锈层损伤对低合金耐候钢大气腐蚀行为的影响

The corrosion resistance of low alloy weathering steel in atmosphere derives from the compact and protective rust layer on its surpace, which forms during corrosion. However, the protective rust layer is frequently damaged during service of steel structures. The application prospects of weathering steel atre largely determined by further corrosion behavior of weathering steel with damaged rust layer. The damage will be induced on purpose in rust layers on surfaces of samples with different composition and corroded for different time in different environments, through frezzing, heat shock, pressing and so on to simulate natural processes in the planned study. Then, further atmospheric corrosion will be carried out. By electrochemical test, mechanical measurement and direct observation, the further corrosion behavior around the damaged local rust layers, especially chemical composition, phase composition, specific surface area, defects, PH values, moving rates of corrosion fronts and incubation and development of local corrosion in steel substrates will be analyzed. In term of the experimental results obtained from the investigation, the main influence factors which decides the local corrosion around damaged rust layers will be determined. It is beneficial to designing of weathering steel with self-repairing ability to damaged rust layer. The main difficulty involved in inducing damage in rust layer is degree controlling of damage. The advantage of frezzing, heat shock and pressing suggested in the planned study is able to control damage quantitatively, while it also simulates the practical service.

低合金耐候钢的耐大气腐蚀性能源于其表面在腐蚀过程中形成的致密保护性锈层。但是在钢结构的服役过程中，该保护性锈层常常受到损伤。耐候钢在锈层损伤后的继续腐蚀行为，在很大程度上决定了耐候钢的应用价值。本申请拟模拟自然过程，通过l冷冻、热震、挤压等方式，在成分不同、且在不同条件下腐蚀不同时间的耐候钢表面锈层中人为引入损伤，然后继续进行大气腐蚀。通过电化学实验、力学实验和直接观察，分析损伤处的继续腐蚀行为，特别是损伤处新形成的腐蚀产物的成分、相组成、比表面积、缺陷形态、PH值变化、腐蚀前沿推进速率以及局部腐蚀在钢基体显微组织中的发生发展规律。通过所获得的实验结果，确定影响锈层损伤处局部腐蚀行为的主导因素，为设计具备锈层损伤自修复性能的耐候钢提供支持。在锈层中制造损伤的最大难度在于损伤程度控制。本申请提出的冷冻、热震、挤压等损伤制造方式，不仅模拟了实际服役过程，还能够定量控制损伤程度。

本项目详细研究了耐候钢表面锈层中几种典型损伤的特征，及其对耐候钢继续腐蚀行为的影响。有以下发现：（1）挤压造成集中于局部的损伤，而热震、冷冻、预氧化等则造成分布于全锈层各处的多发性损伤。当挤压应力超过一定阈值，将在锈层中造成贯穿式的裂纹，导致损伤处锈层保护性完全丧失，并使该局部的腐蚀在随后的腐蚀过程中持续加重，这一现象可借助损伤所造成的大阴极、小阳极而由电化学理论得到解释。（2）热震和冷冻会在外锈层中造成明显损伤，其中热震不能导致裂纹形核，但可以使得原有裂纹扩展，而冷冻则由于外锈层中大尺寸孔洞中所含液态水结冰时的体积膨胀效应在外锈层中引入裂纹。内锈层由于裂纹、孔洞密度较低，抗热震、冷冻损伤能力较强。由于外锈层中的裂纹能扩展到内锈层，去除了外锈层的内锈层抗热震、冷冻损伤能力优于全锈层。（3）裂纹在内锈层倾向于沿平行钢基体/锈层界面的方向扩展。钢基体/锈层界面的断裂韧性高于锈层本身的断裂韧性。（4）在继续腐蚀过程中，经历热震、冷冻的样品倾向于通过在损伤处形成新的致密锈层的方式恢复保护性，耐候钢锈层损伤修复能力明显优于碳钢。（5）预氧化所形成的致密氧化皮在腐蚀初期对钢基体有一定保护作用，但在经历长时间大气腐蚀的情况下，氧化皮会失稳和转化，成为锈层中的缺陷，并在一定时间段内加速大气腐蚀过程。（6）插入大气腐蚀过程中的电偶腐蚀，在阴阳极电位差低于一定值的情况下，不仅不会对大气腐蚀锈层造成损伤，还会提高大气腐蚀的均匀性，但同时会延缓锈层稳定化进程。（7）内锈层由平行于钢基体/锈层界面，成分呈周期性涨落的条带构成，裂纹倾向于沿条带间界面扩展。（8）醇吸附-脱附方法是一种可以定量表征锈层缺陷的非破坏性方法。（9）通过预腐蚀方法控制锈层缺陷，有可能实现对锈层颜色的选择和调控。