Corrosion Protection Mechanism of Novel Copper Alloy Resistant to Formicary Corrosion

Abstract

Copper tubes are widely used in heat exchangers for air conditioning apparatus owing to their superior heat conductivity, formability, and corrosion resistance. However, in spite of their remarkable corrosion performance, copper tubes could undergo a specific type of indoor corrosion called “formicary corrosion”. This is still the most important concern of using copper tubes because increasing numbers of early failure are considered to be due to this attack. Formicary corrosion, also called “ant nest corrosion”, is a form of localized copper corrosion that progresses insidiously and rapidly. The corrosion morphology is literally ant-nest-like shape, resulted from randomly branched and interconnected small tunnels that propagate independently of grain boundary and crystallographic orientation. Early detection is difficult because the opening of the corrosion pit is too small to be seen with unaided eye. Formicary corrosion is caused by organic acids, typically formic acid and acetic acid, which are often released from construction materials. Thus, there is a growing interest in development of a new copper alloy resistant to formicary corrosion. It is widely recognized that phosphorus deoxidized copper (PDC) containing a slight amount of phosphorus (0.015 to 0.040%) is more sensitive to formicary corrosion than oxygen free copper (ODC). However, our group have extensively studied the effect of phosphorus content on the corrosion sensitivity and discovered that more than 0.2mass% of phosphorus addition dramatically enhances the corrosion resistance without spoiling mechanical properties, such as drawability and form rollability. Copper alloys that comprise 0.22 to 0.29 mass% of phosphorus were formed to inner grooved tubes with 0.25 mm wall thickness. PDC and OFC tubes were also prepared for control. To assess the formicary corrosion resistance, specimens were suspended in 2L plastic bottles with 100 ml of 0.01 or 0.1vol% formic acid so that the specimens were exposed to formic acid vapor. These bottles were thermally cycled between 40 ºC for 22 hours and room temperature for 2 hours except weekend. After corrosion tests, corrosion products were removed using 5% sulphuric acid. Dye penetrant inspection was conducted to detect the tiny corrosion pits. Strongly dyed points were chosen and cross-sectioned to examine the corrosion morphology and pit depth. It is noteworthy that the strength of the indication is proportional to the pit depth. If the pit penetrates the wall thickness, the indication is distinctively strong and easy to distinguish. Cross-sectional observation revealed that under 0.1vol% formic acid environment both PDC and OFC suffered typical and severe formicary corrosion, penetrating the tube wall less than 20 days. However, even after 80 days of exposure, corrosion depths of the new alloy tubes were hampered below 150 micrometer. Furthermore, their corrosion morphologies were wide and shallow pitting-like attack, which was clearly different from formicary corrosion. The corrosion inhibition effect of phosphorus is attributed to the formation of phosphoric acid released from the alloy, as demonstrated by ion chromatography and EPMA.