Freshwater corrosion testing

TfflS CHAPTER PROVIDES an overview of standard corrosion testing procedures for freshwater systems. Additional information is provided in Chapter 31—Freshwater. Freshwater can be defined simply as water that is not salty brackish water as defined by the dictionary is water having a somewhat salty taste. A more quantitative definition of brackish water is water with a salinity between 0.5 and 17 parts per thousand [/]. Then freshwater can be further defined as water with a salinity of less than 0.5 parts per thou-setnd. For the purposes of this chapter, ft shwater systems include potable water, heating/cooling, steam, condensate, rivers, streams, lakes, and wastewater. 

The expected forms of corrosion can be anticipated once the water chemistry, operating conditions, materials of construction, and system layout are deBned. A corrosion test matrix can be designed by determining the expected forms of corrosion. Some of the common forms of corrosion found in freshwater systems are described in the chapter on Freshwater Testing in Section V of this manual. A list of test methods is presented in this chapter. The following section describes the applicability and significance of these standardized tests. 

Table 1 shows the most common forms of corrosion that affect metals in the freshwater environment. These corrosion forms are described in ASTM G 15, Terminology Relating to Corrosion and Corrosion Testing. 

In designing an effective laboratory test program, it is important to consider factors that influence corrosion in a freshwater environment. The following discussion summarizes these factors, but the complexity of the topic does not permit a detailed discussion in this chapter. Details on corrosion processes affecting metals in freshwaters are found in the references provided at the end of this chapter [7--5]. 

NACE RP-0775-87 Preparation and Installation of Corrosion Coupons and Interpretation of Test Data in Oilfield Operations Discusses the use of weight loss coupons, interpretation of data, and analysis Principab are appUcable to freshwater systems. 

Crevice corrosion, and concentration cell corrosion, susceptibility can be measured directly, by exposing test specimens to the environment. ASTM F 746, G 48, and G 78 can be used for evaluating these forms of corrosion. These methods were designed to measure crevice corrosion in severe electrolytes such as salt solutions, but can be modified for freshwaters. 

ASTM A 262) provide relative susceptibility only for austenitic stainless steels. These tests have little usefulness in freshwater environments. A metallographic analysis performed after test samples are exposed provides the most useful information about susceptibility to intergranular corrosion (ASTM E 3 and E 80). 

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