I do think that your topic is a little too broad. To create the most well defined essay you need to narrow your scope. (The other posters are correct.) If you are wanting to make the greatest impact, you could look at police corruption in your own area. The information is relevant and will hit close to home.
Another possible way to narrow the topic could be to examine how a history of corruption affects hiring and use of law enforcement officials in a given location. Does Chicago have different training requirements or procedures for officers on the job than does an urban center that has not had the history of illegal relationships between the law and the government and/or the law and the criminals?
You might try to define police corruption as well. Recently, in New York City, there has been a scandal over police officers conspiring to "fix" traffic tickets for friends and family. Is that corruption? Where is the line to be drawn? That would be an important issue to address in an essay on corruption.
In America, the police do tend to be less corrupt in general than in some parts of the world. It depends on the level of corruption in the government overall.
There are several directions you could take to make your topic more specific. You could write about the effects of corruption on a developing country, for example. You could also write about police corruption cases in America. There’s also what corruption does to a community. Usually corruption means bribes and abuse of power. What would that do to a community?
My first question to you is are you talking about police corruption in general as in all or any place in the world, or do you plan to choose this country and one or two cities within this country? If you plan to discuss this topic without narrowing your focus, this essay will be difficult to write. Think about what you want to narrow in on--a specific city, management corruption, police on the street corruption, or how any corruption affects the city or country in which it takes place. For example, in Bolivia, South America, you can usually get a traffic ticket dismissed by paying off the officer who stopped you. With the lawlessness of the drug cartels affecting the country of Mexico and how the world sees this country, tourism is dropping because of the fear of violence. In this country, efforts have been made in many cities to clean up corruption. In the 1930's, St. Paul, Minnesota was a playground for the mob as many vacationed there and were protected by the local police. Since then, much has been done to make St. Paul an exemplary city with an honest police force. You just have to narrow your focus so that it's what you really want to talk about and then outline and write your essay.