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For porous metallic structures , there are two main approaches , based on surface coating and surface corrosion .
 engineering feature: porous metallic structures; concept or principle: surface; application: coating; concept or principle: surface; concept or principle: corrosion
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: manufacturing standard, machanical property, manufacturing process, material, engineering feature, process parameter, machine or equipment, application, process characterization, enabling technology, biomedical, concept or principle 
Text: {0} 
Answer:
 engineering feature: porous metallic structures; concept or principle: surface; application: coating; concept or principle: surface; concept or principle: corrosion