Atlas Piers of Atlanta, Inc.

Atlas Piers of Atlanta, Inc.
P.O. Box 3313
Alpharetta, GA 30023
Phone: 770-740-0400
Fax: 770-740-1513
Email: info@atlaspiers.com

CORROSION GUIDE

Based on the CORROSION GUIDE of Atlas Systems, Inc.

IMPORTANT RECOMMENDATIONS FOR USE OF THE FOLLOWING CORROSION STUDY

As an introduction to the effects of the soil on metallic materials, Atlas Systems, Inc. prepared an overview study on corrosion. We strongly suggest that the references given on Page C9 and other publications be read to fully evaluate the effects of corrosion on metallic materials in your area. We recommend consulting a qualified Registered Professional Engineer to perform a field evaluation and to design a protection plan when using Atlas Foundation Repair Products in an environment known to be corrosion aggressive.

Corrosion Analysis of Steel Foundation Products

CORROSION ANALYSIS AND PROTECTION SYSTEMS

OVERVIEW STUDY OF CORROSION OF STEEL FOUNDATION PRODUCTS

EXAMPLE OF CORROSION ANALYSIS FOR GALVANIZED HELICAL PIER

TYPICAL SPECIFICATION FOR EPOXY CORROSION PROTECTION

PART 1 - BACKGROUND

It is widely recognized that the placement of metallic materials below ground can result in a continuous process of corrosion. Corrosion is the destruction of a metal (loss of metallic structure by chemical or electrochemical reaction with its environment). The surfaces of these buried metallic structures are attacked through the migration of ions from the surface, resulting in a reduction in the wall thickness of the structure and over time, a reduction in the structural capacity of the element. Ultimately, if the extent of corrosion is significant enough, the element may fail. Corrosion is one of the key factors in limiting the life expectancy of steel foundation structures.

The causes of corrosion for buried metallic structures are generally understood, but this knowledge base does not always permit an accurate prediction of a design life when placed in a corrosive environment. This report provides a general overview of corrosion in the context of the use of Atlas Systems, Inc. foundation support products. This report is not intended to be used as a design guide, but rather to provide guidance in establishing whether corrosion could be a critical factor in a foundation support application. Where foundation support products are to be used in a known corrosive environment, a qualified engineer, knowledgeable in design for corrosion environments, must be retained.

PART 2 - FACTORS INFLUENCING CORROSION

2.1 CORROSION DEFINITION AND PROCESS
Corrosion is defined as the deterioration of the metallic structure due to its interaction with the surrounding environment. This phenomena occurs by an electrochemical process. In order for corrosion of the underground metallic structure to occur, the following conditions must be met:

2.1.1 A DIFFERENCE IN ELECTRICAL POTENTIAL: A difference in electric potential must exist between two points on the metallic structure (This can be caused by strains, contact with different soil types, inhomogeneities in metal, etc.). This potential difference causes the development of anodes and cathodes in the metal surface. The anode and cathode must be electrically connected.

2.1.2 ELECTROLYTE: A surrounding electrolyte, which is the moisture within the soil structure, may contain dissolved chemical elements (ions). Availability of oxygen (aeration) is also essential to the corrosion process. The presence (or absence) of these ions, as well as their nature and concentration, determines the electrical conductivity, or resistivity of the electrolyte. Resistivity is measured in units of ohm-cm. The most corrosive soils have the least resistivity.

Under the conditions described above, metal ions will migrate from the anodic locations to the cathodic locations. It is the loss in metal at the anodic locations that result in the degradation of the underground metallic structure.

2.2 CONTROLLING FACTORS FOR CORROSION

2.2.1 SOIL TYPE: Some soil types are more corrosive than others. The physical and mineralogical

2.2.2 SOIL pH: While soil corrosivity can exist within a broad range of soil conditions, the extent of acidity or alkalinity of a soil, as expressed by pH, does influence corrosion susceptibility and rates. pH is a measure of the degree of hydrogen-ion concentration. The pH range and classification is shown below:; Corrosion of metals within soils can occur over a broad range of pH’s.

2.2.3 OXYGEN AVAILABLILITY: In addition to soil moisture, free oxygen must be available in the corrosion process. Oxygen combines with the metal ions to form oxides, hydroxides and metal salts. Corrosion will not occur below underground groundwater tables (GWT) since free oxygen is not available.

2.2.4 SOIL RESISTIVITY: Soil resistivity has a strong influence on the corrosion rate. Generally, the higher the resistivity (measured in ohm-cm), the lower the corrosion rate. Soil resistivity arises from a number of factors, but fine grained soils (silts, loams, clays, and peats) have the lowest resistivities and thus the greatest corrosion susceptibility.
In-situ field measurements of soil resistivity are the most accurate way to determine resistivities. The recommended technique is to use the 4-pin resistivity test according to ASTM G57-78 standard specification.

