to support future implementation for design guidance, assessment, mitigation and repair of buried structures for the various related industries.