PART 3 - PREDICTION OF CORROSION POTENTIAL ENVIRONMENT

3.1 GENERAL TECHNIQUE: As indicated previously, there are a number of variables that influence the corrosion potential for underground metallic structures. The National Bureau of Standards 1 (now the National Institute for Standards and Technology) has conducted extensive field testing of buried metal structures for evaluating corrosion levels related to the more significant variables. These results, along with similar field experimentation results 2 , were used to develop Figure 1. Figure 1 provides a technique for assessing those situations for which concern and design consideration for corrosion must be taken into account when metallic structures are placed below ground. In addition, if the specific information on a soil is available (soil type, pH, resistivity), Figure 1 can be used to provide a preliminary estimate for metal corrosion loss of bare steel. The NBS publication 1 can also be used to find a comparable soil and condition for estimating the rate of corrosion.

3.2 EXAMPLE OF SITE SPECIFIC SOIL ASSESSMENT - Tulsa County, Oklahoma: The United States Department of Agriculture has developed and published soil survey maps and reports for most counties in all of the United States. These maps are generally available through the local Soil Conservation Office. Contained in these reports are physical and chemical descriptions of the soil type(s), properties, features and behavior under certain conditions.

Most of these soils are underlain by shale, sandstone or limestone bedrock. Table 1 gives a summary of the corrosion related properties for these soils as well as some observations.

PART 4 – CORROSION CONTROL TECHNIQUES

Depending upon the classifications as to the corrosion potential for a soil environment, several alternatives are available to deter the corrosion cycle and extend the performance life of the underground steel element. These control measures can be split into categories.

Passive Control – for use in soils classified as slight to moderate corrosion potential
Active Control – for use in soils classified as moderate to severe corrosion potential

4.1 PASSIVE CONTROL – Manufactured Metallic Coating

Atlas Systems, Inc. provides a triple coat corrosion protection (3 mil coating) as a standard feature on the AP-3500 series Pier Pipe. The triple coating consists of:

Hot Dipped Zinc
Galvanized Chromate Coating and 3.) Electrostatic Bonded Polyurethane Finish Coating. The triple coating can significantly reduce the corrosion process by mechanically preventing access of oxygen to the steel surface of the pipe. Because of the thinness of this film and possible scratching of the coating, this technique should not be used in soils classified as severe corrosion potential. Thicker coatings (5 mils) have shown extended life three of five to fifteen years. Hot dipped galvanizing provides a thicker coating and is available as an option on Atlas Resistance Piers and Atlas Helical Foundation Piers. The galvanized coating serves as an anode to provide cathodic protection.
The results of the studies conducted by the National Bureau of Standards indicated that a galvanized coating (zinc) was effective in delaying the onset of corrosion in the buried steel structures. Typical conclusions drawn from this study for the 5 mil (3 oz./ft ²) galvanized coating include:
- Adequate for more than 10 years corrosion protection for inorganic oxidizing soils.
- Adequate for more than 10 years corrosion protection for inorganic reducing soils.
- Insufficient for corrosion protection in highly reducing organic soils and cinders. (Typically offers 3 to 5 years of protection)
It was noted, however, that the use of a galvanized coating significantly reduces the rate of corrosion of the underlying steel structure once the zinc coating was destroyed.
The observed rates of corrosion for the galvanized coating were different (less) than that for bare steel in the NBS study. For galvanized coatings (zinc) of 5 mils., the following (Equation 1) can be used to estimate the corrosion (weight loss) rate.

For the thinner galvanized coating of 3 mils (1.8 oz/ft²), the rate of galvanized coating loss is 2 to 3 times that determined from Equation 1.

4.2 PASSIVE CONTROL – Bituminous and Other Coatings

Bituminous as well as other materials have been used as coatings on buried steel elements for years as a corrosion protection technique. The primary requirements of a bituminous coating are good adherence (permanence), continuous coating and resistance to water absorption. The bituminous coating can either be heat baked onto the pier or field applied just prior to installation. As is the case for the manufactured coatings, this coating technique prevents oxygen and water from contacting the metal surface, thus preventing or retarding the corrosion process. Atlas Systems, Inc. has prepared a standard specification for bituminous coatings of steel piers. This specification is given in Appendix I, and is intended as an added corrosion protection for the thinner coat (3 mils) galvanized Atlas Resistance Pier (AP-3500 series) in order to extend its life. That portion of the specification that relates to the tar-epoxy enamel can be used to provide a corrosion coating for the entire pier assembly and shaft. An example of a representative vendor and approximate coating thickness for corrosion protection is given below:

E22 Hi-Mil SHER-TAR EPOXY ENAMEL (A one-coat, cured coal tar epoxy enamel; brush or spray application of 16 to 24 mils; available from Sherwin-Williams Co.)

A limited amount of available data indicates that bituminous coatings can extend the performance life of underground steel piers by 5 to 15 1,3,6 years, depending on the soil environment and the thickness of the coating. For the vast majority of Atlas Pier underpinning applications, the use of coating techniques (galvanized or bituminous) will provide a sufficiently long-term solution for corrosion protection.

SOIL SURVERY REPORTS

The primary purpose in use of soil survey reports is to gain a preliminary indication of the corrosive potential of soils where Atlas Piers may be place in underpinning projects. Given a potentially corrosive soil, a next step would be to measure the pH and resistivity a of the soils(s) in the approximate location where the piers are to be placed. This information can then be used along with Figure 1 to further establish the corrosive potential of the soil(s). In such cases, specific corrosion control efforts must be designed into the Atlas Pier underpinning project. It is recommended that a qualified professional engineer be employed for such projects.

4.3 EXAMPLE OF CORROSION LIFE ANALYSIS – Passive systems

4.3.1 BACKGROUND
An access bridge is being designed to cross a wetland area. Because the area is a wetland, Atlas Systems, Inc. Helical Foundation Piers are being installed for the bridge foundation. The Atlas product chosen is AHSG-RD3500.300 /10/12/14/. Soil borings at the site indicated that approximately the first 10 feet of the soil profile is an organic silt with low Standard Penetration Test (SPT) blow counts (N = 2 to 4 per foot). These results raised a concern about corrosion effects on the helical pier. Additional testing of the organic silt indicated the following results:

pH = 5.0 Soil Resistivity = 960 ohm-cm
Reference to Figure 1 (see page seven in the Corrosion PDF file) indicated that these test results place the organic silt in the severe corrosion environment region.

The helical foundation pier, shaft and components will receive a 5 mil thick (3 oz/ft²) galvanized coating as added corrosion protection. An estimate is required of the life of the pier tube shaft on the assumption that it can experience a maximum 10 per cent loss in wall thickness.

4.3.2 ESTIMATED LIFE OF GALVANIZED COAT

Using Equation 1 with a resistivity of 960 ohm-cm results in a weight loss per year due to corrosion of:

CL3 = 0.25 - 0.12 log (960/150) = 0.25 - 0.12 (0.806) CL3 = 0.15 oz/ft² per year

The estimated life of the galvanized coat is:
Life = 3 oz/ft²/ 0.15 oz/ft² = 20 years

4.3.3 ESTIMATED ADDITIONAL LIFE OF STEEL

Reference to Figure 1 indicates a corrosion weight loss range for bare steel of approximately 5 to 15 oz/ft² for a 10-year period. In this case (also checking the NBS data) an estimate was used of 10 oz/ft² for 10 years or 1.0 oz/ft² per year. A 10 per cent weight loss of the wall thickness of the steel results in:

10% x [0.300 in / 12 in/ft] x 489.6 lb/ft³ x 16 oz/lb = 20 oz/ft²

The estimated additional life becomes:

Life = 20 oz/ft² / 1.0 oz/ft³ /year = 20 years

4.3.4 SUMMARY

The results of this analysis indicate that based on the assumptions indicated, the Atlas-Helical Pier system will experience an approximate 40 year life. Each project and soil condition is different. It is strongly recommended that a professional engineer, experienced in corrosion behavior, be consulted for corrosion life analysis and recommendations.

4.4 ACTIVE CONTROL – Cathodic Protection

As indicated previously, corrosion is an electrochemical process that involves a flow of direct electrical current from the corroding (anodic) areas of the underground metallic structure into the electrolyte and back onto the metallic structure at the non-corroding (cathodic) areas. In situations where metallic structures, such as Atlas Foundation Support Products, are to be placed in a moderate to severe corrosive soil environment, an active corrosion control technique should be used. This active control technique is termed cathodic protection.

The basic principle in cathodic protection is to apply a direct current of higher electromotive potential than that generated by the corroding metallic structure, thus effectively eliminating the corrosion process. Two options are available for cathodic protection:

4.4.1 SACRIFICIAL ANODES

Magnesium, zinc and aluminum are the most commonly used sacrificial anodes. The sacrificial anode (galvanic) is attached to each underground metallic structure by a metallic conductor (cable) and placed within the common electrolyte (soil medium). The sacrificial anode works best when a small amount of current is needed and/or when the soil resistivities are low.

In designing and using sacrificial anode systems, the soil profile conditions as to the type of soil, resistivities, soil pH and location of the groundwater table (if present) must be determined. Among the design considerations for the system:

Use of wire type or canister type anode
Selection of the appropriate anode material (magnesium, titanium, etc.)
Designing the ground bed (location, dimensions, horizontal vs. vertical, depth of placement, type of backfill, etc.)
Determining the number of piers per anode
Type, size and connections between pier(s) and the sacrificial anode

4.4.2 IMPRESSED CURRENT

In areas of the most severe corrosion potential, where a larger current is required and/or in high resistance electrolytes, an impressed current system is used which requires a power source, rectifier and a ground bed of impressed current anodes. These systems require a continuous external power source.

The majority of applications where Atlas Foundation Support Products may be specified will not require an active corrosion protection system. In those cases where the combination of soil and electrolyte conditions require an “active system”, the sacrificial anode protection system will likely be the most economical approach.

Active cathodic protection systems must be individually designed to the specific application. The major variables are soil moisture content, resistivity of soil and pH. Each of these items influences the final selection of the cathodic protection system. Typical design life for the cathodic protection is 10 to 20 years, depending upon the size and length of the anode canister.

Atlas Piers of Atlanta, Inc.
For more information please contact us at info@atlaspiers.com

Office: 770-740-0400

Fax: 770 740 1513